Dynamics of Local Group galaxies: Evidence for a past Milky Way–Andromeda Flyby?

Indranil_BanikThe 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.

Figure_1I 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.Figure_2new

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.

Figure_3The 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.

Figure_4In 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.

Figure_5

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.

Figure_6

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.

Figure_7I 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.

The paper for which I won the Duncombe Prize is available here: http://arxiv.org/abs/1506.07569

The peer-reviewed version has appeared in the Monthly Notices of the Royal Astronomical Society, volume 459, issue 2, pages 2237 to 2261.

The Weizmann Experience: discussions on the future of cosmology

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.
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 infront of th Department.
Chairs and a pond infront 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.
Part of the Weizmann Institute as viewed from the top of the Koffler Accelerator Building.
The "bubble" housing the conference room in the tower of the Koffler Accelerator.
The “bubble” housing the conference room in the tower of the Koffler Accelerator.
The Group at the Koffler Accelerator. From right to left: Benoit Famaey, Francoise Combes, Mordehai Milgrom and Pavel Kroupa.
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.

We visited Caesarea:

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.
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 thriving thousand-yearold 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.

The ruins of Caesarea. King Herodot had his palace here.
The ruins of Caesarea. King Herod is supposed to have had his palace here.
The author amongst the ruins of Caesarea. "What was the fate of Caesarea's inhabitants when it fell to the Mamluks?"
The author amongst the ruins of Caesarea. “What was the fate of Caesarea’s inhabitants when it fell to the Mamluks?”
The Group in front of the Roman ampitheater in windy Caesarea, nearly but not quite ready.
The Group in front of the Roman ampitheater in windy Caesarea, nearly but not quite ready.
The Group in Caesarea, ready. From right to left: Mordehai Milgrom, Francoise Combes, Benoit Famaey, Pavel Kroupa.
The Group in Caesarea, ready. From right to left: Mordehai Milgrom, Francoise Combes, Benoit Famaey, Pavel Kroupa.

 

 

Acre, once a blossoming port and a gate-way to the holy lands for christian pilgrims.
Acre, once a blossoming port and a gate-way to the holy lands for christian pilgrims.

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: the remains of the Crusader port.
Acre was under the administration of the Knight's Hospitaller who helped arriving pilgrims and food was served in this Crusaders Refectory.
Acre was under the administration of the Knight’s Hospitaller who helped arriving pilgrims and food was served in this Crusaders Refectory.

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.
Old Jaffa, which dates back to a history of 4000 years and where alrady the Egyptian empire stationed a garrison.
Old Jaffa.
Old Jaffa.

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).
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.
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.
At the home of Moti in Rehovot. From right to left: Moti, Benoit and the author.

 

Final comments

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 detection of gravitational waves, predictions, MOND and my visit to the Weizmann Institut in Rehovot, Israel

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.

 

The future

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.

First Workshop on Progress in Modelling Galaxy Formation and Evolution in Milgromian dynamics — first results achieved with the Phantom of Ramses (PoR) code

[Note: This web-page is being updated continuously:
current status: 26.09.15]

LOCATION and TIME:
Observatoire astronomique de Strasbourg, Universite de Strasbourg, CNRS UMR 7550, Sept. 21st - 25th 2015

Below are provided
1.BACKGROUND/MOTIVATION
2.HOW TO REGISTER
3.PARTICIPANTS
4.HOTELS
5.PROGRAMME
6.PHANTOM WIKI
ORGANISERS: Benoit Famaey (Strasbourg) and Pavel Kroupa (Bonn)

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 2006Angus & 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.

Now much more involved and more numerous studies has become possible with the first publicly available Milgromian dynamics computer code including star formation, i.e. baryonic physics (Lueghausen, Famaey & Kroupa 2015) with which even full-scale simulations of cosmological structure formation have become achievable, PoR being an official patch to Teyssier’s RAMSES code. A similar computer code (RAyMOND) has been developed independently by a Chilean research group (Candlish, Smith & Fellhauer 2015).

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.
3.PARTICIPANTS (preliminary):

Garry Angus (Brussel, Belgium)
Indranil Banik (St. Andrews, UK)
Christian Boily (Strasbourg, France)
Joerg Dabringhausen (remotely from Concepcion, Chile)
Benoit Famaey (Strasbourg, France) [SOC]
Martin Feix (Paris, France)
Hector Flores (Paris, France)
Alistair Hodson (St. Andrews, UK)
Rodrigo Ibata (Strasbourg, France)
Tereza Jerabkova (Praha, Czech Rep.)
Pavel Kroupa (Bonn, Germany) [SOC]
Fabian Lüghausen (Bonn, em.; tbc)
Marcel Pawlowski (Cleveland, USA)
Florent Renaud (Surrey, UK)
Jean-Babtiste Salomon (Strasbourg, France)
Ingo Thies (Bonn, Germany)
Guillaume Thomas (Strasbourg, France)
Yanbin Yang (Pairs, France)
HongSheng Zhao (St. Andrews, UK)

Conference Photo (24.09.2015):

PoR_group
Left to right:  Yanbin Yang, Indranil Banik, Ingo Thies, Guillaume Thomas, Garry Angus, Jean-Babtiste Salomon, Tereza Jerabkova, HongSheng Zhao, Rodrigo Ibata, Marcel Pawlowski, Hector Flores, Alistair Hodson, Florent Renaud, Benoit Famaey, Fabian Lueghausen, Pavel Kroupa
4.HOTELS:

Hotel Esplanade
ETC Hotel
Hotel Roses
Hotel21
Au Cerf d’Or
des Princes
5.PROGRAME:
The programme, abstracts and list of participants are available here as a pdf file:
PoR_Programme.pdf


PROGRAM (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
ACCUEIL
******* 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) LUNCH (12:15-14:45) 15:00-16:15 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) LUNCH (12:15-14:45)   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) LUNCH (12:20-14:45) 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
ACCUEIL
******* 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) LUNCH (12:15-14:45) 14:45-15:15  Salomon_PoR.pdf: The tangential motion of the Andromeda System (Jean-Babtiste Salomon) 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) LUNCH (12:15-14:45) 15:00 Final discussion and FAREWELL
6.PHANTOM WIKI

PhantomWIKI
This wiki is dedicated to supporting the research making use of the “Phantom of RAMSES” (PoR) patch.

Can one say anything, even the most obviously wrong things, to discredit an alternative to the standard model? An incident: cosmology at CalTech

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.

 

Milgrom1983citations
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 mond_djorgovski_forum-1screen 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 answer by Prof. George Djorgovski:

(In the forum “Take the world’s best courses, online, for free“)

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.

 

Concluding remarks:

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.

 

See the overview of topics in The Dark Matter Crisis.

Dark Matter in the innermost regions of the Milky Way?

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 huge amount of press coverage.

Rotation curves of two spiral galaxies (images in the background). The black line illustrates the Newtonian expectation for the rotation curve based on the observed baryons (stars and gas), the blue line is the MOND fit.
Rotation curves of two spiral galaxies (images in the background). The black line illustrates the Newtonian expectation for the rotation curve based on the observed baryons (stars and gas), they are clearly not high enough to explain the data points (small circles). The blue line is the MOND fit for which the mass-to-light ratio is the only free parameter. Credit: Stacy S. McGaugh, private communication

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 Galaxy1, 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.

wc_fit
Credit: Fig. 2 of Ioco et al. (2015).

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). Note the excellent prediction of the observed rotation curve given the observed distribution of baryons only which is achieved in MOND; the Galaxy appears entirely Newtonian within the innermost 2 kpc.
Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line). Note the excellent prediction of the observed rotation curve given the observed distribution of baryons only which is achieved in MOND; the Galaxy appears entirely Newtonian within the innermost 2 kpc. Credit: McGaugh (2008)

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.

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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).

 

See the overview of topics in The Dark Matter Crisis.

Pavel Kroupa on "The vast polar structures around the Milky Way and Andromeda "

In case you, like me, have missed Pavel Kroups’s recent talk at the Joint Astronomical Colloquium in Heidelberg, you now have the opportunity to watch a movie of the event and download the slides. The movie is quite long (more than an hour), but it is worth watching it to the end. While the talk is titled “The vast polar structures around the Milky Way and Andromeda”, Pavel talks about much more, starting with tidal dwarf galaxies and ending with a discussion of indications for an alternative model of gravity.

This presentation is very similar and in most parts identical to Pavel’s presentations held at Monterey at the conference “Probes of Dark Matter on Galaxy Scales” and in Durham at the “Ripples in the Cosmos” conference. The latter talk resulted in quite a discussion on Peter Coles’ (aka Telescoper) blog “In the Dark”, following his criticism of Pavel’s talk as being “poorly argued and full of grossly exaggerated claims”. The video of a very similar presentation now offers everybody the opportunity to develop their own opinion on the issue. Given the numerous questions Pavel got during his talk and afterwards, people must have thought that it was worth the effort to argue with him, in contrast to Peter’s opinion.

 

See the overview of topics in The Dark Matter Crisis.