Winnie-the-Pooh came to a village far away from Hundred Acre Wood in which, as he had heard, stood a great jar of the very best honey to be found anywhere. Bears and honey, oh boy. The jar of honey was very famous. Everyone far and wide was saying — because they had had it heard being confirmed by everyone all the time — that this honey is especially very special because it is invisible. And so many other inhabitants of the woods and meadows came to look at The Jar, even from very far places no-one had ever heard of. Many of the rich visitors even hoped to be able to get a taste of this famous invisible honey. The jar was famously called “The Jar Full of Invisible Hunny”. Winnie-the-Pooh wanted to later take his friends, Christopher Robin and Piglet, to also see this Jar Full of Invisible Hunny. Maybe Christopher could buy some of the famous invisible hunny for Winnie-the-Pooh? But it must be very very expensive, so Winnie-the-Pooh thought he should first have a look.
Winnie-the-Pooh (copyright A.A. Milne and E.H. Shepard; credit: Winnie The Pooh 2011 51st Disney’s classic).
Winnie-The-Pooh was very impressed by the great size of The Jar of Invisible Hunny and really wanted to get a taste of this very best honey. But no-one was allowed to look into the The Jar, and when someone did manage to sneak a glance in, they confirmed that it is truly The Jar of Invisible Hunny. Being a clever bear albeit with a very slow mind, Winnie-The-Pooh thought, after some time of looking at The Jar Full of Invisible Hunny, that he might learn a little more about this invisible hunny by knocking on the side of The Jar. He went up to The Jar, embraced it to show his gratitude of being near it, and quickly, so no-one noticed, knocked. The sound he received made him raise his eyebrows – the clanking he heard in return sounded as if The Jar was empty!
Somewhat disconcerted and very thoughtful, Winnie-the-Pooh sat back and looked at The Jar supposedly Full of Invisible Hunny. After some time (the Sun had moved from the left to the right) of very intense thinking, a thought slowly and unstoppably formulated in the bears mind: might it be that The Jar Full of Invisible Hunny is (supposedly) full of “Invisible Hunny” because there is no hunny inside The Jar? Very bothered with his new very Uncomfortable Thought, Winnie-the-Pooh got up and walked around the village and spoke to anyone who would be willing to listen to him. And every time Winnie-the-Pooh explained the Uncomfortable Thought, the listener stopped listening, calling the bear a very impolite bear who should stop having such Uncomfortable Thoughts, behave as everyone else and just accept that The Jar Full of Invisible Hunny is FULL of Invisible Hunny. This has been confirmed by many who have had it confirmed with utmost certainty by others, and there is no question about The Jar Full of Invisible Hunny being FULL of Invisible Hunny. EVERYONE knows this!
No-one believed Winnie-the-Pooh. Some even suggested that the bear be removed from the village and put into a hole as the bear’s Uncomfortable Thought might spread to those dimm wits with lesser minds. After all, the village wants everyone to come because the visitors bring affluence. Winnie-the-Pooh became very unhappy, and sat down again in moonlight as the Moon started to climb up its ladder. By the time Winnie-the-Pooh had to look straight up to see the Moon, a plan had formulated in the bear’s mind:
"If I am not allowed to taste the invisible hunny, I can at least see if I can remove the Uncomfortable Thought from my thoughts and become like everyone else, by dropping a small stone into The Jar Full of Invisible Hunny. If the stone falls through to the bottom or The Jar ever more rapidly, as if it were falling outside of The Jar Full of Invisible Hunny, then the Jar Full of Invisible Hunny is a jar without hunny in it and my Uncomfortable Thought would become a Bother. But if the stone falls into The Jar Full of Invisible Hunny and stops for a while before sinking down slowly to the bottom of The Jar Full of Invisible Hunny then The Jar Full of Invisible Hunny is indeed full of invisible hunny."
The Winnie-the-Pooh Test
Winnie-the-Pooh became very proud of himself because he had managed to have such a great idea for such a complicated test, but he became very worried about the possible outcome of this ultimate experiment.
So when everyone was sleeping and the Moon had climbed down on the other side, Winnie-the-Pooh collected a little stone, cleaned it in a nearby stream, borrowed the same ladder and climbed up the Jar Full of Invisible Hunny. Looking around to make sure no-one saw his very secret and surely forbidden experiment, Winnie-the-Pooh dropped the little stone into the jar. It fell right through landing on the jar’s bottom with a clanking sound. The Jar Full of Invisible Hunny was completely empty.
Winnie-the-Pooh returned the ladder to where he had gotten it from so as to have no-one raise suspicion and sat down. His tummy was by now rumbling away and also increasingly unhappy, because it was very hungry. So Winnie-the-Pooh first needed to get his tummy, his best pal, happy, and he decided to look for a non-empty pot of honey, or some condensed milk, or something else that would make tummy happy.
Winnie-the-Pooh left the village with The Jar Full of Invisible Hunny which he now knew was an empty jar. He reasoned with himself: “oh bother, there’s nothing I can do about the large jar empty of hunny. No-one listens to me and no-one cares about knowing the truth, I do not want to end up in a hole and my tummy will have none of it anymore”.
In The Dark Matter Crisis by Elena Asencio, Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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I have set up a mailing list to discuss the idea that we are living in a large local void and that this solves the Hubble tension, as discussed here:
The mailing list is for anyone who wants to stay updated about related work, even if they are not working on this hypothesis or related topics. The idea is that if someone wants to advertise an upcoming talk or an article they have recently posted but they are at an early career stage and do not know everyone who might be interested, they can just send an email to the mailing list. Also if some discussions between more senior researchers take place through this list, then any early career researchers signed up to it will be included in the conversation – even if nobody thought explicitly to include them in the conversation. Regardless of whether you are signed up, you can send an email to the mailing list and everyone on it should receive the message. Note this mailing list is of course also for people who want to share their thoughts on why the Hubble tension cannot be solved with a local void. Alternative solutions can be discussed briefly, but obviously this is not the ideal place to have detailed discussions about alternative proposals for the Hubble tension.
Please contact Dr Indranil Banik if you want to sign up to this mailing list and thus receive the emails sent to it. He will be in charge of sending an invitation link which you need to accept in order to complete the sign up:
Any request to join this list would take only a little time and you would not be asked for any reasons for why you want to sign up, so just write to the above address and join now! Please do not send join requests to the localvoidworkers address as this will go to everyone on the list. It is meant primarily for astronomers working on related topics, but people who are just interested to follow along with the discussions are welcome as well. There are some journalists and members of the public on the similar mondworkers mailing list, and that is totally fine.
Most likely, this list would not be used all that often for a while, and slowly catch on as more people know about it. Obviously it is not suitable for a great many emails as the sender might only want specific people to see it rather than the whole mailing list. But there are times when people want their email to gain extra visibility, and that is what this is about.
Please advertise this to especially early career researchers, it is intended for sharing adverts for upcoming talks, notifying others of articles and blogs, and discussing research ideas you want to share. In general, it is for anything you want to share with everyone on the list, including asking for advice on articles and research ideas, if you are happy for everyone to know about it.
You can of course be removed from the mailing list if you ask. Thank you to those of you who have already signed up.
In The Dark Matter Crisis by Elena Asencio, Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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The Cosmology Group at the Astronomy Lab of the Aristotle University of Thessaloniki is organising a conference on the above problem. The organisers are PhD student Asvesta Kerkyra with professor Leandros Perivolaropoulos and professor Christos Tsagas.
The conference will take place from June 3rd until June 6th on site in Aristotle University. Further information can be found at this link.
In The Dark Matter Crisis by Elena Asencio, Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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Readers may be interested in two on-line meetings that are coming up:
1. Challenges of Modern Cosmology 2024 (CMC2024): January 18th
CMC2024 is an online discussion panel intended for listening, sharing and learning about challenges of modern cosmology and alternative theories. It will take place on the 18th of January 2024 from 12:00 to 17:00 (CET) and it will be publicly streamed on youtube. The panel will be divided in three sessions: current problems of modern cosmology, modified cosmology and gravity theories, and standard cosmological and gravitational tests. Each of these sessions will include from two to three 10 minutes talks followed by a 20 minutes discussion on the corresponding topic of the talk.
In order to join the discussion panel, registration is still possible until the 10th of January. The discussion can also be followed online without registration by searching for “CMC2024” on youtube or directly through. Before lunch break:
After lunch break:
Participation in social media to share comments or questions is also possible by using, for example, the X-hashtag #cosmos24.
2. Challenging the standard cosmological model: April 15th-16th
Scientific discussion meeting organised by Professor James Binney FRS, Dr Roya Mohayaee, Professor John Peacock FRS and Professor Subir Sarkar. See this link for the details and the registration for researchers in relevant fields.
In The Dark Matter Crisis by Elena Asencio, Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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It seems that experts have a conference once a month somewhere on the globe to discuss why the local Universe seems to be expanding faster than the global Universe. Local expansion is obtained by using standard candles, the type Ia supernovae that explode in galaxies, by associating their distances to their recessional speeds. Global expansion is obtained by fitting the standard cosmological model to the cosmic microwave background (CMB) properties. This Hubble Tension is keeping hundreds if not thousands of astronomers and physicists busy in their jet-setting around the world to meet at repeating conferences at which the latest ideas on exotically complex, time or space-dependent dark energy descriptions are announced as possible solutions to the Hubble Tension. It keeps many bright people busy and some at a very elevated state of fame: “We have discovered a major new mystery of the Universe and only the very brightest of minds will be able to solve it.” Such minds can only be found in Ivy League Universities. I have been at a few such conferences (DMC Nr.77). I noted that it is not a problem for this group of scientists that the standard Einstein/Newtonian–inflation–plus–dark-matter–plus–dark-energy (the LCDM) model (I include here warm and fuzzy dark matter versions as these are extremely similar to the cold-dark matter version) does not conserve energy.
It is simply accepted that this model universe (which nearly everyone thinks is the real Universe) accelerates its expansion driven by a dark energy that no one understands and that creates more space and energy ever faster without an end.
Why is dark energy even needed? Well, assuming Einstein/Newtonian gravitation plus inflation plus dark matter leads to a model universe that would today globally expand too slowly. The only way to fix this overall expansion problem is to include an additional hypothetical process which pulls the model universe apart, and this is dark energy. The draw back of this fix is the above infinite-energy problem.
Interesting in this is also the systematic ignoring of the obvious solution to the Hubble Tension problem: the simple fact that our Local Group of Galaxies (about 10 million light years across) is located in a cosmological void of matter which is some 3 billion light years across. This region contains fewer galaxies than other regions and is observed in all surveys that have been done to date — see Figure 1.
Figure 1: The local void: the y-axis shows, essentially, the ratio between the observed number of galaxies divided by the expected global-mean-value in the LCDM model and the x-axis shows the distance from our Local Group (one pc is about 3.3 light years and h70 is about one). The inset indicates the literature where the data come from. The observational data tell us that at distances further than about 1200 light years (400Mpc) an about constant average space density of galaxies is reached, but that at smaller distances we are seeing a significantly smaller number of galaxies per unit volume. This is the local void (also referred to as the KBC void after Keenan-Barger-Cowie, or the local hole already described by Tom Shanks and his collaborators since 2003. Note that the underdensity is much deeper than allowed by the LCDM model (the shaded region). This figure is from Kroupa (2015).
“Thus, the present 2MASS data suggest the presence of a potentially huge contiguous void stretching from south to north. Not only would this delineate further the limits for the cosmological principle but it would also show the possible presence of significant power on scales of >~300 h-1 Mpc in the galaxy power spectrum.”
In the above Mazurenko et al. (2023) work, a cosmological model was employed to calculate the growth of structures. The model is (yes, obviously) MOND-based and works without cold, warm or fuzzy dark matter (which by now everyone should know does not exist). This cosmological model leads to the growth, as cosmological time progresses, of density differences between different regions. It turns out that the type of under-density, such as is evident in Figure 1, arises naturally. The first-ever hydrodynamical simulations of structure formation in this “nuHDM model” was published by us in Bonn (Wittenburg et al. 2023).
These under-densities (and corresponding overdensities) develop in this MOND-cosmological model because the effective gravitational force is stronger allowing the tiny initial fluctuations observed in the CMB to grow to more pronounced structures than in the LCDM model. These same fluctuations produce a completely uniform and smooth model universe in LCDM when viewed on scales larger than some 600 million light years, while leading to a much more clumpy and irregular model universe in a MOND-based cosmological model on even larger scales. In two previous publications (2021 and 2023) our study in Bonn led by PhD student Elena Asencio and Indranil Banik have also shown that the very massive galaxy cluster El Gordo, observed to already exist at the redshift of 0.87, arises naturally in such a model. In the LCDM model this is quite impossible (the Bullet Cluster, by the way, is also a challenge for the LCDM model but is easy in MOND-cosmology, as explained by Elena in her publications). See DMC Nr. 84.
We thus have the following physical situation: The (correct — in the sense of not having cold or warm of fuzzy dark matter but a realistic gravitational law) MOND-cosmology grows large regions of matter underdensities (as well as major overdensities and massive galaxy clusters). Being in an underdensity, the observer finds that the galaxies in it are falling towards the sides of the underdensity, just like apples fall to the ground on Earth (Isaac Newton would probably have appreciated this — see Figure 2).
Figure 2: The image shows the schematic distribution of matter in space – (blue; the yellow dots represent individual galaxies). The Milky Way (green) lies in an area with little matter. The galaxies in the bubble move in the direction of the higher matter densities (red arrows – imagine these are apples). The universe therefore appears to be expanding faster inside the bubble.
This is the reason why the observer thinks the local Universe is expanding at a slightly faster rate than the global Universe. Given that the underdensity is observed to be there through a lack of galaxies (Figure 1), this solution to the “Hubble Tension” is straight-forward – it is in fact so trivial that the leading minds in cosmology (the elite) appear to be challenged in grasping it — it seems that the whole research community, as shepherded by the brilliant minds in Ivy League institutions, appears to be acting as if it were the Catholic Church some 400 years ago concerning MOND: MOND is a sacrilegious topic not to be talked about by those that want to matter, and any way, every one knows it is wrong and also not a theory (well, the wide-binary-star test of MOND lately causes some heated debate among the few people who have the skills to make calculations in MOND and will be addressed here in the near future).
In any case, given this explanation of the Hubble Tension in a MOND-based cosmological model, our model as published in Haslbauer et al. (2020)makes a prediction! The prediction is: in the model (which accounts for the underdensity — Figure 1 — and the Hubble Tension as published by Haslbauer et al. 2020, and which does not take into account any measurements of bulk velocities AT ALL), galaxies must be moving faster away from us the further they are, after subtracting the Hubble expansion. That is, the model predicts (before the measrurement) that the bulk flow speed of galaxies increases with increasing distance.
And the sensational thing about all of this is: this prediction has now been confirmed! Watkins et al. (2023) measure “the bulk flow in a volume of radii 150-200 h-1 Mpc using the minimum variance method with data from the CosmicFlows-4 (CF4) catalogue.” Figure 3 shows the measured bulk flow: For example, galaxies observed at a distance of 200/h Mpc (about 600 million light years) show a bulk velocity of about 420 km/s (after correcting for the Hubble expansion). The MOND-based cosmological model gives the same velocity if the Local Group (the green dot in Figure 2) is approximately 380 million light years (116 Mpc) away from the centre of the void and is moving with a velocity of about 220 km/s relative to the local bulk flow such that the Local Group moves with a total of approximately 627 km/s relative to the CMB.
Figure 3: The bulk flow of galaxies (the average speed of galaxies in a sphere, y-axis) is plotted versus the distance from the observer on the x-axis. The data from Watkins et al. (2023) are shown as solid black dots. The MOND-based cosmological model is shown as the dotted line assuming the local void has a Gaussian density profile, that the Local Group is located 116 Mpc (about 380 million light years) away from the void centre and that the Local Group is moving with 627 km/s relative to the CMB and about 200 km/s slower than the local bulk flow (within some 150 million light years). In other words, the Local Group’s velocity relative to the CMB has been reduced to 627 km/s by small-scale flows in the local region. Thus, the MOND-cosmology-based bulk flow (dotted black line) is in (stunning) agreement with the data in terms of its amplitude and shape, while the LCDM model predicts bulk velocities (solid red line) that are in major disagreement with the observations. Adapted from Mazurenko et al. (2023).
It thus turns out that once we have a better model of the observed Universe, then (i) the local billion light year underdensity, (ii) the Hubble Tension and, simultaneously, (iii) the high observed bulk velocities of the galaxies at distances of a few hundred million light years are automatically and simultaneously understood. This is trivial in the sense that it pops out of the MOND-based calculations, but it is highly non-trivial because no other known model has been able to achieve this.
What of the future? We now have an improved cosmological model, namely the “nuHDM” model. It accounts automatically for open star clusters, galaxies(e.g. Banik & Zhao 2022, Kroupa et al. 2023), the Hubble Tension, bulk flows as well as the significant density contrasts on scales of some billion light years (this text). But open questions remain:
This “nuHDM” MOND-cosmological model is very conservative by assuming a next-to-identical expansion history as the LCDM model. It thus assumes, like the LCDM model, inflation and dark energy, and assumes the CMB is the photosphere of the hot Big Bang. The one major issue therefore is that it is also not energy conserving. To achieve the same expansion history, the mass content of this model universe needs to be dominated by a sterile neutrino background, which effectively is a hot dark matter component that plays no role in galaxies and is less exotic by being related to the physics of the active neutrino. And it solves the missing mass problem in galaxy clusters. As shown in the first-ever hydrodynamical simulations of structure formation in the nuHDM model published by us in Bonn (Wittenburg et al. 2023), it seems to form galaxies too late and by the present time it is populated by too many hugely massive galaxy clusters, posing two additional problems. Further research on this model is needed reaching to much higher resolution. Given these open questions, a new, and bolder model of the Universe is being studied. I call it the Bohemian Model of Cosmology (it is hinted at in Kroupa et al. 2023). Currently we are testing if this model, which is based on radically moving away from the current Belief Canon of the Cosmological Community, stands up to the observational data, ranging from open star clusters to the largest probed cosmological scales.
To summarise: a major step towards an improved understanding of cosmologically relevant observations has been achieved, even though the majority of scientists are still far from accepting this. The real Universe is significantly better matched by a model based on Milgromian dynamics, from the scale of open star clusters to the billion light-year scale. The challenging prospects are to better understand the fundamental physics underlying Milgromian dynamics which appears to be related to the quantum vacuum, and to develop a model universe which conserves energy. It is quite possible that both of these are different aspects of the same solution. Today is truly an exciting time for young scientists to flex their cerebral muscles, perhaps comparable to the 1920s when quantum physics was being discovered to a large extend in Copenhagen and Goettingen.
In The Dark Matter Crisis by Elena Asencio, Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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Is scientific advance achieved merely through the addition of many small adjustments (adding epicycle after epicycle), or is it achieved by major new bold ideas that take us far beyond currently conceived thinking?
Guest post by David Merritt. Below is a slightly edited version of an essay that appeared on IAI News on Oct. 25th, 2023.
Renowned stellar-dynamicist Prof. Dr. David Merritt perhaps understands better than anyone else how the current problems faced by the dark-matter-based Newtonian/Einsteinian models indicate the need to consider radically different theories. David was a professor at the Rochester Institute of Technology until 2017 and has written the text book Dynamics and Evolution of Galactic Nuclei (Princeton University Press), the award-winning book A Philosophical Approach to MOND (Cambridge University Press, see DMC Nr. 55), and together with Zuzana Parusnikova the book Karl Popper’s Science and Philosophy (Springer Press).
________________________
The universe is vast, and the longer we observe it, the more we learn about its composition and structure. But historians of science tell us that periods of steady growth in science tend to last only so long: they are interrupted by revolutions during which the old assumptions are thrown out and a radically new set are brought in. The initial decades of the twentieth century witnessed a number of such episodes. Classical mechanics and electromagnetism were replaced by quantum electrodynamics, Newton’s theory of gravity and motion was replaced by Einstein’s. The changes in these theories were so radical that even basic concepts like mass and time acquired fundamentally new meanings.
But it has been rather a long time since a major shift of this sort took place, and it is natural to wonder whether scientific revolutions are a thing of the past. Some philosophers seem to think so. For instance, John Worrall wrote in 2007 that “it is reasonable to believe that the successful theories in mature science–-the unified theories that explain the phenomena without ad hoc assumptions . . . are, if you like, approximately true.”
Worrall’s view, which is one aspect of what is now called ‘(critical) scientific realism’, is widely held among the current generation of philosophers. Scientific realists do not just postulate a mind-independent reality. They believe in addition that the central claims of our best scientific theories are probably, or approximately, true, even when it comes to claims about entities that are not directly observable—what philosophers call ‘theoretical entities.’ In the words of Stathis Psillos, scientific realism entails that “the entities posited by [mature theories], or, at any rate, entities very similar to those posited, do inhabit the world”.
The current, standard theory of cosmology—the so-called ΛCDM model—postulates the existence of just such an entity: dark matter. (CDM stands for ‘cold dark matter.’) Dark matter is not a minor component of the standard model; in fact, most of the matter in the universe is said to be dark. And while the detailed properties of the dark matter are not specified, cosmologists almost universally assume that it is composed of some kind of elementary particle. Considerable effort (and money) have been expended over the last four decades in attempts to detect the dark particles, so far without success. Absolutely none of this dark matter, which supposedly constitutes most of what exists, has ever been observed.
But there is an alternate cosmological theory, called MOND, that does not postulate the existence of dark matter. Observations that are explained under ΛCDM using dark matter are explained under MOND by postulating a modification to Newton’s (or Einstein’s) theory of gravity. (MOND stands for ‘MOdified Newtonian Dynamics’.) It has become clear over the last few years that MOND is at least as successful as ΛCDM at explaining our observations of the universe, including those data that are believed by standard-model cosmologists to require the existence of dark matter.
Indeed a compelling case can be made that MOND is the preferred theory, in the sense that MOND has a stellar record of successfully predicting new facts in advance of their observational discovery (as documented here). The ΛCDM theory has rarely, if ever, managed to do that; its ‘successes’ are almost all successes of post-hoc accommodation, not prior prediction.
One might suppose that philosophers of science would be fascinated by this state of affairs, since it has all the earmarks of an incipient paradigm shift. But to the puzzlement of many scientists, the philosophers have mostly declined to engage with the issue. I think that a partial explanation can be found in their ideological commitment to ‘scientific realism’:
The existence of empirical equivalents to existing theories—that is, theories that differ in important ways from accepted theories but that make the same, or nearly the same, predictions about observable phenomena—is difficult to reconcile with a belief in scientific realism. Realist philosophers tend to assume that such equivalents (in this case, MOND) must be contrived or artificial, if they exist at all.
Scientific realists acknowledge that the descriptions of the unobserved entities that appear in scientific theories tend to change over time. To maintain their commitment to realism, they are motivated to search for referential continuity: to argue that the ‘same’ entities are present in a theory, even if the detailed descriptions of those entities, or the detailed manner in which the entities are related to observable phenomena, should change over time. But this is clearly not going to be the case if the standard model is replaced by MOND, since the latter contains no component that could plausibly be related to the dark matter of ΛCDM.
If current theories are assumed to accurately describe the physical universe, it follows that the methods scientists use to arrive at those theories must be reliable. This leads realist philosophers to favor a gradual, typically inductivist, model for scientific progress—as opposed to progress via bold new conjectures, like the conjectures that led to quantum mechanics or relativity (or MOND).
Number three is, I think, the most interesting. So let me elaborate:
The model of scientific methodology that dominated in the early twentieth century was called ‘logical positivism’, and it was based on the old idea that scientists proceed inductively: i.e. that they make (hopefully valid) generalizations from finite data. But it soon became obvious that scientists like Einstein, Bohr and Schrödinger had not proceeded inductively; rather, they made bold conjectures that went far beyond the data that were available at the time.
Starting around 1930, the philosopher Karl Popper argued convincingly that inductive inference simply does not exist, and so could not possibly serve as the basis for a scientific method. He proposed an alternative methodology, which he called ‘conjectures and refutations’: one makes proposals and accepts them only if they stand up to rigorous testing–-that is: if they make novel predictions that turn out to be correct.
Popper argued (contrary to the claims of the inductivists) that it did not matter in the slightest how a scientific hypothesis was arrived at. All that mattered was how well it stood up to critical appraisal. And he argued that bold hypotheses–-which go far beyond any available data-–were always preferable to ad hoc ones that did little more than address a known anomaly.
The attitude of the modern scientific realist is, apparently, “That was then, this is now.” Since they maintain that fundamental changes to our ‘mature’ theories are no longer to be expected, scientific realists have little use for a methodology that encourages bold theorizing. They are motivated instead to favor methodologies that never take theories very far beyond their current (presumed nearly correct) forms.
To find a satisfactory methodology, realist philosophers have had to reach far back in time—to the mid 19th century at least, before the time of Karl Popper. And their currently favored stand-in for Popper is the American philosopher Charles Sanders Peirce (1839 – 1914).
Peirce operated in a world that had not yet experienced the early twentieth century revolutions due to Einstein and Bohr that so strongly influenced philosophers like Popper and Kuhn. Peirce argued that one could claim correctness of a hypothesis simply on the basis that it explains whatever data it was designed to explain. Peirce called this methodology ‘abductive inference.’
Even admirers of Peirce have acknowledged Popper’s point that multiple hypotheses will always be consistent with any finite set of data, and hence that there is a need to select between them. But rather than follow Popper’s advice (bold conjectures followed by critical testing), they have chosen to modify Peirce’s abduction into what is usually called inference to the best explanation, or IBE. Roughly speaking, IBE tells the scientist to accept the ‘best’ explanation among the many possible ones. And (this is the key point) ‘best’ is usually defined as the explanation that requires as little change as possible to accepted theories.
It is easy to see how a methodology like abduction or IBE fits hand-in-glove with scientific realism, which posits that major changes to accepted theories are no longer to be expected. And indeed, promoters of abductive inference, like philosopher Ilkka Niiniluoto, have explicitly stated that dark matter is a better explanation than MOND simply because “the theory [of gravity] is kept constant”; no weight is given to MOND’s enormously greater success at anticipating the data. Inference has seemingly been reduced, here, to the uncritical acceptance of whatever the majority of scientists believe.
All of this smacks of putting the cart before the horse, epistemologically speaking. Fortunately, it is quite possible to be a realist—in the sense of accepting the existence of a mind-independent, objective reality—without signing on to the additional ism’s that are currently lumped together under the rubric of ‘scientific realism’. Popper, a lifetime realist, argued that the existence of a falsifiable, i.e. testable, theory implies the existence of a reality with which it can clash: “Our falsifications thus indicate the points where we have touched reality”. Perhaps ‘touching reality’ ought to be the most we expect from our theories.
In The Dark Matter Crisis by Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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ACT-CL J0102-4915, commonly known as ‘El Gordo‘, is a very peculiar galaxy cluster that stands out due to its extreme properties. This cluster, formed by the collision of two subclusters about seven billion years ago, is considered to be one of the hottest, brightest, and most massive galaxy clusters ever observed. These characteristics have made El Gordo an unsolved mystery in the eyes of the current standard model of cosmology (ΛCDM).
Ever since the first study on El Gordo inferred a mass of 2-3 x 1015 Solar masses for this cluster, several additional studies have attempted to verify and constrain its mass using different observational methods. The most recent studies on this subject (Kim et al. 2021, Caminha et al. 2023, Diego et al. 2023) have determined the mass of El Gordo by measuring the deflection of background light due to the cluster’s gravitational potential, a phenomenon known as gravitational lensing. These studies consistently estimate its mass to be approximately 2 x 1015 Solar masses. Such a high mass at the young cosmological epoch in which El Gordo is observed is very unexpected in the ΛCDM theory of cosmology. This is because, according to this cosmological theory, structure formation relies on smaller structures merging together to form larger structures in a slow process called gravitational accretion. There simply was not enough time to have formed two clusters this massive within striking distance of each other when the universe was only about half its present age.
The other distinctive properties of El Gordo are its X-ray morphology – characterised by its two-tailed shape – and its very high temperature and luminosity. All the hydrodynamical simulations of El Gordo that have been performed so farhave shown that, in order to reproduce these properties, it is required that the two subclusters collided with each other at a very high speed. Explaining how the subclusters could have achieved such a high relative velocity is also a challenge for ΛCDM. This is due to the fact that structures owe their velocities to the expansion of the Universe and to interactions with other structures. Because of dynamical friction, when massive structures interact with less massive structures, they lose energy to these. Therefore, more massive structures are expected to have lower velocities than their escape velocity – but the hydrodynamic simulations indicate this is not the case for El Gordo.
In our recent study (Asencio, Banik & Kroupa 2023), we assessed the probability of observing El Gordo in a ΛCDM universe, assuming a total mass of 2.13 x 1015 Solar masses and a mass ratio of 1.52, as reported by Kim et al. (2021). To do so, we adopted the same method employed in Asencio, Banik & Kroupa (2021). This method consists of using a ΛCDM cosmological simulation to look for analogues of the El Gordo subclusters whose properties could potentially give rise to an object like El Gordo (see the previous post on El Gordo for more detail). This implies that the pair of simulated subclusters must be at a cosmological epoch at least half a billion years prior to that at which El Gordo is observed. They should also be infalling into each other, have a mass ratio smaller or equal to the mass ratio of El Gordo, and have a total mass equal or higher than that of El Gordo. Regarding the minimum infall velocity required for the subclusters to yield a realistic post-collision morphology, we repeated this analysis assuming different values ranging from 1000 km/s to 3000 km/s.
The results of our analysis are summarised in Figure 1. This shows that, in the ΛCDM theory, the El Gordo subclusters would need to have an infall velocity lower than 2300 km/s so that the probability of being observed in their survey region leads to a tension that falls below the 5σ threshold. Events above this threshold correspond to a probability of less than 5.7 x 10-7, which is the widely accepted limit signifying that an event is practically impossible.
Figure 1 showing the probability of observing an El Gordo-like object in a ΛCDM cosmology (y-axis) assuming different infall velocities for the El Gordo subclusters (x-axis). On the right y-axis, this probability is expressed in logarithmic scale (log10 P), and on the left y-axis, the probability is expressed in terms of the Gaussian equivalent number of standard deviations (χ). The solid red line shows χ accounting for the volume of the survey region in which El Gordo was observed. The solid black line shows χ considering both El Gordo and the Bullet Cluster (BC) in their respective survey regions (with χBC = 2.78). The error bars account for the uncertainty in the El Gordo mass. The dotted lines in the same colors represent more conservative estimates assuming the respective survey regions cover the whole sky, in which case χBC = 1.65 (Kraljic & Sarkar 2015).
The tension between the real Universe and the ΛCDM-theoretical universe is even greater when we take into account that El Gordo is not the only object with extreme properties that has been observed in the sky. Another example is the Bullet Cluster, with an estimated infall velocity of 3000 km/s. According to our analysis, the infall velocity of the El Gordo subclusters should be lower than 1800 km/s in order to be accommodated in a ΛCDM cosmology given the observations of both El Gordo and the Bullet Cluster in their corresponding discovery survey regions.
The tension would be reduced if we consider the probability of observing El Gordo and the Bullet Cluster in the full sky, and not just in their discovery survey regions. But this would imply that El Gordo and the Bullet Cluster are the only problematic objects for ΛCDM that have ever been observed, which is an unrealistic assumption. Other surveys – including full-sky surveys like Planck – have already found more clusters with outstandingly high masses. One of the most extreme examples is Abell 1758, with an estimated mass of 3.3 x 1015 Solar masses.
Considering these results, the following question arises: would it be realistic for El Gordo to have an infall velocity lower than 1800 km/s? Studies like Kim et al. (2021) claim that a configuration similar to that of El Gordo could be reconciled with an infall velocity as low as 450 km/s. However, in order to achieve this result in their statistical analysis, they imposed as a condition that only infall velocities that are expected from ΛCDM simulations should be considered. So it is not too surprising that they inferred an infall velocity for El Gordo that is below any significant tension with ΛCDM.
Up to this day, there are no hydrodynamical simulations of El Gordo that support such a low infall velocity. The most recent and thorough simulations of El Gordo were carried out by Zhang et al. (2015). In their best fit models that assume a mass of about 2 x 1015 Solar masses with a mass ratio of about 2 – similar to those obtained from the latest observations – they infer a preferred infall velocity of 3000 km/s to simultaneously reproduce its morphology, luminosity, temperature, and relative radial velocity. This infall velocity would imply that El Gordo is in 5.4σ tension with the ΛCDM theory (5.7σ when accounting for the Bullet Cluster). Other hydrodynamical simulations (Donnert 2014, Molnar & Broadhurst 2015) might not reproduce the properties of El Gordo as accurately as Zhang et al. (2015), but all their best fit models agree that infall velocities of 2500 km/s or even 2600 km/s are required in order to match its morphology and high luminosity and temperature to some extent. These are still in more than 5σ tension with the ΛCDM model, thus falsifying this theory of the Universe.
In conclusion, we found that even for the lower mass estimate yielded by the new analyses of the El Gordo mass, El Gordo still surpasses the 5σ falsification level at any plausible infall velocity. This result is consistent with the preliminary observations of the James Webb Space Telescope (JWST), which suggest that several high mass galaxies at very early epochs have already been found in its first data sample, galaxies which are in more than 3σ tension with the ΛCDM model. Independently, Haslbauer et al. (2022) showed that the distant candidate galaxies observed with the JWST have significantly more stellar mass than the galaxies formed in ΛCDM simulations at such early times. If the early JWST calibration is correct, this – together with the results of our analysis for the El Gordo cluster collision – would strongly imply that the formation and evolution of structures in the Universe is more efficient than predicted by the ΛCDM theory.
In The Dark Matter Crisis by Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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(Guest post by Dr. Indranil Banik, a postdoctoral researcher at the University of Saint Andrews. Perhaps check out his YouTube channel).
I am delighted to announce that I will be hosting the next global MOND conference in Saint Andrews (UK) on 5th – 9th June 2023 (website) with the help of the local and scientific organising committees and my University’s accommodation and events (ACE) team. Registration and abstract submission are now open. This conference aims to consider a broad range of opinions on the validity of MOND, with several confirmed invited speakers. It is also aimed at being broad in its scope, not restricting to galaxy dynamics but considering smaller and larger scales as well. In addition to considering the results of tests conducted so far, some time should also be spent considering future tests of MOND. For those new to MOND, I will try my best to negotiate a better introduction in the first talk and will also run an online question and answer session advertised through the MONDcommunity mailing list in the month before the conference to answer questions about how the theory works, which should help you make the most of the conference. I also hope to take forward lessons learned from organising the past such conference. It would be helpful if you could download the conference poster from the website (single A4 colour page) and use it to advertise the conference, if this is feasible for you. Hope to see many of you there!
Figure: The conference logo. Can you identify what the different parts represent?
In The Dark Matter Crrisis by Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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(Guest post by Dr. Martin Lopez-Corredoira, who is a researcher at the IAC on Tenerife and at present Visiting Scientist at PMO in Nanjing (China). Martin has been publishing on the issue of cosmological models — we had covered his recent book in DMC Nr. 74. Very recently his Tenerife team published the research paper (“Virial theorem in clusters of galaxies with MOND“), and this contribution describes its contents. The setting: in Newtonian/Einsteinian gravitation, galaxy clusters show a mass discrepancy of a factor of a few hundred. In Milgromian dynamics (MOND), the mass discrepancy is nearly entirely removed, with only a factor of two in mass still missing. At least that is the current general concensus. Martin’s team from Tenerife has re-visited this problem pointing out that the boundary conditions matter, and that galaxy clusters may not even be missing mass in Milgromian gravitation. Martin’s research is supported by the Chinese Academy of Sciences President’s International Fellowship Initiative grant number 2023VMB0001.)
In present-day astrophysics, many lines of investigation support the existence of large amounts of non-baryonic dark matter (DM) particles in galaxies and in the Universe at large, the most obvious example being the asymptotically flat rotation curves of galaxies, which indicate the existence of massive DM haloes. Considerable fine tuning is required, however, to justify their observed properties, the most striking example possibly being the baryonic Tully–Fisher relation (McGaugh 2005). Because of this, over the years more than one proposal has been made to find alternative explanations not involving DM. In particular, it has been shown that a specific modification of Newtonian dynamics, known as MOND, is able to describe many observations without the need for DM in several astrophysical objects including (in increasing order of size) wide binary stars, globular clusters, dwarf galaxies, elliptical or spiral galaxies, including our Milky Way, satellites around galaxies, pairs of galaxies, groups of galaxies, gravitational lensing. MOND in the regime of very low accelerations creates a field (made by a ‘phantom mass’ that is not real matter though) which has an effect dynamically similar to the presence of non-baryonic DM in Newtonian gravity. Figure 1 shows as an example the galaxy cluster MACS J0717.
Figure 1: Galaxy cluster MACS J0717. Credit: NASA, ESA, CXC, C. Ma, H. Ebeling and E. Barrett (University of Hawaii/IfA), et al. and STScI.
However rich clusters of galaxies have been more problematic for MOND. A first problem is related to the offset between X-ray and weak-lensing distributions, like in the case of the Bullet Cluster. This is a problem for MOND because one would expect the phantom mass that mimics the role of the non-baryonic dark matter to be distributed around the baryonic matter traced by X-ray emission, and it is not the case here. Nonetheless, proposals to solve this MOND problem in terms of the existence of sterile neutrinos work (Banik & Zhao 2022, and references therein). The off-set of the neutrinos with the baryonic mass is explained because of an internal collision of two components of the cluster. Not a very elegant solution, because this forces MOND to admit the existence of some non-baryonic dark matter (although a known one, neutrinos, rather than a mysterious matter never observed), but these kinds of ad hoc solutions are not worse, and if anything they are better motivated from standard particle physics, than the usual ad hoc corrections in the standard ΛCDM cosmology. Moreover, precisely with the Bullet cluster and another massive cluster called “El Gordo“, there is a fast collision between individually rare massive clusters which is unexpected in standard ΛCDM cosmology and is ruled out at the 6.2σ confidence level in ΛCDM cosmology (Asencio et al. 2021), so apparently Bullet Clusters and other similar cases can be used more as an argument against the standard dark matter cosmological model than in its favour.
A second long-standing problem was the virial theorem in clusters of galaxies, which works for standard Newtonian gravity within the usual assumption of the existence of non-baryonic DM as predicted by ΛCDM models, but it had not worked for MOND till now: there was a deficit (by a factor around two) of predicted dynamical mass derived from the virial theorem with respect to observations. The usual explanation for this is that galaxy clusters are so massive and huge, that they capture sterile neutrinos which are elementary particles needed to explain why normal neutrinos change their mass. These sterile neutrinos add the factor of two in missing mass (in MOND a sterile neutrino needs to have a rest mass of about 11eV/c2, while in the standard ΛCDM model sterile neutrinos can be much lighter). The currently favoured MOND-cosmological model assumes that there is about 5 times as much mass in sterile neutrinos than in ordinary matter and thus “νHDM cosmological model” has the same expansion history as the standard ΛCDM model (Angus et sal. 2013,Katz et al. 2013).
In our work recently published by Martin Lopez-Corredoira et al. (2022), new analytical calculations of the virial theorem in clusters are developed, applying pressure (surface) corrections for non-closed systems and including recent calibrations of mass profiles, allowing free parameters to range within the observational constraints. It is shown that solutions exist for MOND in clusters that give similar results to Newton + DM within the range of parameters compatible with the known distributions of (only) baryonic matter. Figure 2 shows how well these new Newtonian calculations in which there is about 10 times as much non-baryonic dark matter than normal matter reproduce the data, and the paper demostrates that MOND can get a solution very close to the Newtonian one. This last theory does not even have sterile neutrinos, but accounts for the boundary conditions relevant for the galaxy clusters. Figure 2 and Figure 3 show how well these new MOND calculations reproduce the data.
Figure 2: This is figure 5 in López-Corredoira et al. (2022). Each data point is a galaxy cluster, represented by a colour in dependence of its redshift. The standard, dark-matter cosmological models are shown by the black solid line. These need a few hundred times more mass in exotic dark matter particles to be in the cluster than is observed in the hot X-ray emitting gas and in galaxies. The best power-law fit is shown by the black solid line (x is the variable on the x-axis, i.e. M500 in units of 1014 solar masses). The dashed red curve corresponds to Newtonian+DM theory (NFW is the mathematical form of the dark matter halo profile), and the MOND model for some parameters (without any additional sterile neutrinos) reproduces approximately this red line.Figure 3: This figure is not in López-Corredoira et al. (2022) but has been made for this contribution. It is as Figure 2 above except that the MOIND models are now plotted here as the red dashed line. The uncertainty ranges on the black line (the best fit to the data) are shown here in darker shade. Note that the MOND models are very similar to the Newtonian+DM models in Figure 2 and in fact lie closer to the best fit line. The MOND models have no contribution from sterile neutrinos (and of course also no dark matter).
Our results (López-Corredoira et al 2022). show that they can reconcile MOND with the virial theorem in clusters. This agreement is obtained when: 1) the pressure term is taken into account in the virial theorem, which gives a 10–15 per cent higher velocity dispersion of matter in the galaxy clusters for MOND than for Newton + DM; 2) we explore a range of possible parameters in the baryonic matter profile rather than adopting a fixed one. In particular for MOND, we predict velocity dispersions equivalent to Newton + DM by adopting an isothermal β model with β = 0.55–0.70, and core radii rc < 0.3*r500 (r500 is the radius of the sphere for which the average density inside it is 500 times the critical density ρc;; β is a parameter of the baryonic density distribution, namely: J(r)=(1+(r/rc)²)-1.5β), which is in agreement with the known observations. This can increase the velocity dispersion by another factor 10–15 per cent with respect to previous calculations.
This last effect is easy to understand in MOND: lower concentrations enhance the MOND effect because the galaxies spend a longer time during their orbits in the MOND regime of low (<a0) accelerations, i.e. in the outer regions of their clusters. Also, the greater pressure term for MOND is due to a lower concentration of gravitating matter (i.e. of the baryons = hot gas plus galaxies) than in the standard Newtonian plus dark matter models. Given that the dynamical mass is proportional to the third power of the dispersion of velocities, this means dynamical masses were 40–60 per cent lower in former studies, and this would explain the discrepancies found previously.
Thus, at least numerically speaking, the problem is solved and one can say that the virial theorem in clusters can be explained with MOND without needing to add extra unseen matter into the clusters.
In The Dark Matter Crrisis by Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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The observed disagreement between the rotational velocities of galaxies and their photometric masses made the concept of dark matter “mainstream”, and in fact revolutionised the field of cosmology. Previously I found that an automated analysis of galaxy handedness shows galaxy spins to be distributed non isotropically in the local Universe (Shamir 2012, PhLB). Here I describe a simple observation that shows that the velocity of galaxy rotation does not fit its physical property as measured through photometry. An analysis of thousands of galaxies shows that the brightness we see a galaxy to have appears to depend on its spin direction relative to the Milky Way. This has implications of the value of the Hubble-Lemaitre constant and tension, and also poses theoretical questions not understood today.
The idea of dark matter was first proposed in the first half of the 20th century, by astronomers Fritz Zwicky, Jan Oort, and others. That work was followed by the precision analysis of the galaxy rotation curve by Vera Rubin, showing disagreement between the expected rotational velocity of galaxies and the amount and distribution of their visible mass. According to these observations, the rotational velocity of galaxies could not be explained by Newtonian physics, unless assuming that galaxies have far greater mass than its visible matter alone. These observations made the concept of dark matter “mainstream”, and revolutionized the field of cosmology. The presence of dark matter, however, is still an assumption, and has not been proven.
The “mainstream” explanation to the galaxy rotation anomaly might seem trivial: if the rotation curves of galaxies do not fit the known physics, it is convenient to just assume that the physics of galaxies does not fit their rotational velocity. On the other hand, the assumption that galaxies have dark mass is equally trivial. In fact, the assumption of dark matter has been tailored to fit the observations in a manner that can be viewed as conceptualizing no-knowledge as knowledge.
The observation described here is independent from the galaxy rotation curve, and therefore does not rely on assumptions that explain the galaxy rotation curve anomaly. This same single observation might also directly impact the tension in the Hubble-Lemaitre constant.
Brief history of the galaxy rotation curve anomaly
The galaxy rotation curve anomaly is the primary observation that led to the assumption of dark matter. The contention that the rotation of galaxies does not follow Newtonian physics was identified already in the first half of the 20th century. For instance, Jan Oort (1940) studied NGC 3115 and NGC 4494, and noted that “the distribution of mass in the system appears to bear almost no relation to that of light”.
These observations were ignored for about five decades, until Vera Rubin profiled the galaxy rotation curve, making the non-Newtonian nature of galaxy rotation widely accepted. According to Vera Rubin (2000), one of the major reasons for ignoring the previous observations was that preeminent astronomers such as Gerard de Vaucouleurs (1959) assumed that galaxy rotation followed Newtonian physics. That is, although it was clearly incorrect, the Newtonian nature of a galaxy’s rotation curve was considered “truth” because the contentions made by the authority could not be challenged.
Once the authority finally accepted the idea that galaxy rotation does not follow Newtonian dynamics, explanations were proposed to account for the anomaly. The most common theory to explain the galaxy rotation curve anomaly is that dark matter is distributed just in the correct manner to make the rotation curve flat. Another common explanation is the notion of modified Newtonian Dynamics (MOND), and there are some other less common theories. Despite several decades of research, the galaxy rotation and the physics that enables it is still a mystery.
Link between the spin direction of a galaxy and its brightness
The spin direction of a galaxy is merely the perspective of the observer. That is, a galaxy that seems to spin clockwise to an Earth-based observer might seem to spin counterclockwise to an observer located at a different galaxy. We can therefore assume that the brightness and color of a galaxy that spin in one direction is expected to be the same as the brightness and color of an identical galaxy that spins in the opposite direction. I first tested that assumption about a decade ago (Shamir, 2013), and found that it might not be the case. By using over 100,000 galaxies with spectra from the Sloan Digital Sky Survey, I found small statistical differences in the color of galaxies that spin clockwise and galaxies that spin counterclockwise. While I had no explanation for that at the time, now I think that the reason is that the Sloan Digital Sky Survey imaged a dense population of galaxies with spectra at the part of the sky around the Northern galactic pole. The difference was therefore between galaxies that spin in the same direction as the Milky Way, and galaxies that spin in the opposite direction. Due to the Doppler shift, galaxies on the Galactic pole that spin in the same direction as the Milky Way should have a slightly different brightness compared to the brightness of galaxies spinning in the opposite direction compared to the milky Way.
The statistical signal of the analysis was mild, but another experiment (Shamir, 2016) that used Likelihood statistics showed a much stronger statistical signal. Later, other telescopes such as Pan-STARRS (Shamir, 2017) and HST (Shamir, 2020) also showed similar differences. An attempt to identify the part of the sky where the difference in brightness peaks showed that the peak is very close to the Galactic pole. That can lead to the contention that the reason for the surprising brightness difference might be the rotational velocity of the Milky Way compared to the rotational velocity of the observed galaxies. For a simple analysis with a relatively small number of over 6,000 galaxies, we can observe the magnitude of galaxies imaged in the COSMOS field of Hubble Space Telescope. Galaxies that spin counterclockwise have an average G magnitude of 23.077±0.019, which is significantly brighter than the 23.131±0.019 magnitude of galaxies spinning clockwise (with the probability that the two are equal being P=0.023). The raw data can be accessed at http://people.cs.ksu.edu/~lshamir/data/assym_COSMOS. Examples from many other telescopes are available in the papers mentioned above and here.
Tension between galaxy rotational velocity and its brightness
Due to the Doppler shift, a galaxy that spins in the opposite direction *relative to* the Milky Way is expected to be slightly brighter (to an Earth-based observer) than an identical galaxy that spins in the opposite direction. The observed photon flux F of a galaxy can be defined by:
F=Fo(1+4(Vr/c))
where Fo is the flux of the luminous object when it is stationary, Vr is the rotational velocity of the object relative to the Milky Way, and c is the speed of light. Because the speed of light c is obviously much larger than the rotational velocity Vr, the difference between F and Fo is expected to be small. Assuming the rotational velocity of the Milky Way, the maximum possible difference in the brightness of identical galaxies spinning in opposite directions can be ~0.006 magnitude.
Naturally, it is impossible to measure the brightness difference directly using a single galaxy. That is because the Milky Way and the observed galaxies rotate, and therefore we cannot measure the stationary flux Fo. But if we analyze a very large number of galaxies and sort them by their spin directions, we can expect that the mean stationary flux of the galaxies that spin with the Milky Way would be the same (within statistical error) as the mean Fo of galaxies that spin in the opposite direction compared to the Milky Way. Therefore, any difference between the mean brightness of galaxies that spin with the Milky Way and the mean brightness of galaxies that spin against the spin direction of the Milky Way can be attributed to the effect of the Doppler shift.
Obviously, sorting tens of thousands of galaxies manually is impractical. Manual annotation is also subjected to human bias and might therefore lead to biased results. To sort the galaxies, we used two automatic methods for identifying the spin direction of the galaxies. The main method was Ganalyzer, which is explained in detail in Shamir & McAdam (2022) and several other papers. Ganalyzer is a simple model-driven algorithm that works by defined mathematically symmetric rules. It does not make use of machine learning, and due to its simple “mechanical” nature it is not subjected to complex non-intuitive data-driven rules that are often difficult to define or understand. To compare the results, another algorithm that we used was SPARCFIRE. Both algorithms provided similar results.
The following table shows the average brightness in three photometric pass bands (G, R, Z) of more than 42,000 galaxies in the 60×60 degree window centred at the Northern galactic pole. The galaxies are sorted by their spin directions into galaxies spinning clockwise and galaxies that spin counterclockwise. Galaxies spinning clockwise have a spin in the same direction of the Milky Way. The galaxies are taken from the DESI legacy Survey.
Band
Mag counterclockwise
Mag clockwise
Mag diff
P value
G
20.06525±0.010
20.10073±0.010
0.03548
0.01
R
18.98522±0.008
19.01481±0.008
0.02958
0.01
Z
18.2934±0.007
18.31783±0.007
0.02443
0.01
What we can see is that galaxies that spin clockwise are fainter than galaxies that spin counterclockwise. The difference is statistically significant if we assume the maximum possible difference to be 0.006 magnitudes.
Now let’s take a quick look at the same analysis in the Southern galactic pole. In the Southern galactic pole we have a much higher number of nearly 180,000 galaxies imaged by DECam.
Band
Mag counterclockwise
Mag clockwise
Mag diff
P value
G
20.13622±0.004
20.11937±0.004
0.01685
0.003
R
19.08793±0.003
19.07216±0.003
0.01574
0.0002
Z
18.38424±0.003
18.37225±0.003
0.01199
0.0047
This shows that in the Southern hemisphere the difference in brightness is also statistically significant, but inverse. That can be expected, because a galaxy that seem to spin clockwise in the Northern pole would seem to spin counterclockwise if it were instantly moved to the Southern pole. When doing the same analysis in parts of the sky that are perpendicular to the galactic pole, the difference in brightness disappears.
Similar analyses were done with other telescopes such as HST, SDSS, DECam, and Pan-STARRS, and different analysis methods, including manual and automatic annotations of the galaxies. It is interesting that even when using the old Galaxy Zoo crowdsourcing annotations, the same brightness differences were observed. The galaxy Zoo annotations are heavily biased by the human perception, and the agreement can be no more than a coincidence. But Galaxy Zoo annotations definitely do not conflict with the brightness differences.
Asymmetry in the distribution of galaxy spin directions
The difference in brightness can be directly related to another closely related topic, which is on the differences between the number of spiral galaxies spinning clockwise and the number of spiral galaxies spinning counterclockwise. While the number is expected to be the same (within statistical error), reports arguing that it is not necessarily the case started as early as the mid 1980’s, and are still being continued. In summary, these reports argue that the number of spiral galaxies in the sky that spin clockwise is not equal to the number of spiral galaxies that spin counterclockwise. This asymmetric distribution exhibits a cosmological-scale dipole axis. A recent analysis of multiple telescopes with over one million galaxies can be found in Shamir (2022). Similar results were shown with telescopes such as HST, DECam, SDSS, Pan-STARRS, and DES. Some studies showed opposite results, and the reasons for these observations are explained in Section 1 of Shamir (2022), Section 5 of Shamir (2022c), or in Shamir (2022b).
The following figure shows the analysis of the statistical significance of a dipole axis exhibited by the distribution of the galaxy spin directions on the sky (the x-axis in the figure is the Earth’s equator). The different telescopes show different dipole axes. Panel (g) is different from the other panels in the sense that it does not visualize an attempt to fit the axis statistically. Instead, it shows a direct measurement of the number of clockwise and counterclockwise galaxies in the hemisphere around each point in the sky. That analysis was done with 1.3 million galaxies, and is explained in Shamir (2022). The similarity to the galactic plain might not be coincidental.
The following figure (again the x-axis is the Earth’s equator), taken from Shamir & McAdam (2022), shows the locations of the most likely dipole axes from different experiments, including Longo (2011) and Galaxy Zoo. The green dot is the galactic pole. The dipoles are concentrated around the location of the galactic pole.
If there is a link between the dipole axis in the distribution of galaxy spin directions and the difference in brightness, the reason for that link is clear: If in one hemisphere galaxies that spin clockwise are fainter than galaxies that spin counterclockwise, more galaxies spinning counterclockwise will be imaged in that hemisphere. That will lead to a dipole axis formed by galaxy spin directions.
Galaxy rotational velocity explanation to the Hubble-Lemaitre constant tension
Another mysterious anomaly is the Hubble-Lemaitre constant tension. When measuring the Hubble-Lemaitre constant by using Ia supernovae and Cepheids, the expansion rate of the Universe is different than when using the cosmic microwave background radiation. Since the different probes measure the expansion of the same Universe, it is possible that one of these probes has a slight bias.
Ia supernovae and Cepheids are unique in the sense that their absolute brightness is known, and therefore their apparent brightness can be used to determine their distance from Earth. But if their brightness depends on the rotational velocity of the host galaxy, then that might also lead to a small but systematic bias in the distances measured by Ia supernovae.
Supernovae are explosions of stars, and therefore they inherit the rotational velocity of the star they were created from. If the apparent brightness of an Ia supernova changes based on the spin direction of its host galaxy, then this can change its estimated distance. That can lead to an inaccurate Hubble-Lemaitre constant. A simple test whether the Hubble-Lemaitre constant is related to galaxy rotational velocity can be done by repeating a previous experiment. But instead of using all supernovae and Cepheids of the original experiment, the new experiment uses just a subset, comprising those whose host galaxies rotate in the same sense as the Milky Way. That selection reduces the possible effect of the Doppler shift. The following table shows the results:
Rotation direction of host galaxies
Ho
Original
73.758±1.943
Same direction as the Milky Way
69.049±3.42
Opposite direction to the Milky Way
74.182±3.2
The table shows that when using all 96 Ia supernovae the Ho is 73.759, which is far higher than the Ho expected when using the CMB. But when using just the 22 supernovae the host galaxies of which have the same spin direction as the Milky Way, Ho reduces sharply to 69.049, and is much closer to the Ho determined by the CMB. When using the 36 Ia supernovae that rotate in the opposite direction relative to the Milky Way, the Ho does not decrease, but instead it increases.
The relatively small number of Ia supernovae makes it difficult to determine that the Hubble-Lemaitre constant tension problem is indeed solved when the host galaxies rotate in the same direction as the Milky Way. But the observation agrees with the contention that galaxy rotational velocity affects the brightness. Further analysis with a higher number of supernovae will be required to determine whether it indeed explains the Hubble-Lemaitre constant tension.
Large-scale structure explanation
Another way to explain the difference in the brightness is by an anomaly in the large-scale structure of the Universe. In that case, the axis observed around the galactic pole is not the result of differences in the galaxy brightnesses, but the result of a mysterious alignment in the spin directions of galaxies in the Universe. While this idea is foreign to the standard cosmological models, there are several other observations with several different probes that also show the existence of a cosmological-scale axis. The cosmic microwave background radiation is perhaps the most discussed probe that might form such an “axis of evil”, but other probes also showed a cosmological-scale anisotropy. These probes include LX-T scaling, gamma ray bursts, acceleration rates, Ia supernova, distribution of galaxy morphology types, and polarization of quasars.
These observations would possibly lead to a shift from the standard models, requiring new theories. One of the interesting theories is black hole cosmology, according which the Universe is the interior of a black hole. The theory is supported by the agreement between the radius of a black hole (the Schwarzschild radius) and the radius of the Universe, and might explain the accelerated expansion of the Universe without the assumption of dark energy. Because black holes spin, a Universe hosted inside a black hole is also expected to spin, which would suggest the axis.
The physics of galaxy rotation is not fully understood. Some of the current theories and tensions could depend on that physics. That might lead to assumptions that can be inaccurate due to our incomplete understanding of the physics of galaxy rotation. For instance, if distances measured using Ia supernovae depend on the rotation of the host galaxies relative to the Milky Way, current tensions that are based on these measurements might be explained. It is obvious that far more work and experiments will be required, but galaxy spin-direction might be one single solution that solves several mysteries, and therefore efforts to understand it might be justified.
In The Dark Matter Crrisis by Moritz Haslbauer and Pavel Kroupa. A listing of contents of all contributions is available here.
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The Dark Matter Crisis 