A large number of dwarf galaxies in the Fornax cluster (Figure 1) appear to be disturbed, most likely due to tides from the cluster gravity. In the standard cosmological model (ΛCDM) , the observable structure of the dwarfs is barely susceptible to gravitational effects of the cluster environment, as the dwarfs are surrounded by a dark matter halo. Because of this, it is very hard to explain the observations of the perturbed Fornax dwarfs in this theory. However, these observations can be easily explained in MOND, where dwarfs are much more susceptible to tides due to their lack of protective dark matter halos and the fact that they become quasi-Newtonian as they approach the cluster center due to the external field effect.
Figure 1: Fornax galaxy cluster. The yellow crosses mark all the objects identified in the Fornax deep survey (FDS) for this region of the sky, the black circles are masks for the spikes and reflection haloes, and the red crosses mark the objects that pass the selection criteria to be included in the FDS catalog. Image taken from Venhola et al. 2018.
The impact of tides on what the dwarfs look like is illustrated in Figure 2, which shows the fraction of disturbed galaxies as a function of tidal susceptibility η in ΛCDM and MOND, with η = 1 being the theoretical limit above which the dwarf would be unstable to cluster tides. Moreover, there is a lack of diffuse galaxies (large size and low mass) towards the cluster center. This is illustrated in Figure 3, which shows how at low projected separation from the cluster center, dwarfs of any given mass cannot be too large, but larger sizes are allowed further away. Figure 3 thus shows a clear tidal edge that cannot be explained by selection effects, since the survey detection limit would be a horizontal line at 1 on this plot such that dwarfs above it cannot be detected. Diffuse dwarf galaxies are clearly detectable, but are missing close to the cluster center. Another crucial detail in Figure 3 is that dwarfs close to the tidal edge are much more likely to appear disturbed, which is better quantified in Figure 2 in the rising fraction of disturbed galaxies with tidal stability η. The tidal edge is also evident in Figure 2 in that the dwarfs only go up to some maximum value of η, which should be close to the theoretical stability limit of 1. This is roughly correct in MOND, but not in ΛCDM.
Figure 2: Fraction of disturbed galaxies for each tidal susceptibility bin in MOND (red) and ΛCDM (blue). Larger error bars in a bin indicate that it has fewer dwarfs. The bin width of the tidal susceptibility η is 0.5 in MOND and 0.1 in ΛCDM (each data point is plotted at the center of the bin). Notice the rising trend and the maximum η that arises in each theory.
Figure 3: Projected distances of Fornax dwarfs to the cluster center against the ratio Re/rmax, where Re is the dwarf radius containing half of its total stellar mass, and rmax is the maximum Re at fixed stellar mass above which the dwarf would not be detectable given the survey sensitivity. The dwarfs are classified as “disturbed” (red) “undisturbed” (blue). The black dashed line shows a clear tidal edge – at any given mass, large (diffuse) dwarfs are present only far from the cluster center. This is not a selection effect, as the survey limit is a horizontal line at 1 (though e.g. some nights could be particularly clear and allow us to discover a dwarf slightly above this).
We therefore conclude that MOND and its corresponding cosmological model νHDM (see blog post “Solving both crises in cosmology: the KBC-void and the Hubble-Tension” by Moritz Haslbauer) is capable of explaining not only the appearance of dwarf galaxies in the Fornax cluster, but also other ΛCDM problems related to clusters such as the early formation of El Gordo, a massive pair of interacting galaxy clusters. νHDM also better addresses larger scale problems such as the Hubble tension and the large local supervoid (KBC void) that probably causes it by means of enhanced structure formation in the non-local universe. These larger scale successes build on the long-standing success of MOND with galaxy rotation curves (“Hypothesis testing with gas rich galaxies”). MOND also offers a natural explanation for the Local Group satellite planes as tidal dwarf galaxies (“Modified gravity in plane sight”), and has achieved many other successes too numerous to list here (see other posts). Given all these results, the MOND framework appears better suited than the current cosmological model (ΛCDM) to solve the new astrophysical challenges that keep arising with the increase and improvement of the available astronomical data, which far surpass what was known in 1983 when MOND was first proposed.
In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.
(Guest post by Dr. Jörg Dabringhausen, Charles University in Prague, Dec. 18th 2020)
The hypothesis of dark matter in galaxies was originally brought up by observations. Zwicky (1933) first found out that galaxies were usually moving too fast to stay in the observed galaxy clusters, if the luminous matter was all there is in galaxies. With “luminous matter”, essentially all stars were meant. Stars are understood well in terms of how much mass in a star leads to a certain light strength, or luminosity. But if the light emitted by the galaxies in a galaxy cluster is translated to a stellar population similar to the stellar population of the Milky Way, the stellar population would not have enough mass by a factor of a couple hundreds to keep the galaxies bound to the cluster. Thus, the galaxy clusters would have dispersed billions of years ago, and today we would be surrounded by a uniform distribution of galaxies. But that is not what we see: galaxies are still in galaxy clusters today.
But the problem was not only with galaxy clusters. Rubin & Ford (1970) found out, that the Andromeda Galaxy rotates so fast, that its stars would disperse if only the standard gravity would keep them together. And the Anromeda galaxy turned out be the rule rather than the exception; all spiral galaxies that were studied later on showed similar trends (for example Rubin et al. 1980). So, not only galaxy clusters would disperse, but also the (spiral) galaxies themselves. It is like the riders (that is the stars) on a merry-go-round (that is the galaxy). Forces keep the riders on circles around the merry-go-round, and if the forces for some reason become weaker or cease to exist (for example because the link between the rider and the merry-go-round breaks), the riders would move away from it. But again, this is against our observations: There are large spiral galaxies everywhere around us (including our Milky Way), and the stars in them move on stable orbits.
In general, the problem of missing mass in galaxies is nowadays omnipresent. It arises because there are different ways to estimate masses in astronomy. One such way is to make educated guesses about the age and the composition of the stellar population of a galaxy, and calculate from there how much units of mass it should have per unit of luminosity. Astronomers call this a stellar mass estimate. Another way is to measure the radius of a galaxy and how fast stars move on average in it, then make some educated guesses about the dynamics of the galaxy, and calculate the ratio of mass to light from there. Astronomers call this a dynamical mass estimate. Ideally, stellar and dynamical mass would agree for the same galaxy, because the galaxy only has one real mass (within uncertainties, of course). In practice however, the dynamical mass is usually larger than the stellar mass, and the factor ranges from slightly above one to 10000 or so. Apparently, the error lies somewhere in the guesswork leading to the two different mass estimates. Astronomers tried to solve the problem of the missing visible matter in two general ways: Either by adding more matter, so that the matter in total would produce the observed gravitational force, or by changing the laws of gravity themselves and saying that the visible matter is all the matter there is in galaxies.
Adding more matter is mathematically the simpler solution, which is also why many people favoured it at first. The gravitational force is then linear in the critical range of values, that is weak to moderate gravity. This means that if there is twice the matter, there is also twice the gravitational force, independent of the total amount of matter there is. Note that from this point of view, the type of matter does not matter, as long as it is invisible, or nearly so. Also the Earth is near invisible next to the Sun, even though they both consist basically of the same kind of matter (that is atoms, not something exotic). It is only a matter of temperature that makes the Sun brighter than the Earth. Indeed, there was a theory that the missing matter are earth-like bodies (that is free-floating planets and brown dwarfs), until the needed quantity of those bodies was observationally excluded. More and more alternatives for the additional matter were excluded as well, so that we are today at the Lambda-Cold-Dark-Matter Model (LCDM-model) for this class of models. However, the LCDM-model requires exotic dark matter beyond the standard model of particles. But this kind of matter has not been discovered yet, including in the largest accelerators like CERN. Nevertheless, this first group of physicists still believes the LCDM-model to be true in general (even though there are some changes to be made) and therefore they continue to search for the so far still hypothetic dark-matter particle.
The second group of physicists rather correct the law of gravity than adding a hypothetic particle beyond the standard model of particle physics. It is like whichever way you go, you have to expand a theory which has been extremely succesful so far: you either have to give up the standard model of particle physics in order to save the LCDM-model, or have to have to give up general relativity, with Newtonian gravity as its limiting case for weak and moderate gravity. This new theory of gravity is, unlike Newtonian gravity, not linear in the critial range. This means that twice the matter does not necessarily mean twice the gravity when the gravitational force is weak enough. This has a funny consequence, which is in contrast to our daily-life experience, namely that the same amount of matter suddently looks like it becomes more gravitating when you spread it out thinly enough. Lüghausen et al. (2015) therefore called it “phantom dark matter”, because this dark matter is a mirage that disappears when the real matter is put close enough together. (Of course, inside the Solar system, the matter must be on average dense enough for the gravitational force to be linear – otherwise we would not be able to send spaceships with high precision to other planets using Newtonian gravity.) This second set of theories leads to Modified Newtonian Dynamics or Milgromian Dynamics (MOND).
Here, I will concentrate on the “missing” matter of elliptical galaxies – “missing” in the sense that there is usually less matter if seen from a stellar perspective than if seen from a dynamical perspective on the same galaxy. Are there alternatives to adding exotic dark matter to the visible matter, and thus supportive to the second group of physicists?
First of all, let’s start with the question of what an elliptical galaxy is. A very short answer would be that they are more or less like the spiral galaxies, but without the disks that contain the spirals. So, only the central bulge is there, and hence, they are called ellipitical because of their elliptical shape. That central bulge can however be very massive, and the most massive elliptical galaxies are even more massive than the most massive spiral galaxies (bulge and disk of the spirals together)!
Going a bit more to the details of elliptical galaxies, they show however some diversity in their mass and radius. I will distingish them into three different kinds of objects, namely ultra-compact dwarf galaxies (UCDs), conventional elliptical galaxies (Es) and dwarf spheroidal galaxies (dSphs), and discuss the invisible matter in each of them. We will see that the invisible matter is just a mirage in some of them, while others contain really some more matter than originally accounted for, but not the exotic dark matter predicted by the LCDM-model.
UCDs (Figures 1 and 2) stand a little apart from the other elliptical galaxies, and some doubt that some of them really are galaxies, and not just very massive star clusters. The reason lies in their compactness, which makes them look much like very massive globular clusters. However, their compactness also places them deeply in the Newtonian regime, so there is literally no room for the phantom dark matter of MOND. Yet, it was claimed that they may contain dark matter (see for example by Drinkwater et al 2004 and Hasegan et al. 2005).
The reason for that is that at the turn of the millenium, it was popular among atronomers that the stellar initial mass function (IMF) is universal (see for example Kroupa 2001). What this means is that all stellar systems formed with a fixed ratio of massive stars to light stars, and only the age of the stars and their chemical composition may change from stellar system to stellar system. This is not to say that people back then were unaware of the influence that, for example, different temperatures and chemical composition had on the process of star formation. Rather, they were looking for different IMFs, but did not find supportable evidence for them in resolved stellar populations. However, when modeling a UCD (or any other kind of stellar system) with the universal IMF, there is maximum ratio between stellar mass and stellar light that can be reached for any reasonable stellar ages and chemical compositions. Nevertheless, there are many UCDs above that limit, and Dabringhausen et al. (2008) showed that this is not just a statistical uncertainty. So, there must be a reason for this unseen mass, and the exotic dark matter that comes with the LCDM-model was a proposition.
However, Murray (2009) voiced serious doubts that the LCDM-model could accomodate enough exotic dark matter inside the tiny radii of UCDs. This is even though the dark-matter halos around the galaxies can be very massive in the LCDM-model. However, the LCDM-model then also predicts that the halos would be very extended, and thus the density (that is mass per volume) of the dark-matter halo would be very thin. So, the total mass of the dark-matter halo may be gigantic, but the fraction of its mass inside a UCD would be tiny because of the small radius of the UCD, and this tiny amount of dark matter inside the UCD would not influence the internal dynamics of the UCD much. Thus, in short, it is not the exotic dark matter of the LCDM-model that increases the mass of the UCDs. It is then likely “conventional” matter, for example from a different IMF. Thus, the word “universal” IMF is then misleading because the IMF is in fact not universal, but “standard” IMF or “canonical” IMF are pretty good replacements. After all, this IMF pretty much seems to be the standard in our immediate surroundings (in an astromical sense); that is regions whose mixture of chemical elements is like that of the Sun and which do not form so many stars at present.
In UCDs, the conditions under which star formation took place were probably far away from those we know to produce the standard IMF. Thus, Dabringhausen et al. (2009) proposed that the UCDs may have formed with an IMF that had a different shape than the standard IMF, namely one that formed more massive stars. (IMFs that have more massive stars than they should have according to the standard IMF are called “top-heavy”.) These massive stars are known to be short-lived, and after they have burned all their nuclear fuel, they leave remnants which produce little or no light compared to their mass. These remnants exist of course in any aged stellar population, but if the IMF had more massive stars once, it has more stellar remnants now. The stellar remnants thus increase the ratio between mass and light, and make a UCD “darker”. Dabringhausen et al. (2012) also tried an alternative way to detect those additional stellar remnants by looking for systems, where a stellar remnant accretes matter from a companion star. Those stellar systems become distinctive X-ray sources, and are thus countable. They compared the numbers they found in UCDs to the numbers they found in globular clusters (that is stellar systems more or less like UCDs, but less massive), and they found more X-ray sources in UCDs than they expected. This as well could indicate that there are more high-mass stars per low-mass stars in UCDs. Based also on their works, Marks et al. (2012) proposed an IMF that changes with the mass of the stellar system (that is from globular clusters to UCDs) and with the chemical composition. Thus, they gave up the notion of the universal IMF, but explained changes in the ratio between mass and light in UCDs with changes in their IMFs.
Another way to increase the mass of UCDs, but not their emission of light, are central massive black holes. In a black hole so much mass is kept, that nothing that comes too close to it can escape it, not even light. Black holes are a prediction of general relativity and known to exist. For example, very massive stars become black holes when all their nuclear fuel is burned, and the pressure from stellar radiation no longer opposes the pull of gravity. Or, as another example, there is a massive black hole at the center of the Milky Way, and many other galaxies as well, even though it is less clear than for massive stars how those came to be. (This year’s Nobel Prize for physics was about the detection of this central black hole.) But if massive black holes are common at the centers of galaxies, why can’t UCDs have them as well? However, a massive central black hole is easy to overlook at the distance of known UCDs. That is because at the distance of UCDs, the stars look like they are almost located at a single point in space, whereas the mass of the central massive black hole is precisely located a this single point. Thus, if seen from Earth, there is not much difference in the distribution of matter, while the central massive black hole would still add its mass to the mass of the stellar population. Therefore, only by careful observations with the telescopes with the best optical resolution, one has a chance to detect them. Nevertheless, massive central black holes were indeed proposed as a solution for the problem of the missing mass in UCDs; for example by Mieske et al. (2013) and Janz et al. (2015). Seth et al. (2014) then observationally confirmed a massive central black hole in a UCD for the first time. Later, massive black holes were also discovered in other UCDs, see for example Afanasiev et al. (2018).
Naturally, also a mixture of non-standard IMFs and central massive black holes is possible to explain why UCDs are so massive for their light. However, what is important here is that there are less far-fetched alternatives to exotic dark matter in UCDs.
2.) Conventional elliptical galaxies
The conventional elliptical galaxies are not only usually more massive than the UCDs, but also far more extended. What I mean with “conventional” is that they were among the first galaxies to be identified as galaxies – this was in the 1920ies, when people like Hubble first discovered that some “nebulae” are not just gas clouds inside the Milky Way, but distant stellar islands just like the Milky Way. It is unclear what mass exactly is required for an elliptical galaxy in order to be coventional, perhaps 108 Solar masses or so. This unclearity is because there is an extension of elliptical galaxies to even lower masses, which are however not (compact, star-cluster-like) UCDs, but (extended, galaxy-like) dwarf Spheroidal galaxies (dSphs). However, there are some specialities on dSphs about dark matter and its seeming existence, and therefore I will treat them in an own section. What I will not do, though, is to distinguish the elliptical galaxies into dwarf elliptical galaxies and elliptical galaxies proper, because this distinction in merely historical in my eyes (see also Ferguson & Binggeli 1994 about this). The most massive of all galaxies (about 1012 Solar masses) are conventional elliptical galaxies, too.
So, how much exotic dark matter do elliptical galaxies contain, if any? Cappellari et al. (2006), for instance, found out that the conventional elliptical galaxies they observed had on average 30 percent too much mass for the IMF they assumed. They suggested that the missing mass could be the dark matter predicted by the LCDM-model. However, for this finding, they also assumed that the standard IMF is universal for all star-forming regions. Tortora et al. (2014) later tried to fix this without exotic dark matter, but MOND. They also failed with a universal IMF, but not if the IMF was changing with the mass of the galaxy. So, the real question is: Can the IMF change with galaxy mass or is the standard IMF also the universal IMF?
For answering this question, let’s look at star clusters, which are the building blocks of galaxies. Could a star cluster have a star more massive than the cluster itself? Of course not. Actually, Weidner et al. (2010) found out that the mass of the most massive star of a star cluster is much lower still. An impressive example of this was observed by Hsu et al (2012): They compared a large cluster of some mass with several adjacent small star clusters with the same mass in total. All the other parameters like age, chemical composition, and so on are the same, just how the total mass of the stars is bundled is different. However, the massive star cluster has heavier stars than the several small star clusters. This would not be a problem by itself, if the overall star formation was the same in all galaxies; that is when all galaxies form the same number of light star clusters per massive star cluster. But this is not the case. Weidner et al. (2004) found that the mass of the most massive cluster that can form in a galaxy depends on its star formation rate; that is how many stars form in a galaxy per time unit. Low-mass elliptical galaxies have low star formation rates and massive elliptical galaxies have high star formation rates. Thus, low-mass conventional elliptical galaxies have a lack of massive stars. This already is an argument against a universal IMF in all star clusters and in all galaxies.
The galaxies with the highest star formation rates (that is also the most massive galaxies) produce also star clusters in the mass range globular clusters and UCDs. Now, lets assume that these most massive star clusters are in fact UCDs and that these UCDs have IMFs with more massive stars per low-mass stars than “normal” star clusters (see the section about UCDs). Then the real IMF deviates from the once-thought universal IMF not only in low-mass star clusters (by not having any massive stars), but also in high-mass star clusters (by having too many massive stars). Now, remember what we have said about IMFs with more massive stars than the standard IMF: when they grow old, they produce less light per unit mass than the standard IMF. Or when a certain amout of light is observed, a stellar population with more massive stars and a certain age must have more mass to produce it. The stellar populations of elliptical galaxies are usually that old that the massive stars (which are short-lived) have already evolved into dark stellar remnants, and only the light stars continue to shine. So, if the IMF behaves with the star formation rate of the galaxies like it is assumed nowadays (see for example Kroupa & Weidner 2003 or Fontanot et al 2017), then the low-mass elliptical galaxies have a little less mass than assumed with the standard IMF for their light, and the massive elliptical have a little more mass than assumed with the standard IMF. This goes up to about twice the mass for the most massive conventional elliptical galaxies, and the point where the mass estimate is equal to that for the standard IMF is at approximately 109 Solar masses. Thus, for most conventional elliptical galaxies, the mass estimates are above the mass estimates for the standard IMF, and the “missing” mass is about the mass detected by Cappellari based on the standard IMF. (See also Dabringhausen et al. 2016 if you want to follow the brightness of elliptical galaxies with their mass, and Dabringhausen 2019 if you wish to go deeper on elliptical galaxies and non-standard IMFs). Thus, again like with UCDs, there is an alternative, more down-to-earth explanation for the excess mass of those elliptical galaxies.
3.) Dwarf speroidal galaxies (dSphs)
Dwarf spheriodal galaxies (dSphs, Figure 3) are in a way the low mass extension to “conventional” elliptical galaxies, because in a plot of their radius against their mass, they continue the line established by the conventional elliptical galaxies to lower masses. However, the brightest ones are in light and mass like UCDs, but way more extended than UCDs. In other words, there is a gap in radius between dSphs and UCDs (see Gilmore et. al 2007), in contrast to conventional elliptical galaxies and dSphs.
If it is true that dSphs are in fact very low-mass conventional elliptical galaxies, then we would expect them to be about 20 percent or so lighter than expected based on their light with a standard IMF. But in fact, they are way more massive. Just in order get a feeling for the numbers we are dealing with: Let’s say the standard IMF would predict a ratio of mass to light of 2 for a dSph, the ratio for the corrected IMF would then give 1.5, but the measured value is 2000 (all numbers are in Solar units). So, how can we be wrong to a factor up to approximately 1000 (even though in many cases less)?
This is where MOND finally kicks in, because the visible matter in dSphs is actually thin enough, in contrast to UCDs and Es. MOND can rise the ratio of the mass of a dSph over its light from values of a few (that is a stellar population in Newtonian dynamics) to values up to about 100. This fits the dynamical values of many dSphs, which would contain plenty of “dark” matter in Newtonian dynamics. Thus, in MOND, their dark matter is actually phantom dark matter – it would disappear if the matter was denser. Or, in other words, the difference between stellar and dynamical mass estimates disappears for those dSphs, and all is well. The precise value for a given dSph depends on which value the mass-to-light ratio of the stellar population would have according Newtonian dynamics and on how many stars are distributed over which volume, that is the density of visible matter. Estimates for the mass-to-light ratios in Newtonian and MONDian dynamics for a number of dSphs are for example given in Dabringhausen el al. (2016).
But it is also visible in Dabringhausen el al. (2016) that even MONDian dynamics cannot explain the mass-to-light ratios of the few dSphs, which have a mass-to-light ratio far beyond 100. So, have we finally found a failure of MOND? Not necessarily. So far, we have implicitly always assumed that the galaxies are in virial equilibrium. What this means is for instance the absence of tides because of other distracting souces of gravity. The tides on Earth are the best-known example, even though Earth is dense enough to be near tidal equilibrium, given the gravitational forces from the Moon and the Sun. We only see them so well because because in this case, the tides are happening right under our noses. Ultimately, there are tides on Earth because the Earth is an extended body. Thus, the gravitational force from the Moon pull on the near side of the Earth a bit stronger than on the far side, and the Earth is being stretched a bit by the tides. There are ebb and flow of the oceans on Earth, because the Earth also rotates, while the tides are always directed towards the Moon. There of course also other sources of gravity on Earth which cause tides (the Sun for instance), but the Moon is the strongest.
Also UCDs and conventional elliptical galaxies are dense enough to be nearly unaffected by neighboring galaxies, which are the potential reason for tides in them. But the internal gravity is comparatively weak on the thin matter of dSphs, so that they are easy to stretch by outside forces of other galaxies. Thus, the tidal forces form gigantic tidal “waves” consisting of stars. Every encounter with another galaxy pulls on the galaxy, because the gravitational force is stronger on the near side of the encounter than on the far side. This heats the galaxy up, meaning that the galaxy is being pulled out of virial equilibrium by the encounter and that the average velocities of the stars get faster with enconters. Finally, the tidal forces from encounters with other galaxies make the galaxy break apart.
Now, what would an observer from Earth see? The observer could for example see a dSph that has been heated up by a recent encounter with another galaxy, and is thus out of virial equilibrium. Or the dSph has found its virial equilibrium again, but at the cost of stars which have left the dSph, and are now moving faster or slower than the stars which are still bound to the galaxy. But the observer could be ignorant of this fact, and assume that all the stars (s)he sees are bound to the galaxy. Or the dSph has dissolved already completely, but the stars still move all along on similar orbits, even though they are not bound to each other any more. The radius in which the stars are is then just much larger than it would be, if the stars were bound to each other. If the observer then wrongly assumes the dSph to be in virial equilibrium, all these effects increase the dynamical mass estimate (not the real mass!) (s)he makes for the mass of the galaxy. And those effects could indeed raise the dynamical mass estimate by the required factor. For a discussion of tidal heating of dSphs under Newtonian gravity, see for example Kroupa (1997). McGaugh and Wolf (2010) made a similar study with MOND. Notably, they found for observed dSphs surrounding the Milky Way that if a dSph is more susceptible to tidal forces, it is also more likely to be outside virial equilibrium for MOND. For an interesting theoretical discussion of how a dissolving star cluster in a tidal field could be mistaken for a much more massive (but evidently not more luminous) dSph, see Dominguez et al. (2016).
However, the dSphs which are out of virial equilibrium far enough to increase the dynamically estimated mass-to-light ratio by a few or more compared to the real mass could just be a few dSphs out of a larger sample. For the majority, the effect would simply be too weak now, although their time to dissolve will also come. In other words, this scenario is highly improbable if gravity was Newtonian, because then all dSphs around the Milky Way must be in dissolution. However, if gravity is MONDian, only a few would be near their dissolution, while most would be in or near virial equilibrium – see Dabringhausen el al. (2016).
There is also another argument against dark matter in dSphs. Galaxies are usually not by themselves, but surrounded by other galaxies. Together, these galaxies form gravitationally bound galaxy clusters. But how do these galaxy clusters form? According to the LCDM-model, this happens by the infall of galaxies from all directions. They can come, the dSphs included, with any amount of exotic dark matter into a galaxy cluster. We will call those galaxies “primordial galaxies” from now on, because there is also another way to form galaxies that look like dSphs to an observer. This other way is through close encounters of already existing galaxies. In such encounters, matter is pulled away from the existing galaxies by gravity though tides (Figures 4 and 5), and new small galaxies can form from this matter. We know that this process happens. Otherwise, the elongated streaks of matter of, for instance, the Antennae Galaxies and the Tadpole Galaxy would be difficult to explain. Simulations of interacting galaxies, which are set up to reproduce situations like in the Antennae Galaxies, show also those streaks of matter like the ones observed (see for example Bournaud & Duc 2006 or Wetzstein et al. 2007). They are called tidal tails for obvious reasons. The Tadpole Galaxy even has a new small star-forming regions in its tidal tail, which may become dSphs. If aged enough, these dwarf galaxies may be difficult to distinguish from primordial galaxies of the same mass, though (see Dabringhausen & Kroupa 2013). However, in the following, we call galaxies of tidal origin “tidal dwarf galaxies”, in order to distinguish them from primordial galaxies. The tidal dwarf galaxies cannot contain the exotic dark matter of the LCDM-model, even if their progenitor galaxies did. The reason is that all matter that ends up in a tidal dwarf galaxy, whether visible or not, must have occupied similar regions of space with similar velocities also before the encounter of the existing galaxies. The total amount of the exotic dark matter may be huge, but most dark matter had other velocities and other locations, and therefore does not qualify to be bound to the tidal dwarf galaxy. After all, simulations of galaxy encounters by, for example, Barnes & Hernquist (1992) show that most visible matter that is to become a tidal dwarf galaxy comes from the disks of spiral galaxies. This visible matter does not only form a thin disk, as opposed to the presumed dark matter halo, but it also moves with the same velocity in the same direction, again in contrast to the presumed dark matter halo. Also, the tidal dwarf galaxies that form in an encounter of galaxies can only move in the plane of the encounter (because of the conservation of angular momentum). Thus, there is an easy way to distinguish the dSphs in the LCDM-model: those which move in a plane and those which cannot be assigned to a plane. Those in a plane are very likely tidal dwarf galaxies and cannot have any exotic dark matter. Those, however, which cannot be assigned to a plane might also be primordial and can thus contain dark matter (see for example Kroupa et al 2010). Now, what do observations tell us about the pattern of motion of the dSphs? In the Milky Way, it was shown by Lynden-Bell (1976) and by Kroupa et al. (2005) that the then known dSphs are most likely arranged in a plane. Later, additional objects and also velocities were added, but the long-lasting disk of Satellites was always confirmed (see for example Pawlowski et al. 2012 and Pawlowski & Kroupa 2020). This was according to some proponents of the LCDM-model just an exception, while other, they said more normal galaxies would have dSphs with random motions around them. However, it was shown then that also the Andromeda Galaxy has a disk of dSphs around it (for example Ibata et al 2013), and Centaurus A as well (Mueller et al 2018). In short, disks of satellites around major galaxies are more the rule than the exception, see for example Ibata et al (2014) for an attempt of a census. Thus, galaxies in these planes must manage their high dynamical mass-to-light ratios without exotic dark matter, despite numerous claims to the contrary from the LCDM-community. If MOND is the correct description of gravitation, then the large gravitating (phantom) masses of the satellite galaxies, as opposed to their small masses in stars, is beautifully resolved.
I have discussed the reasons for “dark” matter in elliptical galaxies, which comes ultimately from the comparison of different mass estimates. Also, some assumptions which were used for the lack of better knowledge have been proven wrong by now. This concerns the theory of a universal IMF in all star-forming regions, which was leading to a mismatch between the mass estimates from stellar populations and from the dynamics in UCDs and conventional elliptical galaxies. If the “one-size-fits-all” IMF is replaced by a more elaborate picture of the IMF, those differences disappear easily without using exotic dark matter or MOND. For dSphs, the situation is different. They cannot have exotic dark matter because it could not bind to them, but neither can their extreme mass-to-light ratios be explained with different stellar populations. Here, MOND and tidal fields offer an answer. Thus, adding more exotic dark matter to all galaxies until their dynamics is fitted might appear the simpler solution on first sight, but it is not necessarily the correct one. The seemingly more complicated solution without exotic dark matter stands a better test result here.
In The Dark Matter Crisis by Joerg Dabringhausen. A listing of contents of all contributions is available here.
We have not blogged for some time and an update on some of the developments concerning The Dark Matter Crisis has been posted here. Below are recent scientific developments which strongly suggest that the standard model of cosmology (the SMoC) which relies on the existence of cold or warm dark matter (C/WDM) particles is not a correct description of the observed Universe. Note that the SMoC which is based on the hypothesis that cold dark matter particles exist comprises the currently widely accepted LCDM cosmological model, while the SMoC which assumes warm dark matter particles exist constitutes the currently less popular LWDM cosmological model. The difference of both models in terms of structure formation and the type of galaxies formed is minimal, which is why both are referred to as the SMoC.
Why has the Cosmology Crisis become catastrophic?
First of all, C/WDM particles have still not been found after more than 40 years of searching! The account of the situation published on October 11th, 2020, on the Triton Station by Stacy McGaugh is worth reading. Stacy writes “… the field had already gone through many generations of predictions, with the theorists moving the goal posts every time a prediction was excluded. I have colleagues involved in WIMP searches that have left that field in disgust at having the goal posts moved on them: what good are the experimental searches if, every time they reach the promised land, they’re simply told the promised land is over the next horizon?“. In view of the available evidence challenging the existence of C/WDM particles, it is stunning to read “The existence of Dark (i.e., non-luminous and non-absorbing) Matter (DM) is by now well established” in Sec. 26.1.1 of the 2018 version of the Review of Particle Physics. Some five years ago I had dared to suggest to the editors and section authors to change this very statement to “The existence of Dark (i.e., non-luminous and non-absorbing) Matter (DM) is currently a leading hypothesis” or similar, but the short reply was quite unpleasant. It is unfortunate that only the cosmological argument leads one to the C/WDM particle hypothesis, there being no independent (non-cosmological and non-astronomical) evidence. Such evidence could have come from indications in the Standard Model of Particle Physics, for example, but this is not the case. Put in other words, if we had not known about cosmology or galaxy rotation curves, we would not be contemplating C/WDM particles. Thus, by the astronomical evidence having gone away (follow the Dark Matter Crisis), the physicists are left with nothing apart from belief. I would argue that the words “belief” and “opinion” should be banned from the language of natural sciences. Note that the situation is different for the fast collisionless matter (FCM, or “hot dark matter”) which appears in MOND-cosmological models (Angus 2009). Independetly of the astronomical evidence, the experimental fact that neutrinos have mass and oscillate suggests the existence of an additional sterile neutrino. Candidates for FCM particles thus arise independently of astronomy or cosmology. FCM particles do not affect galaxies as they are too low mass, so even at their maximum allowed phase space density as set by the Tremaine-Gunn limit, they cannot be dynamically relevant to the masses of galaxies. Returning to the SMoC: the lack of experimental verification of C/WDM particles comes in hand with additional failures of the SMoC:
Testing for the presence of the speculative C/WDM particles through the very well understood physical mechanism of Chandrasekhar dynamical friction leads to the conclusion that the dynamical friction through the putative dark matter halos around galaxies which are, in the SMoC, made up of C/WDM particles, is not evident in the data (Angus, Diaferio & Kroupa 2011; Kroupa 2015; Oehm & Kroupa 2017). That is, a galaxy which falls towards another galaxy should be slowed down by its dark matter halo, and this slow-down is not seen. The galaxies pass each other with high velocities, like two stars passing each other on hyperbolic orbits, rather than sinking towards each other to merge. This evidence for the non-existence of C/WDM halos around galaxies is in-line with the above mentioned lack of experimental detections (point 1 above). Customarily, an image of two strongly interacting galaxies is automatically interpreted as being a galaxy merger. But this is an over-interpretation of such images, since the implied mergers are not happening in the frequency expected in the standard dark-matter-based theory. Renaud et al. (2016) calculate ant document the theoretical description of an observed strongly interacting galaxy pair in the C/WDM framework and in MOND. Indeed, that the population of galaxies does not evolve significantly since a redshift of one has been found by Hoffmann et al. (2020) and has already been described by Kroupa (2015). This lack of evolution and the hugely vast preponderance of disk galaxies, of which a large fraction is without bulges, means that galaxies merge rarely as mergers nearly always transform the involved disk galaxies into earlier types of galaxies (disks with massive bulges, or even S0 or elliptical galaxies).
The Hubble tension is now much discussed. The Hubble Tension comes about as follows: the Hubble constant we should be observing today can be calculated assuming the standard dark-matter based SMoC is correct and that the Cosmic Microwave Background (CMB) is the photosphere of the Hot Big Bang (but see also point 6 below). The actually measured present-day value, as obtained from many independent techniques including supernovae 1a standard candles, gravitational lensing time delays, and mega-masers, comes out to be significantly larger though. The evidence is compiled in Haslbauer et al. (2020). The observer today sees a more rapidly expanding Universe than is possible according to the SMoC. More on the Hubble tension below (point 7).
The planes of satellites (or disk of satellites) problem has worsened: Our own Milky Way has been found to have a more-pronounced disk of satellite galaxies around it than thought before (Pawlowski & Kroupa 2020; Santos-Santos, Dominguez-Teneiro & Pawlowski 2020). Andromeda has one (Ibata et al. 2013, Sohn et al. 2020) and the nearby Centaurus A galaxy too (Mueller et al. 2018). The majority of other galaxies also show evidence for such planes or disks of satellites (Ibata et al. 2015). That the three nearby major galaxies simultaneously show such disks of satellite galaxies, and that disks of satellite systems are indicated by the majority of more distant galaxies, where the SMoC expects such satellite planes only in very rare cases (Pawlowski et al. 2015; Pawlowski 2018), eliminates with de facto complete confidence (i.e. much more than 5sigma) the SMoC, given that the satellites are in the great majority of cases ancient and void of gas such that they must have orbited their hosts many times. The Milky Way satellites also seem to be on almost circular orbits, strongly suggestive of a dissipative origin (Cautun & Frenk 2017) similar to the process that forms solar systems.
Astronomical data have uncovered, with extremely high confidence (more than 5sigma), that the strong equivalence principle is violated on the scale of galaxies (Chae et al. 2020 ), exactly in-line with a central expectation by MOND (Milgrom 1986), and in contradiction to the SMoC. While apparently not receiving much attention (e.g. via news coverage), this work by Chae et al. (2020) is a game-changer, a break-through of the greatest importance for theoretical physics. Independent evidence for the violation of the strong equivalence principle is also evident in asymmetrical tidal tails around globular clusters (Thomas et al. 2018). Gravity therefore behaves non-linearly on galaxy scales, preventing a simple addition of the fields contributed by different masses. This is a consequence of the corrected, generalised Poisson equation (Bekenstein & Milgrom 1984) which these authors point out is also found in classical theories of quark confinement.
Possibly a “nuclear bomb” nuked standard cosmology: Although the SMoC is only valid if the Universe is transparent, observations show there to be dust between galaxies. This intergalactic dust is ancient, and it radiates as it is heated by photons from the surrounding galaxies. Vaclav Vavrycuk (2018) has added all photons from this dust in an expanding Universe (i.e., in the past the intergalactic dust density was higher in a warmer Universe) and found the photon emission received by us to be very (nearly exactly) comparable to the measured CMB with the correct temperature of about 2.77K. For an explanation of his research paper see this YouTube video by MSc student Rachel Parziale at Bonn University. Note that the measured weak but large-scale magnetic fields around galaxy clusters and voids produce a correlated polarisation signal. The total number of infrared photons received at Earth is an integral over the time evolving density distribution along the line of sight such that the observed mass distribution within a small redshift around us should not correlate with the overall fluctuation of photon intensity seen in projection on the sky. The calculations by Vavrycuk thus suggest that CMB=cosmological dust emission, rather than being the photosphere of the Hot Big Bang. CMB research comprises an incredibly precise science, but the role of intergalactic dust needs to be considered very carefully and by avoiding pre-conceptions. Note that even if only a few per cent of the CMB were to be due to ancient intergalactic dust, then this would already bring down the SMoC.
The Universe around us contains far too few galaxies out to a distance of about 0.3 Gpc. This Keenan-Barger-Cowie (KBC) void falsifies the SMoC at more than 6sigma confidence. The KBC void kills the SMoC because the SMoC relies on the Universe starting off isotropically and homogeneously with the observed CMB fluctuations at the redshift z=1100 boundary condition about 14Gyr ago and cannot evolve density differences to the observed KBC under-density at z=0 which is the present time. Combined with the Hubble tension, the SMoC is falsified with more than 7sigma confidence. Newtonian gravitation plus the hypothetical C/WDM particles are together nowhere near strong enough to generate the observed density contrasts and the observed velocity differences between neighbouring Gpc-scale volumes. The next blog by Moritz Haslbauer will explain this situation. Note that here we still treat the CMB as the photosphere of a Hot Big Bang, but this may need to be reconsidered (see point 6 above).
The SMoC relies on the Universe having no curvature, but Di Valentino, Melchiorri & Silk (2020) find the enhanced lensing amplitude in CMB power spectra to imply a closed and thus curved Universe. However, this could be related to structure formation being more efficient than is possible in the SMoC (see point 7 above).
Cosmic isotropy is challenged at the 5sigma confidence level by X-ray selected galaxy clusters (Migkas et al. 2020), with the implication that the Universe appears to expand faster in a certain direction. A discussion of this evidence is provided by Scientific American. Cosmic isotropy is also challenged by the significant evidence for a dipole in the number counts of quasars beyond redshift one (Secrest et al. 2020). Independently of this, Javanmardi et al. (2011) also found evidence for a directionally dependent expansion rate.
Last for now but not least, the observation of massively interacting galaxy clusters such as the El Gordo cluster at high redshift (z=0.87) independently falsifies the SMoC with more than 6sigma confidence. In the SMoC, galaxy clusters cannot grow to such masses by this redshift – there is not enough time, or alternatively, Newtonian gravitation is too weak even with the help of the hypothetical C/WDM particles. This is shown by Asencio, Banik & Kroupa (2020). Elena Asencio is researching for her MSc thesis in the SPODYR group in Bonn.
Combining the above KBC void/Hubble Tension/El Gordo falsifications with the previously published tests (Kroupa et al. 2010, Kroupa 2015; see the figure below taken from Kroupa 2012) means that it has become, by now, wrong to still consider the standard dark-matter based cosmological model, the SMoC, as being relevant for describing the Universe. The falsification of the SMoC has reached well above the 7 sigma confidence — Remember: the Higgs Boson was accepted as having been discovered once the experimental confidence rose to 5sigma. It is important to emphasise that independent tests on very different scales lead to the same result, the SMoC being ruled out by many tests with more than 5sigma confidence.
The loss of confidence until 2012 in the Standard Model of Cosmology (SMoC) with each documented failure (numbered here from 1 to 22 and explained in Kroupa 2012) which has never, to date, been resolved. Thus, if each such failure (meaning the SMoC prediction is falsified by observational data) is assumed very conservatively to lead to a loss in confidence of only 30% that the SMoC is valid, then, by today (including the catastrophic >6sigma falsifications described in this blog) the statement that the SMoC describes the real Universe can be defended with a confidence=epsilon, with epsilon being arbitrarily close to zero (taken from figure 14 in Kroupa 2012).
The above list, but more importantly, the very high significance of the results, seem to indicate that a paradigm change may be under way in the sense that our current understanding of the Universe may be entirely rewritten at a very fundamental level. This is already indicated by gravitation being Milgromian. The paradigm shift would be epochal (see also this previous blog on the historical context) if the suggestion by Vavrycuk concerning the physical nature of the CMB were correct (point 6 above) because in this case our very concept of a Hot Big Bang and the origin of matter would be up in the air. There is independent evidence that a once-in-a-century paradigm shift may be under way:the Universe is much more structured than allowed by the SMoC. Thus, the Local Group of Galaxies (on a scale of 3Mpc across, Pawlowski, Kroupa & Jerjen 2013 ) shows a frightening symmetry in its matter arrangement (I call this frightening because there is currently no known theory to explain this distribution of matter). The arrangement of galaxies (Peebles & Nusser 2010) in the nearby cosmological volume (20Mpc across) does not correspond to the SMoC model and these very galaxies show a history of star-formation which appears to be far too tuned and non-varying (Kroupa et al. 2020). This begs the question how they manage to do so? The entire local Universe appears to be engaged in a significant bulk flow generated by major voids and over-densities (Haslbauer et al. 2020; Hoffmann et al. 2020).
Galaxies provide formal and precise observational data that allow us to correct the work of Newton and Einstein on gravitation, who did not have these data at their disposal. Rather, they formulated the currently assumed theories of gravitation subject to Solar System constraints only, which are now many decades if not centuries old. In his book “A Philosophical Approach to MOND“, David Merritt (2020)addresses the formal philosophical measures concerning how the Newtonian/Einsteinian formulation of gravitation needs to be assessed in terms of its success in describing the observed Universe in comparison with the correction to the law of gravitation through incorporation of galaxy data as formulated by MilgrOmiaN Dynamics (MOND). (Next sentence added Jan 3rd, 2021:) In Merritt (2017) we read his conclusion “The use of conventionalist stratagems in response to unexpected observations implies that the field of cosmology is in a state of ‘degenerating problemshift’ in the language of Imre Lakatos.” This would tend to close a circle: if Newtonian/Einsteinian gravitation needs to be revised, then we cannot use Einsteinian gravitation to formulate the evolution of the Universe, which opens the whole issue of how it started, what are the boundary conditions and how does it evolve? The Catastrophic Crisis in Cosmology (i.e. the fact that the observational data do not fit to the SMoC) is thus merely exactly the statement that we may well be in the process of a very major paradigm shift.
The big challenge for the future will be to find out how the Universe truly does work. The next blog by Moritz Haslbauer will indicate how a step towards this goal might have been achieved by Haslbauer, Banik & Kroupa (2020).
In The Dark Matter Crisis by Pavel Kroupa. A listing of contents of all contributions is available here.
…and are we at the beginning of a major historical paradigm shift?
(by Pavel Kroupa and Moritz Haslbauer, 07th Nov. 2020; 15:00)
There have not been posts on this blog for some time. The reason is certainly not that the dark matter crisis has gone away. Quite the contrary — the dark matter crisis, or more generally the cosmological crisis, has worsened and is now quite catastrophic. More on this in the next blog “The Crisis in Cosmology is now catastrophic”. With this contribution we provide an update on recent developments and some philosophical contemplation concerning paradigm shifts.
As a reminder: this blog on the Dark Matter Crisis was started in 2010 through the pressure (which I first resisted) by staff of the journal Spektrum der Wissenschaft in Germany (equivalent to Scientific American) who wanted Marcel Pawlowski (then a PhD student in the SPODYR group in Bonn) and me to blog about the developing crisis. This was related to the research I was involved with at that time leading me to the conclusion that the astronomical data rule out the standard dark-matter-based cosmological model as being relevant for a description of the Universe. This was in tension with my peers.
In January 2013 the blog was moved, along with all English blogs on Spectrum, to Scilogs.com. Later this same year there was a temporarily successful attempt by an amateur-science blogger (a sworn MOND enemy) to have the Dark Matter Crisis close down. This failed and the Dark Matter Crisis continued, simply because it’s content is scientifically solid. In 2016 SciLogs.com decided not to host the English Spektrum blogs any longer, and they were transferred to WordPress.com, where they are now. We have not blogged since this last move which had not gone perfectly well technically, with quite a few images having been lost. Just now we repaired most of the losses after some historical digging and with the help of Srikanth Togere Nagesh, MSc student at the University of Bonn. The corrections are continuing, and we are finding that some old links out of the Dark Matter Crisis blogs do not work any longer – we are trying to update them as far as possible and given the limited time available. This has taught me that documentation developed for the internet is fleeting. But we hope the WordPress platform will remain stable.
Much has happened since the move to WordPress: Indranil Banik, who had contributed the last piece obtained his PhD and is now an Alexander von Humboldt Fellow in the SPODYR group in Bonn. Marcel Pawlowski obtained a Hubble Fellowship and is now a Schwarzschild Fellow at the Leibniz-Institute for Astrophysics in Potsdam, Germany. Moritz Haslbauer, who is now researching towards his PhD in the SPODYR group at Bonn University, joined our editorial team just now and will publish his first post in the next contribution based on his own research on the Keenan-Barger-Cowie void and the Hubble tension. He already published two other research papers, one on galaxies lacking dark matter in cosmological simulations (Haslbauer et al. 2019a), and one on ultra-faint dwarf galaxies in cosmological simulations (Haslbauer et al. 2019b), both finding that the observed galaxies are in conflict with the standard model of cosmology (the SMoC). Concerning myself (PK), I have taken up a joint affiliation with Charles University in golden Prague and have been spending much time travelling there and beyond. I guess the beer, the knedliky and the scientific and cultural importance as well as the open atmosphere at the institutes and the multi-cultural nature and safety of historically extraordinarily beautiful Prague resonate with me. In Bonn, we hosted the large international conference BonnGravity2019: The functioning of galaxies in 2019 and I disjoined myself from the astronomers and have administratively joined a pure-baryonic-physics institute, namely the theory group at the Helmholtz-Institut für Strahlen- und Kernphysik at the University of Bonn. In this context:
Scientists have explorative minds and we know science evolves into new and often unforeseen directions and we should keep our minds open to these in order to allow science to progress rather than stopping scientific advance. It is also important to continue discussions between people working on different ideas without being dismissive. History shows that changes of paradigm can last decades and for those involved it may be impossible at the time to know if they are on the right track.
from Tereza Jerabkova
But are we in a paradigm shift and are we on the right track? The indications for being on the right track come, of course, from constant comparison of the theory one is developing with the observational data, and this blog will be covering this in the future. But are there perhaps some apparently unassociated hints or indications for an ongoing true major paradigm shift?
From the historical record: Very major paradigm changes in world view (religious, scientific) seem to be associated with significant relatively rapid transformations in the arts and with dramatic historical upheavals. Examples of this are (1) the fall of the Roman Empire went along with large-scale change to benign [thou shalt neither lie to nor kill anyone, but love and forgive everyone and all are equal in front of God] monotheism in Europe which improved local social cohesion, removed slavery from Europe and constituted an essentially critical mental step in abstracting the workings of the Universe. This abstraction is critically important because, simply put, until the abstraction there was a deity for every phenomenon (e.g. god of war). (2) The [first] 30 year war in the 17th century which was associated with the Keplerian revolution. In music, the first opera “L’Orfeo” by Monteverdi appeared in 1607. (3) In the early 19th century, the social transformations and associated Napoleonic wars with their large orchestrated battles outside of cities and the “Revolutions of 1848” appear to go in-hand with the development of thermodynamics and electricity as well as the emergence of romantic music and the symphonies by large orchestras (Schumann, Verdi, Wagner, Bruckner, Brahms, Tschaikowsky, and others). (4) The [second] 30 year war in the 20th century (i.e. the first and second world wars combined) happening in-parallel to the Einsteinian/Planckian revolution and being accompanied by the appearance of the twelve-tone technique by Schönberg and the music by the Russian composers Shostakovich, Prokofjew, Stravinsky, and Rachmaninow. (5?) The current world-wide geopolitical developments which appear with rising tensions and increasing dissociation of the power-blocks from each other, the accelerating demographic and potentially negative cultural-religious shifts in Western Europe, the societal changes concerning personal individualism, cancel culture and political correctness, and all of this in combination with the accelerating over-population, climate, micro-plastic-pollution crisis and on-going mass extinction, do seem to be suggestive of a major upheaval which is in the process of unfolding.
The next blog explains why cosmology is in a catastrophic crisis.
Given my affiliation with Charles University, I have been travelling to Prague and beyond frequently and now the CORONA Pandemic has stopped this flying about the planet — I have already written about the first wave and my getting marooned on a beautiful island next to the Strand. Being this time stranded in Bonn without a Strand during the second wave, I have a little more time on my hands I guess. So here we are, back to the Crisis.
In The Dark Matter Crisis by Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
The following is a guest post by Indranil Banik. Indranil is a PHD student at the University of Saint Andrews, part of the Scottish Universities’ Physics Alliance. He was born in Kolkata, India and moved to the UK with his parents a few years later. Indranil works on conducting tests to try and distinguish between standard and modified gravity, especially by considering the Local Group. Before starting his PhD in autumn 2014, he obtained an undergraduate and a Masters degree from the University of Cambridge with top grades. There, he worked on understanding the dynamics of ice shelves, and on a Masters project on the thick disk of the Milky Way, as well as on a few other problems.
I recently won the Duncombe Prize from the American Astronomical Society’s Division on Dynamical Astronomy for a detailed investigation into the Local Group timing argument. This was to present a recently accepted scientific publication of mine (link at bottom of article) at their annual conference in Nashville, Tennessee.
The timing argument takes advantage of the fact that the Universe has a finite age of just under 14 billion years. Thus, everything we see must have started at a single point at that time, which we call the Big Bang. Due to the finite speed of light, by looking very far away, we are able to look back in time. In this way, we observe that, shortly after the Big Bang, the Universe was uniform to about one part in 100,000. Thus, we know that the expansion of the Universe was very nearly homogeneous at early times. This means that any two objects were moving away from each other with a speed almost proportional to the distance between them. This is called the Hubble law.
The Hubble law also works today, but only on large scales. On small scales, the expansion of the Universe is no longer homogeneous because gravity has had a long time to change the velocities of objects. As a result, our galaxy (the Milky Way, MW for short) and its nearest major galaxy, Andromeda (or M31) are currently approaching each other. This implies that there must have been a certain amount of gravitational pull between the MW and M31.
Although this has been quantified carefully for nearly 60 years, my contribution involves analysing the effects of the MW and M31 on the rest of the Local Group (LG), the region of the Universe where gravity from these objects dominates (out to about 10 million light years from Earth). Recently, a large number of LG dwarf galaxies have been discovered or had their velocity measured for the first time (McConnachie, 2012). We took advantage of this using a careful analysis.
We treated the MW and M31 as two separate masses and found a trajectory for them consistent with their presently observed separation. We treated the other LG dwarf galaxies as massless, which should be valid as they are much fainter than the MW or M31. For each LG dwarf, we obtained a test particle trajectory whose final position (i.e. at the present time) matches the observed position of the dwarf. The velocity of this test particle is the model prediction for the velocity of that galaxy.
The basic feature of the model is that the expansion of the Universe has been slowed down locally by gravity from the MW and M31. At long range (beyond 3 Mpc or about 10 million light years), this effect is very small and so objects at those distances should essentially just be following the Hubble law. But closer to home, the results of this model are clear: the MW and M31 are holding back the expansion of the Universe, and objects within about 1.5 Mpc should be approaching us rather than moving away (see figure above). By comparing the detailed predictions of our model with observations, we were able to show that, for all plausible MW and M31 masses, a significant discrepancy remains. This is because a number of LG galaxies are flying away from us much faster than expected in the model.
An important aspect of these models is that the MW and M31 have never approached each other closely. Although one can in principle get them to have a past close flyby in Newtonian gravity if they are assigned very high masses, there are several problems with this. Such high masses are unreasonable given other evidence. More importantly, if there had been such a flyby, the dark matter halos of the MW and M31 would have overlapped, leading to a substantial amount of friction (of a type called dynamical friction, which is reliant only on gravity). This would have caused the galaxies to merge, contradicting the fact that they are now 2.5 million light years apart.
I was aware of an alternative model for galaxies called Modified Newtonian Dynamics (MOND – Milgrom, 1983). This is designed to address the fact that galaxies rotate much faster than one would expect if applying Newtonian dynamics to their distributions of visible mass. The conventional explanation is that galaxies are held together by the extra gravitational force provided by a vast amount of invisible dark matter. Many galaxies need much more dark matter than the amount of actually observed matter. But, so far, this dark matter has not been detected directly. What MOND does is to increase the gravitational effect of the visible matter so that it is enough to explain the observed fast rates of rotation. In this model, there is no longer any need for dark matter, at least in halos around individual galaxies. You can find out more about MOND here on McGaugh’s MOND pages and here on Scholarpedia.
In MOND, the MW and M31 must have undergone a past close flyby (Zhao et al, 2013). In this model, the absence of dark matter halos around galaxies means that there need not have been any dynamical friction during the flyby (remember that the disks of the MW and M31 are much smaller than their hypothetical dark matter halos, which are only needed if we apply Newton’s law of gravity).
The high relative speed of the MW and M31 at this time (about 9 billion years ago) would probably go a long way towards explaining these puzzling observations. This is because of a mechanism called gravitational slingshots, similar to how NASA was able to get the Voyager probes to gain a substantial amount of energy each time they visited one of the giant planets in our Solar System. The idea in this case would be for the MW/M31 to play the role of the planet and of a passing LG dwarf galaxy to play the role of the spacecraft.
This mechanism is illustrated in the figure above. In the left panel, there is a small galaxy moving at 1 km/s while a much heavier galaxy moving at 5 km/s catches up with it. The massive galaxy sees the dwarf approaching at 4 km/s (right panel). The trajectory of the dwarf is then deviated strongly, so it ends up receding at 4 km/s back in the direction it approached from. Combined with the velocity of the massive galaxy (which is almost unchanged), we see that the velocity of the dwarf has been increased to 5 + 4 = 9 km/s.
We do in fact observe many LG dwarf galaxies moving away from us much faster than in the best-fitting dark matter-based model (see figure below, observed radial velocities are on the y-axis while model-predicted ones are on the x-axis). Moreover, based on the distances and velocities of these objects, we can estimate roughly when they would have been flung out by the MW/M31. This suggests a time approximately 9 billion years ago, which is also when one expects the MW and M31 to have been moving very fast relative to each other in MOND as they were close together.
These high-velocity LG dwarfs would have been flung out most efficiently in a direction parallel to the velocity of whichever heavy galaxy they interacted with. Naturally, the MW and M31 have not always been moving in the same direction. But it is very likely that they were always moving within much the same plane. Thus, one test of this scenario (suggested by Marcel Pawlowski) is that these high-velocity dwarfs should preferentially lie within the same plane.
There is some evidence that this is indeed the case. Moreover, the particular plane preferred by these objects is almost the same as what would be required to explain the distribution of satellite galaxies around the MW and M31. This is described in more detail towards the end of this lecture I gave recently about my work.
Even without this evidence, there is a strong case for MOND. One of the astronomers heavily involved in making this case is Professor Stacy McGaugh. I was very pleased to meet him at this conference. We discussed a little about his current work, which focuses on using rotation curves of galaxies to estimate forces within them. For a modified gravity theory which does away with the need for dark matter, it is important that these forces can be produced by the visible matter alone. Stacy was doing a more careful investigation into estimating the masses of galaxies from their observed luminosities and colours (which give an idea of the mix of different types of star in each galaxy, each of which has its own ratio between mass and luminosity, old stars being red and young ones blue). The success enjoyed by MOND in explaining dozens of rotation curves is one of the major reasons the theory enjoys as much support as it does.
This brought us on to discussing how we came to favour the theory over the conventional cosmological model (ΛCDM) involving Newtonian gravity and its consequent dark matter. Stacy explained how it was particularly his work on low surface brightness galaxies which convinced him. This is because such galaxies were not known about when the equations governing MOND were written down (in the early 1980s). Despite this, they seemed able to predict future observations very well. This was somewhat surprising given that the theory predicted very large deviations from Newtonian gravity. In the ΛCDM context, the presence of large amounts of invisible mass makes it difficult to know what to expect. As a result, it is difficult for the theory to explain observations indicating a very tight coupling between forces in galaxies and the distribution of their visible mass – even when most of the mass is supposedly invisible (a feature called Renzo’s Rule). A broader overview of what the observations seem to be telling us is available here (Famaey & McGaugh 2012) and here (Kroupa 2015).
I then explained my own thinking on the issue. I was aware of some of the observations which persuaded Stacy to favour MOND and I was aware of the theory, but I did not favour it over ΛCDM. Personally, what got me interested in seriously considering alternatives to ΛCDM was its missing satellites problem. The theory predicts a large number of satellite galaxies around the MW, much larger than the observed number. Although it is unclear if MOND would help with this problem, that does seem likely because structure formation should proceed more efficiently under the modified gravity law. This should lead to more concentration of matter into objects like the MW with less being left over for its satellites.
Although this suggested MOND might be better than ΛCDM, my initial reaction was to consider warm dark matter models. Essentially, if the dark matter particles were much less massive than previously thought (but the total mass in the particles was the same), then they would behave slightly differently. These differences would lead to less efficient structure formation at low masses, reducing the frequency of low-mass halos and thus making for less satellite galaxies. I hoped this would explain a related problem, the cusp-core challenge which pertains to the inner structure of satellite galaxies.
What finally convinced me against such minor alterations to ΛCDM and in favour of MOND was the spatial arrangement and internal properties of the MW and M31 satellite galaxies. Much has been written in previous posts to this blog about this issue (for example, here), with this 2005 paper by Kroupa, Theis & Boily pointing out the discrepancy between observations and models for the first time.
I have summarised the results in a flowchart (left). Essentially, the hypothetical dark matter halos around the MW and M31 need to be distributed in a roughly spherical way. This is unlike the disks of normal (baryonic) matter in these galaxies. The reason is that baryons can radiate and cool, allowing them to settle into disks. As a result, in an interaction between two galaxies, the baryons with their ordered circular motions in a disk can get drawn out into a long dense tidal tail that then collapses into small tidal dwarf galaxies. But these would be free of dark matter, and they would also be mostly located close to a plane: the common orbital plane of the interacting galaxies. You can see more about this scenario here.
The argument goes that it is difficult to form such planes of satellites in any other way (for example, see Pawlowski et al, 2014). Just such satellite planes are in fact observed around both the MW and M31. Supposedly free of dark matter, they should have quite weak self-gravity and thus low internal velocity dispersions/rotate very slowly. Yet, their observed velocity dispersions are quite high, signalling the need for some extra force to stop them flying apart.
Because the spatial arrangement of these satellites suggests a violent origin, it is unlikely that they have much dark matter. Thus, I became convinced of the need to modify our understanding of gravity. It turns out that exactly the same modification that can help explain galaxy rotation curves without dark matter could also help address this problem (McGaugh & Milgrom, 2013). Although the dark matter plus Newtonian gravity worldview might just about be able to explain galaxy rotation curves (although detailed tests are showing this not to have succeeded: Wu & Kroupa 2015), I do not think it can explain the satellite plane problem. This eventually convinced me to investigate this issue further. I explain some of the more compelling reasons for favouring MOND over ΛCDM in this lecture I gave recently.
Together with Francoise Combes, who was recently appointed as a professor in the most prestigeous institution in France, Le College de France, and Benoit Famaey, who is an expert on Milgromian dynamics and its deeper foundations (e.g. Famaey & McGaugh 2012), we were invited by Mordehai (Moti) Milgrom to spend a whole week at the Department of Particle Physics and Astrophysics in the Weizmann Institute in Rehovot, Israel. A link to the video (dubbed in English) of the inaugural lecture given by Francoise Combes for her new chair and the introduction by Serge Haroche (Nobel Prize 2012 in physics) is available here (alternatives to the dark matter approach are explicitly mentioned by both).
I met Benoit at Frankfurt airport in the very early morning (he was heading in some random direction) since we had booked the same Lufthansa flight to Tel Aviv. We arrived on Sunday, March 6th, and met Moti at his office in the late afternoon.
In the entrance hall of the Department. From left to right: Einstein’s field equation without Lambda, Francoise Combes, Mordehai Milgrom, Pavel Kroupa and Benoit Famaey.
Coming to know the place and first discussions
I am very impressed by the size and beautiful campus of the whole Weizmann Institut, and how pleasant the entire ambiente is.
Chairs and a pond in front of the Department.
The people are very friendly and helpful. And interested. I was staying at the spacious and luxurious San Martin Faculty Clubhouse. At night the various buildings and park areas in the Weizmann Institute are illuminated beautifully, with warm lights setting accents and emphasizing a welcoming atmosphere.
The highly-ranked Weizmann Institute consists of many departments of various natural sciences and seems to be perfectly created for academic pursuit, including leisure areas. Its success in the pursuit of basic research in the natural and exact sciences and in acquiring funding is evident through the architecture, spaciousness, and general design.
There was no planned agenda for us, apart that Benoit was to give a talk on Wednesday, 9th of March, at 11:15, and for Francoise Combes to give a departmental colloquium on Thursday, 10th of March at 11:15. In between these talks we could do either nothing and hang about enjoying the sunshine and exquisite weather and pool, or engage in intense discussions. Perhaps due to the ambiente and of course our comparable research interests, we largely chose the latter.
On Monday, 7th of March, we had a very relaxed day, meeting with Moti at the Department in the late morning and spending our time debating. Typical discussion points (largely between Francoise, Benoit and myself) throughout the visit were the local major underdensity and its possible implications on the value of the cosmological Lambda, the underlying theory of MOND and whether it is due to a “dark” fluid which behaves like dark matter on large scales (e.g. Luc Blanchet’s dipoles and Justin Khoury’s condensate)
Given that Lambda was missing in the equation displayed in the entrance hall of the Department (see first photo above), we began to discuss it. And this is where the “local” underdensity now plays a possibly important role, see this figure from Kroupa (2015),
The underdensity is significant, according to the shown data, and may challenge any cosmological model. From Kroupa (2015).
and in contrast the very recent work by Whitbourn & Shanks where the authors explicitly state agreement with the previous survey by Kennen et al. (2014). The independent finding by Karachentsev (2012) on the local 50 Mpc scale appears to naturally continue the trend evident from the Kennan et al. data (see the figure on the left), IF one assumes the same baryonic to dark-matter ratio as at larger distances. The actually measured stellar density remains similar to the Keenan et al. value at small distance. So the baryonic density (assuming the gas to star ratio and the contribution by dwarf galaxies to remain unchanged out to distances of 800 Mpc [redshift of 0.2]) then within 300 Mpc there is at least a decrease in the baryonic density by factor of two. Conversely, taking Karachentsev’s measurement, we would see a disappearance of dark matter nearby to us since the stellar density remains similar to the Kennen measurement within 150 Mpc while the dark matter density decreases further. So the measurements appear to imply the following picture: within 400 Mpc the luminous (and thus baryonic) matter density decreases significantly by a factor of two. At the same time, the ratio of dark matter to baryonic matter decreases even more. Both findings violate the cosmological principle.
The work by David Wiltshire (his lecture notes) and Thomas Buchert already indicates that inhomogeneities could possibly make the Universe appear to an observer situated within such an underdensity as if it’s expansion is accelerating, although in truth it is not. That is, the inhomogeneities appear to be of the correct magnitude to eliminate the need for Lambda, Lambda (dark energy) merely being an apparent effect mis-interpreted by the supernova type 1a data. The reason lies in that a distant object’s observed redshift depends in reality on the exact paths the photons travel in a universe which consists of time-changing voids and over-densities, and this is a different redshift computed assuming a homogeneous and isotropic expanding Universe.
But we need more detailed calculations taking into account the constraints from the observed under-density shown in the figure to be assured that Lamba=0. It is certainly true that Lambda=0 may be more in line with theoretical ideas than the very small value deduced to explain an apparently accelerating Universe, because it is actually predicted, from quantum field theoretical calculations of the vacuum (for details see e.g. Padilla 2015), to have a value some 60 to 120 orders of magnitude larger. It should be emphasized, though, that “MOND likes Lambda“, in the words of Moti. The reason is that the Lambda derived from astronomical observations (e.g. from supernovae of type 1a observations) and Milgrom’s constant a_0 appear to be naturally related, and MOND may be derivable from vacuum processes (Milgrom 1999).
Within about 300 Mpc, where we can say that we have the best measurements, the Universe is nicely consistent with MOND. The mass-to-light ratios of galaxy groups are less than 10 (Milgrom 1998 and Milgrom 2002), i.e. there is only baryonic matter. The observationally inferred increased density of baryonic matter at distances larger than 300 Mpc would then perhaps be due to cosmological models being inappropriate, i.e. that the currently used red-shift–distance relation may be wrong.
We also debated galaxy evolution, the fraction of elliptical galaxies and the redshift dependence of this fraction. Notably, fig.7 in Conselice (2012) shows that the observed fraction of massive galaxies does not evolve although the LCDM model predicts a strong evolution due to merging. This is consistent with the independent finding by Sachdeva & Saha (2016) that mergers are not a driving mechanism for galaxy evolution, and this is in turn consistent with the independent findings reached by Lena et al. (2014) on the same issue.
We further talked about how LCDM is faring on large, intermediate and small scales, how stellar populations change with physical conditions, the variation of the IMF, as well as political topics. The discussions were far from reaching consensus, we had different views and data sets we could quote on various problems, and time flew by such that we barely noticed.
However, Moti managed to drag us away from his Department, and showed us around the Weizmann institute. An particular station was the famous landmark tower which once housed the Koffler Accelerator and which now houses, in its “bubble”,
The tower which housed the Koffler Accelerator and which now houses a conference room (in its “bubble”) and the Martin S. Kraar Observatory.
a conference room and also the Martin S. Kraar observatory which is also used in international top-level research projects. The director of the observatory, Ilan Manulis, kindly explained to us in much detail its functionality and design for full remote-observations without human interference.
Viewing the lands from the top of the Koffler Accelerator Building. From left to right: Benoit Famaey, Francoise Combes and Mordehai Milgrom.
Part of the Weizmann Institute as viewed from the top of the Koffler Accelerator Building.
The Group at the Koffler Accelerator. From right to left: Benoit Famaey, Francoise Combes, Mordehai Milgrom and Pavel Kroupa
On this Monday Moti took us to lunch at the Lebanese restaurant Petra located in Nes-Ziona, a town 5 minutes drive from the Weizmann Institute. The Lebanese cuisine was fabulous, and I ate far too much.
A diversion to history
And, on Tuesday, 8th of March, Moti and his wife Ivon took us on a drive-around nearby Israel. This trip, involved about 4 hours of driving by Moti, and while driving we discussed, amongst other topics, the new study by Papastergis et al. (2016) in which they use 97 gas-dominated galaxies from the ALFALFA 21cm survey to construct their estimate of the baryonic Tully-Fisher relation showing excellent agreement with the expectations from Milgromian dynamics.
The drive was incredible, as we saw places with many thousands of years of history dating back to the Caananite peoples. It is this land which took the central role in the evolution of the Mediteranean-Sea-engulfing Roman Empire to a Christian empire. It contains the scars of the episodes of the invasion by a newer religion of christian lands, christian reconquest, and reconquest by the newer religion, till the foundation of Israel, issues which remain current to this day.
The author amongst the ruins of Caesarea. “What was the fate of Caesarea’s inhabitants when it fell to the Mamluks?”
Caesarea, once a thriving port for many centuries, from where Paulus was imprissioned and sent to Rome for his hearing at the emperor’s court, was wiped out in the 13th century.
The Group in front of the Roman ampitheater in windy Caesarea, nearly but not quite ready. From right to left: Mordehai Milgrom, Francoise Combes, Benoit Famaey, Pavel Kroupa.
The thriving thousand-year old medieval city of Caesarea, named by King Herod after Octavian (i.e. Augustus Caesar) and which was once the main port in his kingdom, was finally obliterated from existence after a siege by a Mamluk army in the thirteenth century.
Acre: the chief port in Palestine during the crusader epoch still boasting major remains of the huge crusader’s fortress:
Acre: the remains of the Crusader port.
Acre, once a blossoming port and a gate-way to the holy lands for christian pilgrims.
After a wonderful dinner at the seashore between Tel Aviv and old Jaffa at the restaurant Manta Ray, where some action happened just before we arrived judging from the large number of police and other forces around, we visited very beautiful Old Jaffa:
Old Jaffa, which dates back to a history of 4000 years and where alrady the Egyptian empire stationed a garrison.
The restoration of the archeological sites of Caesarea, Acre and of Old Jaffa brings to mind how incredibly rich and beautiful the thousand year old places are along the Mediterranean coast throughout the middle East and northern Africa, if upheld with the corresponding desire to show this history.
Back to science
On Wednesday, 9th of March, we spend the whole day in discussions with staff of the Institute. It began with Benoit Famaey’s presentation on the latest numerical results of modelling the Sagittarius satellite galaxy and its stream in Milgromian dynamics by Strasbourg-PhD student Guillaume Thomas. Natural solutions appear to emerge and this will, once published, clearly add spice to the discussions, given that the only solutions available in LCDM by Law & Majewski (2010) are unnatural in that the dark matter halo of the Milky Way needs to be oblate at right angle to the Milky Way, a solution which poses severe dynamical instabilities for the Milky Way disk. Notably, this polar oblate dark matter halo of the Milky Way alignes with the vast-polar structure (the VPOS) of all satellite galaxies, young halo globular clusters and stellar and gas streams.
In these discussions with the staff members during the aftenoon, we dealt with supernova rates and explosions and types in different galaxies, the relevance to the variation of the IMF in various environments (e.g. metal-poor dwarf galaxies vs metal-rich massive galaxies and the dependency of the IMF on density and metallicity), and cosmological problems such as the local massive under-density mentioned above.
An important point I tried to emphasize repeatedly is that if Milgromian dynamics is the correct description of galactic dynamics, then we must keep an open mind concerning the possibility that all of cosmological theory may have to be rewritten and the large-redshift data may need to be reinterpreted in terms of different redshift–distance and redshift–age relations.
In the evening of Wednesday I tried out the swimming pool on campus, and their sauna as well. I had access to this swimming pool by staying in The San Martin Faculty Clubhouse and the Hermann Mayer Campus Guesthouse – Maison de France. I must admit, that the day was near to being perfect with the sunshine and a closing dinner with Francoise and Benoit again in our meanwhile standard kosher restaurant (Cafe Mada) nearby the San Martin guest house.
On Thursday, 10th of March, Francoise Combes gave her interdepartmental presentation on “The Molecular Universe” which was well visited, and afterwards we went together with some staff of the Weizmann Institute for lunch at Cafe Mada, where a lively and very entertaining discussion ensued on religeos questions. In the late afternoon we joined the Whisky lounge, in which anyone traveling back to Rehovot from abroad can bring a duty-free bottle of Whisky to and donate it to this lounge.
The Local Group of galaxies is highly symmetrical, with all non-satellite dwarf galaxies lying in two planes symmetrically and equidistantly situated around the axis joining the Milky Way and Andromeda. From Pawlowski et al. (2013).
Young researchers meet every Thursday (remember, this is in Israel the end of the week) to sip Whisky and thereby to elaborate on various problems, such as in our case on the local underdensity, or how the two critical constraints we have from the highly organized structure of the Local Group of galaxies and the CMB together constrain the cosmological model.
An interesting statement made was that while one needs about ten LCDM Universes to get one Bullet cluster (Kraljic & Sarkar 2015), an infinite number of LCDM Universes will not give a single Local Group with its symmetries.
At least these are some of the questions we discussed while there on this Thursday. We were also impressed by all the connections of this Department with Princeton, Caltech and Harvard.
Friday and Saturday
Shops begin to close down and it becomes a challenge to find food and Francoise left for France. In the morning I went for a swim and sauna, and for luch Benoit and myself had to go out of the Weizmann Institute (exit Main Gate and turn left) to find a sandwich place.
The Basha Bar in Tel Aviv.
After some work and then in the evening and at about 18:00 we decided to take a taxi to Tel Aviv. We arrived at the Basha Bar by about 18:30 and stayed for three hours (see photo).
The Basha Bar, enjoying a three-hour shisha smoke and many Tuborg beers.
On Saturday, the kosher breakfast in the guest house was as excellent as ever, but it was interesting for me to note that neither the toaster nor the coffee machine were to be used, while the water boiler was on so we could still have hot Turkish coffee (which we also drink in Bohemia, by the way, so not much new for me here). Nearly everything is closed. Benoit and myself met for lunch and walked outside the Main Gate turning right, over the bridge to reach the Science Park finding bistro Cezar for lunch.
In the evening Moti picked us up for a dinner at his home with Ivon, where we had a long discussion also on the dynamic situation in Germany, Europe and the future.
At the home of Moti in Rehovot. From right to left: Moti, Benoit and the author.
Benoit and myself stayed on until Monday, joining the astrophysics journal club which serves lunch at the Department on Sunday. I spent most of the afternoon discussing with Boaz Katz how star clusters may be relevant for type 1a supernovae. In the evening of Monday Benoit and I went again to Cafe Mada for a final dinner and drinks. On Monday, 14.03., we flew out around 16:00, taking a taxi to the Tel Aviv airport at 13:00 from the Department. We shared the same flight back. Again the 4+ hour long Lufthansa stretch without personal-screen-based entertainment system! But, this gave Benoit and myself a chance to further discuss at length the above mentioned Khoury condensate and the Blanchet dipoles as models for galaxy-scale MOND and cosmology-scale dark-matter-like behaviour. But I note that these are not dark matter models. During pauses my thinking was that as the coastal line of Tel Aviv receded in the setting Sun we left a small fraction of the Levant and northernmost Africa, all once pat of the Roman Empire, at a level of civilisation mirrored by the clear, brllliantly lit vast and dynamic power- and resource-hungry central-European night with full autobahns, radiant towns and illuminated football fields in nearly every village. In Frankfurt our ways parted after a last small dinner in the train station, Benoit taking a bus to Strasbourg at about 21:30, and me starting my odessey to Bonn at the same time using the available train connections (German trains all too often run late, these days).
The visit was most memorable for all of us, and Benoit and myself agree that we would like to return. We did not reach any conclusions but we came to know many new people and perhaps helped to underscore the very seriousness of alternative concepts to dark matter and the many failures of the LCDM model.
In closing it is probably fair to say that Milgrom contributed the greatest advance on gravitational physics since Newton and Einstein.
In The Dark Matter Crisis by Pavel Kroupa and Marcel Pawlowski. A listing of contents of all contributions is available here.
The announcement on Feb.11th, 2016, that gravitational waves have been detected is a sensation and it is indeed rather incredible to imagine that space-time is constantly wobbling with and around us all the time because of some cosmic events, as is expected to be the case in Einstein’s theory of general relativity.
Imagine a wave comes though and everything gets distorted. Obviously, we will not measure a change, since also the ruler is distorted. So the way LIGO works is to use two 4 km long rulers or measuring arms angled to each other, and to use overlapping light waves from both arms to seek the tiniest of tiniest relative changes between the two lengths. This is possible because gravitational waves are polarized.
This way and with the truly most incredibly developed hyper-sensitive length-measurement technology, the LIGO team can measure changes in relative length between the two arms that amount to 1/10000 of the diameter of a proton, or 10^-19 m.
In the announced case, two heavy stellar-mass black holes (with masses of about 29 and 36 Solar masses) coalesced about 1.3×10^9 yr ago to an about 36 Solar mass black hole plus about 3 Solar masses in radiated gravitational wave energy, leading to the detection of gravitational waves on Earth.
What is the source of these waves?
There are two possibilities. The rumors that a signal with its properties had been detected by AdLIGO was already available by October 2015 as reported on The Reference Frame by Lubos Motl.
Individual massive star binaries: very fine-tuned solutions?
On Dec. 15th, 2015, Amaro-Seoane & Chen placed predictions on the likely to-be-found-by-AdLIGO events on the arXiv arguing for massive black holes and that these circularise before coalescence due to gravitational wave emission.
One group (Marchant et al.) at Bonn University placed a paper onto the arXiv preprint server on Januray 14th, 2016, predicting essentially the particular waves which were then reported on Feb. 11th, 2016, by the LIGO team.
On Feb. 15th another group (Beczynski et al.) came up with a similar prediction.
Both of these latter contribution demonstrate that the two massive black holes orbiting each other may arise from one stellar binary system in which both stars were very massive and that this system evolved through stellar-wind-driven mass loss of both stars followed by their individual supernova explosions, to form a binary black hole system which is sufficiently tight to merge within much less than a Hubble time through the radiation of gravitational waves. From the above description it emerges that this is a highly fine-tuned problem to work out as the source of the very first observed gravitational wave emission. This scenario does have interesting consequences, namely that it leads to aligned spins of the black holes and that the kicks the black holes receive must be smaller than typically 400 km/s as emphasized by Belczynski et al.
Rather common events: star clusters as engines for making them
But, there is another process which actually makes such black-hole merging events common, to the degree of AdLIGO (the now operating advanced LIGO observatory) observing 31 plus minus 7 such events per year.
The process begins with the birth of a massive star cluster somewhere in the universe. This massive star cluster, being typical in every respect (e.g. weighing 10^4 Mun, having a 1pc radius, with a normal stellar population), has its share of very massive stars which explode, one after another, as type II supernovae. Some of these leave a stellar-mass black hole in the cluster, which consequently and over a time of roughly 3-50 Myr builds-up a population of such black holes. These, being more massive than the stars in the cluster, sink to the centre of the cluster forming, by about 100Myr, a core of black-holes. There they meet and interact stellar-dynamically and they pair up through three-body dynamical encounters: one takes away the energy leaving two black holes in a binary. Such a binary may become tighter (i.e. it shrinks) with time because of the constant perturbations by the other cluster members. The black-hole binary “hardens” over time, until a final strong encounter with another black hole in the cluster center hardens it strongly, in which case the recoil energy may fling it out of the cluster. Independently of whether it is ejected out of its cluster, some such hard black-hole binaries may be so tight and eccentric, that their orbit shrinks due to the radiation of gravitation waves at peri-center. The binary shrinks further and circularizes, until it merges, as was observed by AdLIGO.
Because star clusters are observed everywhere in the Universe in and around galaxies, them being the building block of galaxies, these events become common and not special. The calculations of the process described above have been published in 2010 by a Bonn-University team led by Sambaran Banerjee et al. They perform detailed stellar-dynamical computations of the above processes such that we can estimate the rate of binary black hole mergers at a given time produced by a star cluster. We can then sum up all such events from all star clusters in the Universe (since we know how many star clusters there are per galaxy approximately) to come up for the first time with such a prediction, which appears to have been nicely verified now with the AdLIGO announcement. The above mentioned rate (31±7 events per year) predicted in 2010, may be somewhat larger if less-massive star clusters are incorporated into the calculations. Low-metallicity stars leave more massive black holes, essentially because their weaker winds sweep away a smaller fraction of the star’s initial mass, and so modern stellar-evolution theory readily accounts for black holes more massive than 30 Solar masses in low-metallicity clusters which are abundant. The most massive of these black holes are most likely to dynamically interact near the star-cluster core, producing massive black-hole–black-hole binaries.
The observed rate of wave detections will test these predictions. One important aspect has been raised by Belczynski et al. above, namely that this dynamical star-cluster process predicts the black-hole spins to not be aligned, while the above stellar-binary-process does. So a given gravitational wave detection can be used to assess the particular channel of production of the pre-black-hole merger event.
Gravitational theories (and dark matter?):
MOND: Does the existence of gravitational waves, as predicted by the theory of general relativity, pose a problem for MOND? This is an important question to study now, since the detected signals constrain gravitational theories (a theory which does not allow gravitational waves to propagate is of course ruled out now). The detection of gravitational waves does not prove Einstein’s theory to be right, since there may be another theory which leads to the same effect. But the detection is certainly consistent with this theory. The analysis of the signals implies that the gravitational waves are propagating with a speed which is indistinguishable form the speed of light and this constraints the mass of the graviton to be less than 2.1×10^−58 kg or 1.2×10^−22 eV/c2.
One possible interpretation of MOND is that it is a consequence of gravity being mediated by a massive graviton. Sascha Trippe at Seoul National University discusses this implications in his 2015 paper stating10^−69 kg or 10^−33 eV c−2 as being the mass of the graviton. So this is consistent with the AdLIGO limits.
Also, the detection of gravitational waves does not prove the existence of dark matter at all, in the sense that someone may want to argue that since Einstein’s general theory predicted the waves, their verification now shows that this theory is right, and since this theory implies cold or warm dark matter particles in the standard LCDM or LWDM model of cosmology (which nearly everyone says is right but some of us _ know is ruled out by astronomical data), then dark matter must exist. This would be a false deduction.
The existence and the observed properties of gravitational waves however place important constraints on the theories of gravity which yield the classical MOND limit. Mordehai Milgrom already published a study of this issue in 2014 in PhRvD. Further research is required to test the various formulations in detail, given the observed gravitational waves and their properties.
The Weizmann Institute and my impending visit there:
I am visiting the Particle Physics and Astrophysics group at the Weizmann Institute in Rehovot this coming week (06.-14.03.2016), having kindly been invited by Mordehai Milgrom together with Francoise Combes and Benoit Famaey. Undoubtedly, apart from a planned sight-seeing tour through the incredibly deeply historic and beautiful lands of Israel on one day, we will of course be discussing gravitatonal wave propagation in a Milgromian Universe, as well as the most recent computational results already now obtained on various problems researched in Strasbourg and Bonn with the Phantom of Ramses computer code (the PoR code, the first PoR workshop).
Caveat (not to be taken seriously)
So far so good. But there is one caveat I’d like to very carefully mention finally.
Natural science must be reproducible! As much as we might be excited and thrilled, this is at present not given by the AdLIGO claims. Here, one team reports the detection of a transient signal with their own two observing devices. No-one can ever go back and check if the seen signal actually occurred. We have every reason to believe that the detection is true, but an independently working team would verify or independently observe such events, preferably with their own detectors. Undoubtedly this will happen, when the additional other gravitational wave observatories hopefully being comissioned soon in other countries will begin to listen to the Universe. But is is essential that independent verification be ensured. That AdLIGO is rumored to have been detecting a substantial number of additional events indeed emphasizes that the detections are occurring and that the events are common, as predicted.
Apart from verifying by direct detection that gravitational waves exist, this is a gound-breaking event because physicists now have build new devices to probe the very fabric of space time itself. Once we have full-scale gravitational wave observatories the view we will obtain of the whole Universe is surely going to be something none of us can barely imagine today. In the past, comparable revolutions have occurred. Galileo Galilei’s first-time ever observation of heavenly objects with the first primitive telescope completely changed our world view for ever. Then, 400 years ago, no-one would have even imagined the incredibly powerfull optical observatories operating today and peering right to the beginning of time. The first-ever detection of radio waves from the heavens with the first primitive radio receivers is of a similar scale of events by leading us to the detection of the cosmic microwave background emission, which essentially is an image of the beginning of time if its physical interpretation is correct. When the first radio antennae were put up, no-one would have imagined that we will one day be able to image Solar system scales in distant galaxies, let alone view the Beginnings, as is being done routinely today. Assuming our open inquisitive, equal-human-rights, rational and non-religeous-argument based civilisation still exists, what will we be seeing with gravitational wave observatories in 100 years time?…
The Dark Matter Crisis by Pavel Kroupa and Marcel Pawlowski. A listing of contents of all contributions is available here.
1.BACKGROUND / MOTIVATION: Galaxy-scale data seem to be in accordance with the hypothesis that the extrapolation of Newtonian gravitation by orders of magnitude below the Solar system space-time curvature breaks down completely, and that collisionless astronomical systems behave according to space-time scale-invariant dynamics, as postulated by Mordehai Milgrom (2015). The classical theories of dynamics and gravitation underlying this symmetry, often referred to as MOND theories, show a richer dynamical behaviour with new phenomena which appear non-intuitive to a Newtonian mind. Very successful analytical results have been obtained in this dynamics framework, such as accounting for the hitherto not understood properties of polar-ring galaxies (Lueghausen et al. 2013), accounting for the Bullet cluster (Angus, Fmaey & Zhao 2006; Angus & McGaugh 2008) and the properties of disk galaxies (MOND reviews by Scarpa 2006; Famaey & McGaugh 2012;Trippe 2014) and elliptical galaxies (Sanders 2000; Milgrom & Sanders 2003;Scarpa 2006).
But little understanding of the dynamical behaviour of live Milgromian systems has been gathered. Live calculations, i.e. simulations of galaxies, are required in order to test, to possibly refine or to falsify this approach. The implications for fundamental physics are major in any case!
A series of Milgromian-dynamics workshops is planned to begin remedying this situation.
With this first “Phantom of Ramses” (PoR) meeting, the aim is to bring together the pioneers who have been daring footsteps into applying Milgromian dynamics to simulate live galaxies. First simulations of galaxies within MOND have been achieved with the first Milgromian Nbody code without gas (Brada & Milgrom 1999). Tiret & Combes (2007) re-visited this problem with their own code. The PhD thesis of Tiret is available here (in French). For spheroidal geometries MOND simulations have become possible with the NMODY code by Nipoti, Londrillo & Ciotti (2007), see e.g. the application of this code to the phase-transition of spheroidal systems on radial orbits (Wu & Kroupa 2013). A MOND code has also been developed for studies of cosmological structure formation by Ilinares, Knebe & Zhao (2008). While being highly successful in their ability to represent observed galaxies, all of these attempts have died-off due to a lack of long-term sustainability.
Because non-linear Milgromian dynamics is largely non-intuitive for researchers trained to think within the framework of linear Newtonian gravitation, this group of pioneers needs to find the chance to discuss, in as great depth as is required, the issues arising with initialising, setting-up and evolving Milgromian galaxies in virial equilibrium, including gas dynamics and star formation. The first scientific results which have already been achieved with the PoR code will be discussed at this occasion, but research related to Milgromian dynamics (e.g. by adoption of zeroth-order approximations by adding dark matter particles to Newtonan systems) will also be discussed.
The meeting will take place at the Observatoire astronomique de Strasbourg. We are planning a whole week for this event, whereby there will be one to two (at most three) presentations per day interrupted with long discussion breaks to dwell upon problems that have been encountered and that may need solutions. Also, the breaks are intended to allow new persons to learn using PoR. The meeting will take place in the *MEETING ROOM* (with a capacity of about 20) at the Observatoire, and the presentations can be of any duration, but must have a break after the first 45 minutes if longer. After the last presentation each day discussions may continue at will, and Strasbourg offers many excellent culinary opportunities for the evening entertainments.
2.HOW TO REGISTER / IF INTERESTED:
Please register by sending an e-mail to Benoit Famaey <benoit.famaey_at_astro.unistra.fr> and to Pavel Kroupa <pavel_at_astro.uni-bonn.de>.
Note that this meeting does not have invited talks. The attendance is limited to 20.
The programme, abstracts and list of participants are available here as a pdf file:
PoR_Programme.pdfPROGRAM (with downloadable presentations):
First Workshop on Progress in Modelling Galaxy Formation and Evolution in Milgromian dynamics —
first results achieved with the Phantom of Ramses (PoR) code.
At the Observatoire astronomique de Strasbourg, 21.09.-25.09.2015.
PoR-code talks are scheduled for the afternoons allowing for discussion and learning time. A few scientific talks relevant to the mass-deficit problem are scheduled for the mornings.
******* Sunday, 20th September
evening, approximately 18:00-
Meet for drink and food at Au Brasseur
******* Monday, 21st September
10:00 MORNING COFFEE
10:30 Welcome/Introduction/First presentation and discussion:
Setting the scene:
1. Kroupa_PoR.pdf: Why is the dark-matter approach ill-fated? (Pavel Kroupa)
2. Famaey.pdf: The basics of Milgromian dynamics/MOND (Benoit Famaey)
1. Lueghausen_PoR.pdf: The PoR code (Fabian Lueghausen)
2. Thies_PoR.pdf: Setting up a stable disc galaxy in PoR (Ingo Thies)
16:30 AFTERNOON TEA
17:00-18:00 Open Discussion
******* Tuesday, 22nd September
10:00 MORNING COFFEE
10:45-11:15 (30 minutes) Angus_PoR.pdf: The DiskMass Survey’s implications for MOND, CDM and itself (Garry Angus)
14:45-15:15 (30 minutes) Banik.pdf: The External Field Effect In QUMOND: Application To Tidal Streams (Indranil Banik)
16:10 AFTERNOON TEA
16:30 Thomas_PoR.pdf: Simulating Tidal Streams with PoR (Guillaume Thomas)
PoR Movie (dSph Sgr, slide 19 in presentation): YouTubelink
17:00-18:00 Open Discussion - decision to set up PhantomWIKI
******* Wednesday, 23rd September
10:00 MORNING COFFEE
10:45-11:15 Yang_PoR.pdf: (30 minutes) Reproducing properties of MW dSphs as descendants of DM-free TDGs (Yanbin Yang)
MEETING PHOTO (12:15)
14:15-14:45 Angus2_PoR.pdf: The sub-subhalo connection to M31’s plane of satellites (Garry Angus)
14:45-15:15 Pawlowski_PoR.pdf: (30 minutes) Small-scale problems of cosmology and how modified dynamics might address them (Marcel Pawlowski)
16:00 AFTERNOON TEA
16:30 Renaud_PoR.pdf: Gravitation-triggered star formation in interacting galaxies (Florent Renaud) 17:30-18:00
Open Discussion 18:30--
Workshop dinner at Au Brasseur
******* Thursday, 24th September
10:30 MORNING COFFEE
10:45-11:15 Hodson_PoR.pdf: (30 minutes) EMOND (Extended MOND) and effective galaxy cluster masses (Alistair Hodson)
11:30-12:00 Preliminary results on QMOND forces between point masses (HongSheng Zhao)
14:45-15:15 Salomon_PoR.pdf: The tangential motion of the Andromeda System
15:15-15:45 Dabringhausen_PoR.pdf: Early-type galaxies in Milgromian dynamics (Joerg Dabringhausen, remotely from Concepcion, Chile)
16:15 AFTERNOON TEA
16:45-17:15 Banik2_PoR.pdf: Evidence for Dynamical Heating in The Local Group (Indranil Banik)
17:15-18:00 Open Discussion
******* Friday, 25th September
10:00 MORNING COFFEE
10:30-12:00 Kroupa_IMF_Strasbrourg.pdf Main Seminar of the Observatory: Is the stellar IMF a probability distribution function, or is star formation highly regulated? (Pavel Kroupa)
15:00 Final discussion and FAREWELL
This wiki is dedicated to supporting the research making use of the “Phantom of RAMSES” (PoR) patch.
The answer to the question posed in the title is “Apparently, and sadly, yes.”
In previous contributions we have blogged about sociological problems that arise when attempting to do research in non-standard cosmological frameworks (for example the attempt at closing down “The Dark Matter Crisis”).
Early 2015 an incident occurred which is a contemporary example of this, but which may also possibly be a serious case of scientific misconduct. It appears to be an aggressive act in an attempt to discredit new approaches to cosmology and those working on them. A senior professor at CalTech has expressed, in a public forum, “Take the world’s best courses, online, for free“, directed at students of cosmology, wrong and unacceptable views which are likely to discourage young researchers from studying important theoretical concepts. The statements are derogatory, dismissing and demeaning to those full-time researchers who have been performing research in such fields, and who are without exception very talented physicists.
Prof. George Djorgovski teaches Cosmology at CalTech and his course can be followed by students world wide. In order to dismiss alternatives to the standard cosmological model, he recently used in a public forum (see below) the argument that General Relativity is “conformal”, and that this is “well tested”, while MOND is not. He further writes that “Cosmology tends to attract a certain type of crackpots, and some of them even have PhD’s.” “Some were great scientists, before sinking into the downward spiral” thereby implying, it seems from the context, researchers who work on MOND. He makes other, wrong statements, about MOND.
While there are indeed valid and rational arguments to make about the problems of MOND on large scales and on sub-galactic scales (such as globular clusters), one could seriously wonder whether a respectable institution like CalTech should find it acceptable for someone affiliated with it to make such erroneous statements about physics in a public forum dedicated to an official online lecture.
We remind the reader that a conformal transformation is a transformation preserving the angles but changing the magnitude of the length vectors. While many equations in physics are invariant under conformal transformations, Einstein’s equations are not. If they were, their weak-field limit giving rise to Newtonian dynamics would also be conformally invariant in space-time. Since one of the conformal transformations is the one known as “scaling” (others being related to rotations in space-time), conformal invariance would imply space-time scale invariance. But obviously, Newtonian dynamics is not space-time scale-invariant.
Indeed, assume we seek a trajectory (x,y,z,t): which equations of motion are required such that the trajectory n(x,y,z,t), where n is a number, is also acceptable?
This space-time-scale invariance (Milgrom 2009) actually leads quickly to equations of motion different from Newtonian dynamics, and, remarkably, these strictly imply the baryonic Tully-Fisher relation, flat rotation curves of galaxies as well as the external field effect. The above space-time scale invariance has been noted by Milgrom (2009) to be a new symmetry which may have deep theoretical implications. Milgromian dynamics, or MOND, is a classical framework which contains the Newtonian regime and extends it to the very weak-field regime which is identical to the space-time-scale invariant regime. The interested reader may find additional information in the important review by Famaey & McGaugh (2012) and in Kroupa (2015) as well as in Trippe (2014).
To summarize, it is certainly true that space-time scale-invariance does not imply full space-time conformal invariance. But this is a) of course not a problem, and b) GR is not conformally invariant either.
If on the other hand, Prof. Djorgovski meant that MOND is conformal, while GR is not, this is not true either. And there is certainly nothing “well-tested” about this. So one may be led to conclude that Prof. Djorgovski has either misunderstood some important issues or has a non-scientific agenda when interacting with students, and one could wonder whether a respectable institution such as CalTech ought to accept this, whatever one’s stance on the validity of alternative approaches such as MOND. A rigorously working scientist can only accept objective and evidence-based arguments when testing hypotheses.
Personal opinion ought not to play a role when testing the possible laws of nature. For nature it is irrelevant what opinion someone may have, or how prestigious the institute is where the scientist is opinionating from. A scientist may decide which field to work in, and which tests to perform, but dismissing hypotheses without a rigorous and solid analysis is unscientific behavior.
But perhaps Prof. Djorgovski used the wrong word (“conformal”) but meant “covariant”. GR is covariant, but the original paper by Bekenstein & Milgrom 1984 also explicitly proposed a covariant MOND theory, so this is obviously incorrect too.
As explained by a high-profile colleague interested in modified gravity theories (who however does not want to be named here, given the quality of Prof. Djorgovski’s statements) below, it is possible that Prof. Djorgovski has been confused by the fact that this first covariant version of MOND proposed by Bekenstein & Milgrom in 1984 involved a conformal transformation between the Einstein metric and the physical metric. This could not reproduce the observed enhancement of lensing, and led Bekenstein to propose a non-conformal relation between the Einstein and physical metric in 2004 (which is not a problem). So, Prof. Djorgovski is likely to have become confused here, leading to his nonsensical sentence. As stated by our colleague below, this again appears to suggest that Prof. Djorgovski may not understand what he is talking about. It would be a rather serious issue for modern cosmology to have ignorant people teaching it to youngsters.
Coming back to Milgromian dynamics, it has proven to be an incredibly rich theoretical approach to understanding the dynamics of galaxies with convincing success. The success in accounting for observations and more importantly in predictions is convincing evidence that Milgromian dynamics needs to be taken very seriously by theoreticians. It is false to claim, as Prof. Djorgovski does, that “epicycles” kept being added to MOND in order “to salvage it”. The classical framework of MOND, written down in Princeton by Prof. Milgrom in 1983, contains one single free parameter a_0 (possibly a new constant of nature, call it Milgrom’s constant, probably related to the properties of the vacuum; is has the value a_0=3.8 pc/Myr^2 approximately, e.g. Kroupa 2015), which is an acceleration, and this parameter can be fixed by one single galactic rotation curve leaving no freedom for further adjustments in other systems. Exploration of how to embed this classical framework into general-relativistic theories do not constitute “adding epicycles” but are important and necessary theoretical and mathematical research at the highest level of intellectual activity (see the comment below by our high-profile colleague and e.g. Zhao & Li 2010).
Indeed, dismissing the possibility that Einstein’s theory of general relativity (GR) may not be correct in the extreme weak-field regime, constitutes an unphysical ideological constitution of the mind in question. It is well known that Einstein’s GR is not unique. It should also be well known that Einstein 1916 put much effort in constraining his geometical interpretation of gravitation to agree with the Newtonion law of universal gravitation in the appropriate limit. But Newton derived this empirical law based on Solar System data only. Even Einstein did not know what galaxies are. Any person claiming that Einstein’s GR is valid on all scales is effectively performing an extrapolation by many order of magnitudes beyond the empirical data which the law was derived from. It is high-school knowledge that such extrapolations are extremely dangerous and are not likley to work. The apparent failure of GR on galactic scales and beyond may thus be the mere break-down of an extrapolation. It may also harald the existence of dark matter particles, which is a resaonable hypothesis a physicist may probe (as done here at great length). But it is not the only hypothesis.
While incredibly successful on galaxy scales, the hardest test of Milgromian dynamics designed until now has come from my (Pavel Kroupa) group in Bonn using globular clusters (Baumgardt, Grebel & Kroupa 2005; see Kroupa 2012 for a discussion). The evidence until now is ambiguous, but Milgromian dynamics appears to be under some stress on these globular star cluster scales.
Another interesting test being followed up now by observational astronomers in Chile has been proposed by Michael Bilek using shell galaxies (Bilek et al. 2015).
World-wide, the interest in Milgromian dynamics is increasing significantly, partly due to its most amazing success in accounting for the properties of galaxies. The increasing interest is shown in the figure below though the rising number of citations of Milgrom’s paper per year.
This chart shows the development of citations to the original research paper by Milgrom (1983). The increase in citations after the year 2004 comes with the break-through by Bekenstein (2004). Source: ADS.
Two independent groups have now created, for the first time ever, Milgromian simulation codes to allow full cosmological computations of galaxy formation and evolution using baryonic physics with feedback and star formation: the publicly available Phantom of Ramses (PoR)code by Lueghausen, Famaey & Kroupa (2015) and the RAYMOND code by Candlish, Smith & Fellhauer (2015). Numerical experiments on galaxy formation and evolution are being started in Concepcion (Chile), Strasbourg (France), Bonn (Germany), St Andrews (Scotland) and other places.
Surely this increasing activity world-wide is not due to “a certain type of crackpots, and some of them even have PhD’s” (me included with a BSc (hon) from UWA, Perth, a PhD from Cambridge University and habilitation from the University of Kiel as well as receiving a Heisenberg Fellowship, amongst other prizes). It is not so very clear where the crackpots actually are. Prof. George Djorgovski teaches Cosmology at CalTech and his course can be followed by students world wide. Questions may be asked in a forum. Early 2015 a very talented MSc student studying Astronomy and Astrophysics at Charles University, Prague, asked Prof. Djorgovski why he discounts MOND (here is the MOND_Djorkovski-1 of the discussion, and here is a screen shot:
Question by a MSc student to Prof. Djorgovski:
In module 7.2 there is short note about the alternavitve explanation of Dark Matter – the MOND. It was the first time I’ve seen such a possibility, so I did some research about it.
1. There is note in the table, that the gravity is modified on large scales, in papers I’ve found about MOND there is wrriten that the non-Newtonian regime should apply not on large space scales but in very weak gravity regime (such as the General relavity in strong gravity regime). Am I correct?
2. Also in the lecture was mentioned that the MOND does not work properly. I tried to find any references, but I did not. Could someone please explain me where is the problem with MOND?
The original formulation of MOND was a purely ad hoc modification of the Newtonian gravity, designed to explain the flat rotation curves, and without any other physical motivation. This made it also predict that galaxy clusters should not exist. More to the point, it was not a conformal theory, and thus in a conflict with the well established (and tested) aspects of the GR. Theoretical proponents of the theory (there are one or two of them) kept adding “epicycles” to it, to salvage it, thus sacrificing any putative elegance to this purported solution, and again, purely in order to save it, and without any other physical motivation.
A very small number (<< 10) of observers keep finding “evidence” that supports MOND, while ignoring any of the problems. Then some other observers point out that this is not the case, and the cycle continues. Most people see it as an exercise in futility.
Why do people persist in such pursuits? I think that this is a matter of psychology, not astrophysics. Cosmology tends to attract a certain type of crackpots, and some of them even have PhD’s. Some were great scientists, before sinking into the downward spiral; the most famous (and most tragic) example was Fred Hoyle, who simply cannot bear the idea that he was wrong about the Steady State cosmology, and he turned what was a brilliant career into becoming an irrelevant crank. Another, lesser, example was Geoff Burbidge, who refused to accept that the quasar redshifts were cosmological, despite an overwhelming and growing evidence, saying how there may be some new physics behind them, but never producing any. There are many more examples, and the proponents of MOND are not nearly as smart as Hoyle or Burbidge were. Once your ego becomes bigger than your ability to be a critical thinker and an honest scientist, so that you cannot admit that you were wrong and move on, it is over.
I should also note that a great majority of theoretical models turn out to be wrong, and simply disappear without a trace – they turn out to be in conflict with some measurements, fail to make good predictions, and that’s that. That is how science works. Sometimes a brilliant, new, original idea does work, or even transforms the physics – e.g., the relativity – by explaining the known facts and by making testable predictions (and surviving those tests). Most do not.
So if you really want to waste your time, go ahead and sift through those 600 papers on arXiv, and make up your own mind, but I think that you could spend your time more productively on other things.
Note by P. Kroupa:
Remarkable are the comments by some of the other students, if this is what they are, as evident in the forum. Noteworthy is Stephen Schiff’s addenda: “unscrupolous people”, “quacks”, “own egos or self-delusion” etc. with Prof. Djorgovski replying “Exactly”.
A commentary by a high profile colleague who is also an expert on modified gravity:
(given the contents of the text above by Prof. Djorgovski this colleague asked to remain anonymous)
This forum post by Mr. Djorgovski is absolute nonsense. To say that “the original formulation of MOND” was “not a conformal theory (sic)” casts serious doubts that he actually understands what he is talking about. I don’t think anyone could even understand what it is for a theory to be “conformal”… Is GR “conformal”? What does he mean? Does he mean it is conformally invariant? Of course, it is not. So what does he mean, then? Probably one should ask him, but the sad and clear truth is that this statement of Mr. Djorgovski simply does not make any sense whatsoever. But it may surely award him a rather high crackpot index. This is rather ironic, given the rest of his comments, which would probably be best applied to himself.
Actually, the problem of the original scalar-tensor theory proposed to reproduce the MOND phenomenology back in 1984 (which is actually what one would now call a “k-essence” scalar-tensor theory) is that it invoked a physical metric (coupled to matter fields in the matter action) which was conformally related to the Einstein metric, and for that reason, while enhancing the dynamical effect (g_00 term of the metric) could not enhance gravitational lensing (through the other space-space diagonal terms) by similar amounts. This is why a disformal transformation, invoking a vector field in addition to the scalar field, was proposed by Jacob Bekenstein 20 years later. This is perhaps what confused Djogorvski. But of course this is not “in a conflict with the well established (and tested) aspects of the GR” (sic). The latter statement relating to a mysterious “conformal” nature of GR, I have still a hard time believing has been written by someone with a PhD in Physics, and not by some random crackpot.
But this so-called TeVeS theory of Bekenstein does have real phenomenological problems, like the fact that without additional non-baryonic matter it has a hard time reproducing the CMB. Much better models in this respect are those recently proposed by Justin Khoury 2014 or Blanchet & Le Tiec 2008 and Bernard & Blanchet 2014.
Regarding his other comments, MOND is obviously not an “ad hoc” modification of gravity, but simply a phenomenological law relating the distribution of baryons to the gravitational field in galaxies. The original Milgrom’s formula is of course not a theory “per se” but a phenomenological law which allows to make predictions on the scale of galaxies. These a priori predictions do work extremely well on these scales, and do of course concern data that were not available back in 1983, which is why it is ridiculous to call it “ad hoc”. Especially so since MOND can be derived from space-time scale-invariance.
Now, the MOND interpretation of these observations is, very generally speaking, just that this fine-tuned relation between baryons and the gravitational field is not a consequence of “gastrophysical” feedback mechanisms (as is usually assumed in the standard dark matter context based on Einsteinian/Newtonian dynamics) but rather a reflexion of something more profound in the Lagrangian of nature, which one usually refers to in the standard context as “dark matter”, and which one also usually conflates with “non-baryonic, mostly collisionless, particles”, which is by no means requested by galaxy-scale data.
It is very true that it is not easy to write a modified action which reproduces this phenomenology, appears natural, and also keeps the most successful aspects of the current standard model such as successes in reproducing the acoustic peaks of the CMB. There are however a few proposed actions which do achieve this such as those proposed by Justin Khoury and Luc Blanchet (see references above), but they still appear a bit unnatural. These should of course just be considered as examples of what kind of Lagrangian can be written to both reproduce the phenomenology of MOND and reproduce the undeniable successes of LCDM on large scales.
Also, to say that MOND predicted galaxy clusters not to exist is of course blatantly wrong. MOND actually leads to galaxy clusters forming more rapidly than in the standard model of cosmology, as has been published years ago. It actually predicted that there should indeed be missing mass there, e.g. in the form of missing baryons such as cold molecular gas clouds, or in the form of hot dark matter with a free-streaming length above galaxies, or that the new degree of freedom in the Lagrangian of nature (see references above in the work of, e.g., Khoury and Blanchet) creating an effective modification of gravity on galaxy scales which is behaving like a collisionless preassureless fluid on these scales, just as it should do to reproduce the angular power spectrum of the CMB.
All of this does of course not mean that “MOND” is right, or in any way a final theory (which it cannot be because it could only come out of a larger theoretical framework), but it is a proof that the criticisms raised by Djorgovski just display ignorance. His comments are, at best, nonsensical.
Until the many challenges to LCDM (see Kroupa 2012; Kroupa 2015) are addressed within the standard model, if they ever can be, it is only a fair scientific endeavor to also consider modifications of the action which could address these issues. That does not prevent people from working on the solutions in the standard context, nor to criticize these alternatives. But when doing so, only rational arguments are admissible. The expressions used by Djorgovski in a public forum are instead completely nonsensical from a physics point of view, demeaning and offensive from a behavioral point of view, and generally unacceptable.
The above episode demonstrates that the cosmological research field is broken. It apparently allows its members to teach students the most blatantly wrong contents as long as they are considered to be defending the “mainstream”. It appears that knowledge of basic physical concepts may not seem to be a requirement to teach cosmology at CalTech anymore. This is both pathetic and terrifying.
This example exemplifies the serious sociological forces acting against the few bright and inquisitive minds who, in the true spirit of science, dare to venture outside the dull beaten track followed by most.
Spiral galaxies rotate too fast. If they would only consist of the visible (baryonic) mass we observe in them and Newton’s Law of gravity is correct, then they would not be stable and should quickly fly apart. That they don’t has been one of the first indications that the galaxies (and the Universe as a whole) either contains large amounts of additional but invisible “dark matter”, or that the laws of gravity don’t hold on the scales of galaxies. One possibility for the latter, Modified Newtonian Dynamics (MOND), proposes that gravity needs to be stronger in the low acceleration regime present in galaxies (for more details see the extensive review by Famaey & McGaugh 2012 and Milgrom’s Scholarpedia article). That the rotation curve (i.e. the function of circular velocity of the galactic disc with radius) of our Milky Way galaxy follows the same trend as the rotation curves of other spiral galaxies has been known for a long time, too. So it appears to be a bit surprising that the Nature Physics study “Evidence for dark matter in the inner Milky Way” by Fabio Iocco, Miguel Pato and Gianfranco Bertone makes such a splash in the international press. That the MW should contain dark matter is not news, but nevertheless the paper got a hugeamount of presscoverage.
Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line).
One thing emphasized a lot by the press articles (and press releases) is that the authors claim to have found proof for the presence of dark matter in the ‘core‘, ‘innermost region‘, or even ‘heart of our Galaxy‘1, not just in the intermediate and outer regions. This might be worrisome for modified gravity theories like MOND, which predict that regions very close to the center of the Milky Way should be in the classical Newtonian regime, i.e. the rotation curve should be consistent with that predicted by applying Newton’s law to the observed mass distribution. The underlying reason is that due to the higher density of baryonic matter in the center of the Milky Way the gravitational acceleration of the baryons there already exceeds the low-acceleration limit. But only once the acceleration drops below a certain threshold the non-Newtonian gravity effect kicks in. Interpreted naively (i.e. assuming Newtonian dynamics), this would mimic dark matter appearing only beyond a certain radial distance from the Galactic Center.
Without even going into the details of checking their assumed Milky Way models, the way the observational data is combined and whether there are systematic effects, a simple look at figure 2 in Iocco et al. already reveals that their strong claim unfortunately is not as well substantiated as I would wish.
The plot’s upper panel is what is of interest here. It shows the angular circular velocity in the Milky Way disk versus the Galactocentric radius. The red points with error bars are observed data for different tracers. The grey band is the range of velocities allowed for the range of baryonic mass distributions in the Milky Way considered by Iocco et al. (that are all consistent with observations). If there would be only baryonic matter and Newtonian Dynamics, the rotation curve of the Milky Way should lie somewhere in this area.
First of all, the figure shows that they did not consider any data in the region within 2.5 kpc. That makes sense because that region will be dominated by the bar and bulge of the Milky Way. Stars in the bulge don’t follow circular orbits, so one can’t measure circular velocities there.
So, what is the core, heart or ‘innermost region’ of the Milky Way? Lets try to come up with something motivated by the structure of our Galaxy. The Galactic disk is often modeled by an exponential profile, with a scale length of about 2.2 kpc. What if we say the core of the MW is everything within one scale length? Immediately there’s a problem with the claim by Iocco: They are not even testing data on this scale.
Lets ignore the phrase ‘core’ or ‘heart’ of the Milky Way and focus on the more general formulation they also use in their paper’s title: “Evidence for dark matter in the inner Milky Way”. Looking at their Figure again, we can see that the data start to leave the grey band at a distance of about 6 kpc from the MW center. Thus, within 6 kpc (almost three scale radii of the Milky Way disk!) the purely baryonic models encompass the data. Consequently, here is no need to postulate that dark matter contributes significantly to the dynamics. The figure clearly shows that there is no need, and therefore no evidence for dark matter within 6 kpc of the Galactic Center, which is as generous a definition of ‘inner Milky Way’ as it gets in my opinion. The authors themselves even write that ‘The discrepancy between observations and the expected contribution from baryons is evident above Galactocentric radii of 6-7 kpc’. In this regard it doesn’t matter whether the majority of the possible baryonic models predict a lower rotation curve: as long as the data agree with at least one baryonic model that is consistent with the observed distribution of mass in the Milky Way, there can not be evidence for dark matter.
I really don’t understand why they then claim to have found proof of dark matter in the innermost regions of the Milky Way. My suspicion is that the authors and their press releases seem to have a (literally) quite broad interpretation of the term ‘innermost region’. Judging from the context, they seem to subsume everything within the solar circle of ~ 8 kpc (the distance of the Sun from the Galactic Center) as ‘innermost’. I don’t think it is an appropriate definition, after all it makes the vast majority of the baryonic mass of the Milky Way part of the innermost region. Half the light of an exponential disk is already contained within less than 1.7 scale length (1.7 x 2.2 kpc = 3.7 kpc for the Milky Way), and all of the bulge/bar is in there, too. But if we nevertheless roll with it for the moment we can see that yes, between 7 and 8 kpc there seems to be need for dark matter … or for a MOND-like effect.
Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line).
So, lets have a look at one MOND rotation curve constructed for the Milky Way (from McGaugh 2008) to see where we expect to find a difference in Newtonian and MONDian circular velocities. The expected Newtonian rotation curve is shown as a black line in the plot, equivalent to the purely baryonic rotation curves making up the grey band in the figure of Iocco et al.. The rotation curve predicted by MOND is shown as a magenta line and the observed circular velocities are the small squares.
The plot immediately reveals that a discrepancy between the Newtonian and the MONDian rotation curves is expected already at small radii, well within 6 kpc. The findings of Iocco et al. that there appears to be some mass missing within the solar circle therefore do not disagree with the MONDian expectation, in contrast to what one of the authors is quoted saying in a Spektrum article. Furthermore, the plot demonstrates that the need for dark matter (or MOND) in the region inside the solar circle was already well known before this new study.
So, in summary, the study doesn’t show all that much new or surprising, the claimed ‘evidence’ for dark matter in the innermost Milky Way is not present in their data (unless you define ‘innermost’ very generously) and some apparent dark matter contribution within the solar circle is not even unexpected based on MOND predictions.
1: The press releases of the TU Munich and Stockholm University even call it a ‘direct observational proof of the presence of dark matter in the innermost part our Galaxy’ (which is clearly wrong, there is obviously nothing direct about it and the innermost part would imply the very center of the Milky Way).