9. State-of-the-art cosmology: the current status

Last week’s contribution “Is LambdaCDM or standard cosmology a 4th order speculation, and ought it be further researched?”  was concerned with the recent suggestion by Prof. Abraham Loeb that alternative approaches should be followed to advance science. But at the same time he proposes the alternative approach “MOND plus netrinos” to be a second order speculation not worth the effort.

Following this logic of Loeb, it becomes immediately apparent that LCDM, or standard/concordance cosmology, is at least a 4th order speculation, with the corresponding implications.

Dr. Garry Angus is a very talented young cosmologist currently at the University of Torino, Italy, but shortly moving to the University of Cape Town, South Africa, who has been working in the field of “MOND plus neutrinos”. Dr. Angus is the recipient of the Cormack Bequest Prize for his 2007 publication on the topic of the Bullet Cluster, Neutrino Dark Matter and Alternative Gravity. This prize is awarded annually to the most outstanding postgraduate student contribution to astronomical research in Scotland.

Below he directly addresses Abraham Loeb’s assertion concerning his field.

It should be noted, before reading Garry’s text, that in the LCDM field (4th order speculative science) whole armies of researchers (hundreds?) have been toiling over the past decade to improve the computations and observations. It is the accepted model of cosmology, and over the past 10-15 years the very major professorships in cosmology or extragalactic astrophysics have been filled with experts in this one specific field. In contrast,  “MOND plus neutrinos” (2nd order speculative science according to Abraham) has been worked on by not more than about 2 researchers, while the other alternative, Modified Gravity (MOG), has been worked on by not many more researchers than that as well.

It nevertheless turns out that LCDM sort of works on large scales, and MOND plus neutrinos does at least as well as far as the existing work allows us to judge. Indeed, as Dr. Garry Angus shows below, the cosmic microwave background (CMB) power-spectrum is fitted perfectly well in MOND + neutrinos. On  scales smaller than about ten million light years LCDM fails however, while non-Newtonian/Einsteinian gravity works brilliantly (Kroupa et al. 2010).

Thus, MOND plus neurinos seems to be the astrophysically most modern and successful cosmological description we have currently. 

Dr. Garry Angus writes:

I’d like to just make a comment on why MOND+neutrinos is not a 2nd order speculation. I  don’t know how familiar you are with the literature on MOND+neutrinos, but no one, to my knowledge, has ever suggested that the CMB can be fit by MOND plus the active neutrinos – be they 2.2eV or 0.1eV. Skordis et al. (2006) clearly showed the apparently high 3rd peak is not compatible with even 3×2.75eV in TeVeS, even if the critical MOND acceleration (a_0) is boosted by a factor of 4.

It should be a well known fact to all cosmologists that replacing  Omega_CDM x h2  with the same energy density in sterile neutrinos gives as good a fit to the CMB. The proviso is that this energy density comes in the form of a single, thermal sterile neutrino species. Given a reasonable mixing angle, it is perfectly possible for these sterile neutrinos to be thermalised in the very early Universe. This means the neutrino has a mass of ~11eV. A figure of the MOND + neutrino CMB calculation can be seen here where we fit both WMAP 7 and ACBAR data. As can be seen, the theortical fit is near to perfect to the CMB data.

This has nothing to do with MOND. In fact, it requires MOND to have no influence at redshift z>1000 and a cosmological constant is still required. It just so happens that an 11eV sterile neutrino would resolve all problems MOND has in clusters of galaxies. At 100~kpc (about 300 thousand light years) in basically all clusters there is a 10:1 ratio of DM:baryons (after accounting for MOND), it is only at distances like 1Mpc that there is a 2:1 ratio. Angus, Famaey & Diaferio (2010) looked at 30+ groups and clusters and made the intriguing observation that the Tremaine-Gunn limit for the 11eV neutrinos is reached in every system, but never need be exceeded for very sensible values of the brightest cluster galaxy’s mass-to-light ratio.

These neutrinos would free stream out of Milky Way type galaxies, so all the successes of MOND at galaxy scales would be unaltered. The ramification of this is that the galaxies must collapse under their own gravity (enhanced by MOND) without the aid of a cold dark matter halo (see Sanders 2008). Linked to this, we have run preliminary cosmological simulations that incorporate MOND and 11eV sterile neutrinos and the conclusion is that they form roughly the correct number of clusters of galaxies as a function of cluster mass. It could just as  easily have ended up in a big black hole or with no structure forming at all.  If we run numerical simulations with the 11eV neutrino and no MOND, then no structures form i.e. MOND is essential for massive neutrinos to work.

Whether the correct number of galaxies form is an incredibly difficult question to answer and the numerical tools are nowhere near ready – basically because the 2 or 3 people with the necessary expertise to develop the codes can’t get jobs for love nor money. However, based on the successes of MOND at galaxy scales, we do expect that MOND+sterile neutrinos can reproduce that observed properties of galaxies with no effort, unlike CDM. For example, as long as a galaxy forms, we know trivially that it will conform to the baryonic Tully-Fisher relation. Furthermore, the highly organised distribution of satellite galaxies surrounding the Milky Way (see Kroupa, Theis & Boily 2005; Metz et al. (2009); Kroupa et al. 2010)  will immediately be explained to be tidal dwarf galaxies. Currently they have no explanation in LCDM.

For these reasons I don’t see how MOND+neutrinos is a second order speculation. If one has MOND, then there is an incredibly high chance that 11eV sterile neutrinos must exist and by the same token, if sterile neutrinos at 11eV exist then MOND is needed. In the former case it is possible that a deeper theory of MOND will spring a surprise which conspires at cluster scales, for the expansion history, during the formation of the acoustic peaks of the CMB and during structure formation to resemble an 11eV sterile neutrino. Abraham Loeb mentioned briefly that the baryonic acoustic oscillations appear at the scale predicted by LCDM. I don’t believe this is fully accurate. They appear at the scale defined by a scale factor that evolves as if it has a dominant dark matter component, hot, cold or warm. The peaks themselves do not require the dark matter to be cold.

A lot of people talk about neutrinos being against the design, spirit or original intention of MOND. I feel this is never very helpful. As Professor Milgrom clearly states above, the state of observational astronomy was very different in the early 80s. That MOND works at all highly disfavours the need for any type of Warm or Cold dark matter, but if that dark matter is hot and hot enough to free stream from galaxies then MOND (in the 80s) made no predictions about its presence. Nowadays, MOND without sterile neutrinos and MOND with sterile neutrinos are two different models with very different predictions for cosmology.

In addition to the cosmological evidence for 11eV sterile neutrinos, there also exists tentative particle physics evidence from Miniboone (see Giunti & Laveder 2008). And, more experiments are in the pipeline, for instance the T2K experiment will be able to put excellent constraints on 11eV sterile neutrinos, with results probably released in early 2012. Unfortunately, Planck will not offer any evidence for the specific mass of sterile neutrinos because the CDM model with a very low mass (say <0.1eV) thermal sterile neutrino would generate an identical power spectrum. It can, however, rule out the existence of further thermalised neutrino species (i.e. if N_eff=3).

I should add that although 11eV sterile neutrinos is my preferred solution that extends MOND to cosmology, there are others. HongSheng Zhao & Baojiu Li are working very hard testing a model that combines the MOND effect, the cosmological dark matter and dark energy into the same field that behaves differently depending on environment, which is a very nice idea. It boils down to the same essential ingredient for cosmology, however, and that is hot dark matter plus MOND.

by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (25.08.2010, “State-of-the-art cosmology: the current statusin “The Dark Matter Crisis – the rise and fall of a cosmological hypothesis” on SciLog. See the overview of topics in  The Dark Matter Crisis.


8. Is LambdaCDM or standard cosmology a 4th order speculation, and ought it be further researched ?

On August 9th, 2010  Prof. Abraham Loebfrom Harvad University  published a stimulating paper on the electronic preprint server with the title “Taking “The Road Not Taken”: On the Benefits of Diversifying Your Academic Portfolio“.

In this paper he takes issue with which type of research ought to be supported, making the much noted suggestion that innovative projects not following the main stream should be invested in by young researchers, in addition to following more secure research directions.  Abraham points out that even if one in a million new ideas bear fruit, this may completely transform our understanding of reality justifying the entire effort of using a certain fraction of funding for new, risky ideas.  To achieve this goal, he recommends that each researcher should spend up to about 50 per cent time on non-standard research, while individuals will naturally develop their own strategy based on personal and social factors. His suggestion is further that senior members of the community should find better strategies for rewarding innovation.

How does this fit in with the dark-matter versus non-Newtonan/Einsteinian dynamics issue?

Indeed, Abraham suggests MOND is a worthwhile non-standard approach in his Fig. 2. In his paper, Abraham states that “Once experiments push the upper limit on the cross-section of Weakly-Interacting Massive Particles (WIMPS) down well below the expectation of most reasonable models, alternative gravity models might apear more apealing.”

But, towards the end of the paper Abraham notes that the approach to cosmology via MOND plus neutrinos (as hot dark matter, see e.g. Angus, Famaey & Diaferio 2010) ought to be avoided as a non-standard research field because MOND and neutrinos are each individually unlikely to be true and thus the combination of both constitutes second-order speculative science.

The general gist of Abraham’s paper finds agreeement with many researchers, and has been commented upon very positively in discussions we are aware of.  For example, at the ESO coffee round, the rough notion of each researcher spending about 10 per cent time on non-standard research was talked about, based on Abraham’s paper.

But, there are two issues with Abraham’s statements:

One issue is that LCDM (i.s. standard cosmology) has been found to already fail on galaxy and somewhat larger scales (Kroupa, Famaey, de Boer et al. 2010), such that the CDM (cold dark matter) part must be wrong. Thus, WIMPS can’t exist. If some experiment were to find evidence for CDM particles, then the failure of LCDM would not be removed, but instead, the situation would become even worse in trying to understand how structures arise in the universe. This is straight-forward logics and has nothing to do with apparently not understood small-scale baryonic physics.

The other issue is that by removing MOND plus neutrinos from the recommended innovative research effort a logical error is made, as noted by Dr. Garry Angus: the current version of MOND and hot dark matter in the form of neutrinos are logically a unit. So, is MOND plus neutrinos a feasible alternative that ought to be followed seriously and supported by the senior scientists?

Abraham does not think so.

Following this paper,  Prof. Pavel Kroupa from Bonn University, while a Visiting Scientist at ESO headquarters in Garching, took the liberty to directly contact Abraham Loeb, applying the logics of Abraham and thereby raising the question whether LCDM (i.e. standard cosmology) should then not also be viewed as nth order speculative science (since it relies on not well understood speculative additions to physics such as inflation, dark matter, dark energy and the dark force – Kroupa, Famaey, de Boer et al. 2010). In this case n=4, ignoring the worrying issue that we do not understand how space-time and matter are coupled at a fundamental level within the LCDM framework.

Following Abraham Loeb’s reasoning, should the LCDM model not be also put on the list of research topics far too risky to bear any chance of success?: given the low probability that any one of the additions are true, the LCDM model is essentially ruled out to yield a correct description of the universe. And indeed, do the failurs of LCDM not already demonstrate this as a fact?

The following documents the e-mail exchange. Contributions are by

  1. Prof. Pavel Kroupa (16.08., 3:16pm)
  2. Prof. Abraham Loeb (16.08., 5:44pm)
  3. Prof. Pavel Kroupa (16.08., 6:28pm)
  4. Dr. HongSheng Zhao (16.08., 6:56pm)
  5. Prof. Mordheai Milgrom (17.08., 10:30am)
  6. Prof. Abraham Loeb (17.08., 1:23pm)

Interesting to note is that Li & Zhao (2009) have developed a theory which unifies the success of LCDM and MOND (see the 4th contribution below).

Feel free to add your comments below on how research is to be supported such that progress is maximised, or on what you think is worthwhile science.

Note also the discussion on this same topic concerning the US American Decadal Survey on which astronomy projects ought to have priority in the coming ten years.

1. e-mail on 16.08., 3:16pm:  Pavel Kroupa  —>  Abraham Loeb

Re: Taking “The Road Not Taken”: On the Benefits of Diversifying Your Academic Portfolio (by Abraham Loeb)

Dear Abraham,

Your recent posting on astro-ph was noted with interest in various places, and was also discussed here at ESO/Garching during coffee. Your suggestion that a fraction of ones’ research work ought to be spent on non-main stream ideas was received with quite some enthusiasm.

However, towards the end you single out MOND and neutrinos as a field of research which ought to be omitted, and you declassify this as “second-order speculations”.

I note this with interest, because it seems logically inconsistent with the rest of the text. And given this, it would cast doubt on whether your personal judgements can be trusted in terms of judging what ought to be done.

It is an established empirical fact that galaxies are MONDian objects. If you prefer to explain galaxies with cold or warm dark matter instead, then you are stuck with the second order speculation that the dark matter physics is dictated by the baryons. That is, you end up having to postulate the existence of a dark force Kroupa, Famaey, de Boer et al. 2010), in addition to all the other speculations in the LCDM model (e.g. Shanks 2005).

Having a hot dark matter component in the universe is also a fact. Neutrino oscillations imply the existence of an additional particle through the seesaw mechanism which may be a more massive contributor to hot dark matter and so hot dark matter with some dynamical importance seems to be in fact well motivated.

Furthermore, from what I have seen the physics community working on in the field of non-standard gravitation is following most beautiful approaches with deep implications for our understanding of space time and mass. I lack this in the LCDM community.

In fact, following your suggested argument: each of the individual speculations making up the LCDM model has a small probability of being true. One would consequently have to deduce that students and young postdocs should stop working in this field.

Therefore your statement is not understandable, and perhaps you could please clarify what you mean by excluding MOND from your list of non-main-stream physics research.

With kind regards, Pavel Kroupa

2. e-mail on 16.08., 5:44pm:  Abraham Loeb   —>  Pavel Kroupa

Dear Pavel,

Thanks for your e-mail. Over the past week I have received many more messages than on any of the few hundred papers that I posted on astro-ph in the past. My article must have touched a nerve in the community and I am grateful to messages such as yours.

As Moti Milgrom can tell you, I am very much in favor of exploring MOND because I am open-minded that it might represent the truth. However, I do not regard it as a mainstream theory as of yet. MOND is more successful than LCDM in explaining the dynamics of stars and gas within galaxies, but it is not as successful as LCDM in fitting the many cosmological data sets that were assembled over the past decade. Note that the bar for a successful cosmological theory has been raised dramatically since the 80s when Moti wrote his first insightful papers about MOND.

My sense is that if MOND is right, then it must have its underpinning in an elegant new theory of nature. The current limits on neutrino masses (<1eV) imply that thermal neutrinos make a small contribution (<1%) to the global mass budget in X-ray clusters. Moreover, the acoustic oscillations in the CMB anisotropies were detected now up to high harmonics and would be difficult to reconcile with the simplest MOND+neutrinos cosmological model.  Baryonic oscillations are observed in galaxy surveys of the present-day Universe at exactly the correct wavenumber that LCDM predicts (based on the response of collisionless dark matter to the imprint of the radiation sound waves on the distribution of baryons at recombination).

I am personally torn between the success that MOND has inside galaxies and the success that LCDM has on cosmological scales.  Most of the cummunity prefers to believe that LCDM will explain galaxies once we understand the complex feedback processes there. But in my mind, it is also possible that LCDM will be replaced by a generalization of MOND to the cosmological context. However, I regard neutrinos as a speculative addition to both MOND and LCDM. The classification of LCDM as a “bond” in my article is based on the much greater popularity of LCDM among cosmologists. As we know, the truth is not always popular. But my current sense (based on all available data) is that MOND could turn true only through a conceptual breakthrough rather than by adding a speculative matter component.

With best wishes,

3. e-mail on 16.08., 6:28pm:   Pavel Kroupa   —>   Abraham Loeb

Dear Avi,

Thank you for your prompt clarification.

I agree with you on all accounts, but take a much stronger and clearer standpoint, also in view of your published statements:

MOND is an overwhelmingly more successful description of the universe where detailed observations on cosmological scales (i.e. galaxies and even clusters) can be made. Even on galaxy cluster scales MOND is only missing a factor of two in mass, and this is nearly a factor of one already.

So research dealing with MOND or related ideas is what ought to be supported very strongly, even at the expense of funding for LCDM work.

LCDM work has led to no successful progress whatsoever on these well-observed scales since more than ten years, and the situation has been deteriorating constantly. In fact, I am amased to have realised that every prediction that the CDM-Ansatz has made has failed, while virtually every MOND prediction I am aware of has been successfully verified through observation. So please excuse my complete ignorance here, but as a scientist it ought to be quite clear which direction ought to be considered the better one, in terms of future funding.

That LCDM is successful on large scales, as you write (where observations are so much more difficult) is somewhat overstated, and even if true, it does not mean that another theory would not also explain the observations (some of which are still debatable though, e.g. recent Sawangwit and Shanks contribution on the CMB).

So your conclusion/suggestion in your astro-ph posting is not helpful at all, I am sad to write. It resonates with those strongly opposed to MOND (and there are many indeed). You will realise that the funding situation is not helped given statements such as you are making, and this can be deemed as being damaging in the historical context, because progress is slowed.

Imagine that 10% of the funding available for LCDM work would be channelled into MOND plus neutrino work. We would get much better quantifications of whether this ansatz works as well or even better than LCDM on large scales. Or, with that level of funding, entirely new theoretical avenues might be found that give MOND. But this is not happening.

That MOND works endlessly better than LCDM on galaxy scales is already an established fact. But all I hear is that funding is not being granted for “second order speculative” science, or that “second-order” scientists are not hired on faculty positions, or that MOND research at a Max-Planck Insitute is not seen to be worthwhile. In trying to understand galaxies even better, we are at this very moment faced with the problem of possibly loosing many years of research effort in galaxy modelling in MOND, simply because funding is not upcoming to keep those with the knowledge in the job. At the same time, many hires are taking place for numerical experts within the LCDM framework, although we already know that LCDM does not work.

This is a disaster from the point of view of how tax-payers money is being used to advance our understanding of nature. It is a dark epoch in the history of science, and the only reason the tax payer does not revolt is because they cannot understand the complex issues at hand.

Best wishes, Pavel

4. e-mail on 16.08., 6:56pm:

HongSheng Zhao  —>  Araham Loeb and Pavel  Kroupa

Dear Avi and Pavel,

Part of your debate seems to be around the (in)feasibility of unification of LCDM and MOND.

I invite you to read an example of such a co-variant generalization of MOND to do (most of) the job of LCDM at large scale without throwing away MOND’s advantage on galaxies.  This model uses same number of fields (vector+scalar) as TeVeS, but these fields explain more phenomena than TeVeS did, e.g., offering an explanation of the Lambda phenomena for free.  The scalar field explains the CDM phenomena in large scale and high density, and its effect is tapered away in galaxies.  We didn’t use a single non-baryonic (neutrino) particle, at least not explicitly in this formulation.

Environment-dependent dark sector” in Physics Rev. D, by Li and Zhao

Comments welcome.


5. e-mail  on 17.08., 10:30am:   Mordehai Milgrom  —> Abraham Loeb

Hi Avi,

Your points are all well taken, at least qualitatively. But I think I would put rather different weights on the different pieces of evidence.

In the first place, to use your own assertion, remember that the bar for successful GALACTIC theory has also been raised dramatically since the time of the first MOND papers. Few people remember, in fact, that when I proposed MOND, and some years after that, there was not a single MOND rotation curve analysis.

The first rotation curve test of the MOND predictions was done only in late 1987 (by Steve Kent), more then 4 years after the advent of MOND, because they had had to await the extended HI rotation curves. And, of course, we have gone a very long way since then. (The TF relation was also not well established then).

I would also remember that every time the bar in cosmology was raised significantly, a new adjustment of the “theory” has had to be made to adjust to the new bar (e.g. the introduction of dark energy, and the huge change in the DM content of the universe from 95% to ~ 20%), while MOND has not required a single adjustment at regards galactic phenomenology from its first day.

Also, when comparing the weight of the MOND successes on galactic scales with those of LCDM in cosmology we should bear in mind the following:

1. I think that, epistemologically, there is far more independent data to be predicted in the context of galactic astronomy then there is in what we call cosmology.

While a single galaxy is certainly not as “important” as the universe at large, each galaxy, in the data sense, is an independent universe.

Roughly speaking, the gravitational field of a galaxy is a function of three variables, while the whole of what we call cosmological data today barely comes to that much. (This is only a rough caricature, of course).

For example the whole CMB really comes down to a few parameters to be explained related to the positions of the first few peaks and their relative heights,  there are also several parameters that go into the fitting.

For example, I think the evidence for cosmological DM in this context is based mainly on the relative height of the third peak. I am very far from disdaining the successes of the standard cosmological model, but I think we should not lose sight of what is involved in this success.

CDM advocates indeed say they hope one day to explain away the apparent contradictions of their predictions with the data on galactic scales. But surely they cannot hope to actually predict, for example, the rotation curve of a  given galaxy from only its baryon distribution. How can they when the relation between baryons and CDM distribution, which largely determines the RC, depends so much on the turbulent history of how this particular galaxy was shaped (mergers, dissipation, feedback, supernovae, etc.)? a history that can never be known. (Tidal dwarfs are the only exception I know of. There, the expected DM content is known because it is determined by a single recent event that erased the past history. As you may know CDM does not fare very well there.)

2. For CDM in the context of galaxies, the basic theoretical ingredients are known already (unless you  want to invoke a non CDM picture)  and that is what you have to work with.

For MOND in the context of cosmology we still have many open possibilities theory-wise.

I personally think that TeVeS (Bekenstein 2004), with all the good things that it does give us, is not really the answer.

BIMOND (Milgrom 2009) has only been explored very superficially in this context. And I agree with you: MOND needs a  fundamental theory, probably based on principles and ideas not thought of heretofore.  But I think we could make a judgment based on what MOND does already do.

We do not need to have a theory of quantum gravity to be rather sure that there is fundamental truth in GR, even if it is most probably not the whole truth.

All the best, Moti

6. e-mail on 17.08., 1:23pm:  Abraham Loeb  —>  Pavel Kroupa


This would be fine with me. [putting up this blog]

You might want to correct a statement you made in your last e-mail (or
simply include this e-mail in the blog). I am not stating that MOND is
second order speculation, but that MOND+neutrinos belongs to that
category. MOND (together with all theories of modified gravity) is
mentioned as Venture Capital in Fig. 2 of my article.


by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (18.08.2010): “Is LambdaCDM or standard cosmology a 4th order speculation, and ought it be further researched ?” on SciLog. See the overview of topics in  The Dark Matter Crisis.

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7. The Train Wreck Cluster – an “anti-Bullet-Cluster”: disproof of Cold or Warm Dark Matter

This is the final part of our small series on the Bullet Cluster (and galaxy clusters in general). In the first part we have already argued that the Bullet Custer can not be used as a “smoking gun” for dark matter and even poses a problem for concordance cosmology. The second part laid out that theories of modified gravity can account for galaxy clusters and expecially for the Bullet Cluster, too.

There is one cluster which, in some respects, resembles the Bullet Cluster: Abell 520 (see also: Cosmic ‘train wreck’ defies dark matter theories).

Source: Chandra X-ray observatory site, Harvard University. http://chandra.harvard.edu/photo/2007/a520/a520_comp.jpg

Similar to the Bullet Cluster two galaxy clusters have collided recently. Consequently, the hot gas is again found in the middle of the clusters. And again one expects Dark Matter, as it is collisionless in contrast to the gas (which, when atoms collide, radiates its energy in the form of light and thus cools and slows down), to be centered on the two galaxy clusters. But to the surprise of the Dark-Matter community, Mahdavi et al. (2007) found a “Dark Core in Abell 520”, that is, there is Dark Matter in the center where no galaxies are.

This object therefore looks like the inverse of the Bullet Cluster. Things look messed up, that’s why the object got the name “Train Wreck Cluster”. We did not find an explanation for it in the literature and one of us, Marcel Pawlowski, even discussed it’s case with standard cosmologists. Up to now, they all agree that we do not understand it in Standard Cosmology. Interestingly, the alternative gravity community has come up with an explanation, such as Moffat and Toth (2009) for MOG.

After finding out about the existence of this “Train Wreck Cluster”, one question cames to mind: How is it that everybody mentions the Bullet Cluster as a proof of Dark Matter, but (almost) nobody ever talks about the Train Wreck Cluster? Does an object for which the theory gives a good explanation have more “evidence-value” than an object which seems to be at odds with the theory?  Isn’t that a bit too selective for scientists? In fact, while during discussions everybody points at the Bullet Cluster, many people and even a lot of astronomers do not even know about Abell 520!

We have to be really careful here. Always pointing at one object as the ultimate proof for dark matter and not mentioning a counter-example isn’t good science. In fact, this selective reporting distorts the evidence especially towards people who do not and cannot acquire the objecitve information – the public gets a wrong impression.

And stating that galaxy clusters can not be explained in modified gravity theories while there are peer-reviewed papers doing exactly that is very bad style and positively unscientific. The whole problem of the existence of Cold or Warm Dark Matter should not be about opinions, but about science. And the evidence is defintely not in-favour of its existence.

A Radical Conclusion

Why can we make such a radical statement depite the vast majority of fellow-scientists expressing the oposing view?

Well, given the material on galaxy clusters presented here it is very clear that the Cold- or Warm-Dark-Matter hypothesis has problems with galaxy clusters, particularly with the Bullet and Train-Weck Clusters. Non-Newtonian approaches on the other hand seem to easily account for them. And, the Local Group of galaxies (and thus us humans) cannot really exist in a Cold- or Warm-Dark-Matter universe.

Putting this together we get a positively dark view of Dark Matter Cosmology, while the alternative models (MOND or MOG or … ?) yield a notably bright window towards a much more superior description of cosmological reality.

by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (11.08.2010): “The Train Wreck Cluster – an anti-Bullet-Cluster: disproof of Cold or Warm Dark Matter” in “The Dark Matter Crisis – the rise and fall of a cosmological hypothesis” on SciLog. See the overview of topics in  The Dark Matter Crisis.

6. The Bullet Cluster and galaxy clusters in modified-gravity theories

This is our second post on the topic of the Bullet Cluster and galaxy clusters in general. Here you can find the first part: “But the Bullet Cluster …“, in which we argue that it can not be used to proof the Dark Matter Hypothesis. The third and last part, about an ‘anti Bullet Cluster’, will be published in a few days.

Modified gravity theories do well on galaxy scales. Even Ethan Siegel accepts that they are better than Dark Matter at these small scales. But can they cope with the Bullet-Cluster observations? Ethan says no. Let us explain why this statement is wrong.

As mentioned before, the relative velocities of the two galaxy clusters are too fast to be consistent with the standard cosmological framework. In theories of modified gravity, in contrast, high relative velocities of galaxy clusters occur because the effective gravitational acceleration is stronger. And this does take into account that there is less mass because there is no dark matter. The increase in acceleration out-weighs this. High impact velocities naturally occur in MOND, as Angus and McGaugh (2008) have shown, while Moffat and Toth (2010) were able to resolve the infall-velocity issue using MOG.

Thus, there is at least one problem on galaxy cluster scales where modified gravity theories do better than Dark Matter. They not only convincingly triumph on the small scales (as even Ethan Siegal submits to and as is demonstrated scientifically in our research paper). Seen this way, one can just as well use the Bullet Cluster as an example against the Cold Dark Matter Hypothesis.

But how to explain the missing mass in galaxy clusters? It can not all be in hot gas because of the observed offset between the lensing mass and the gas emission in the Bullet Cluster. Assuming the validity of Newtonian Dynamics, the mass missing in clusters of galaxies is about a factor of 4 more than what can be seen in luminous galaxies and gas. Life is made much easier in Modified Newtonian Dynamics (MOND) as Sanders (1999) has shown, because in MOND less mass can produce the same acceleration. This reduces the factor of missing mass to only two times the visible mass. This amount of missing mass might be found in neutrinos, or it might not even be real: a factor of two is not much in astronomy. And a systematically larger mass may be obtained through biases hitherto not taken into account.

We know that neutrinos oscillate, therefore they must have a mass. That mass is small. This makes them a form of hot dark matter that we most definitely know to exist. In order to explain the oscillations, particle physics even predicts the existence of more massive, sterile neutrinos, which only interact by gravity. If they exist they might be massive enough to account for the missing mass in galaxy clusters in MOND (and they can fit the first three acoustic peaks in the CMB).

For Moffat’s theory of Modified Gravity (MOG) the situation is different still: Analyzing the Bullet Cluster, Brownstein and Moffat (2007) realized that in MOG, no Dark Matter is needed at all:

“Using Modified Gravity (MOG) theory, the ‘normal’ matter in the Bullet Cluster is enough to account for the observed gravitational lensing effect.”

So, please, do not say the Bullet Cluster or the high speeds of galaxies in clusters kill all alternative gravity theories. They don’t. In fact, they might just be arguments for non-Newtonian gravity.

by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (04.08.2010): “The Bullet Cluster and galaxy clusters in modified-gravity theories” in “The Dark Matter Crisis – the rise and fall of a cosmological hypothesis” on SciLog. See the overview of topics in  The Dark Matter Crisis.