Marcel S. Pawlowski | 22. November 2010, 16:37

On November 18th 2010, Pavel Kroupa and Simon White met in Bonn for a special Bethe Colloquium: "Dark Matter, a Debate". While the video podcast is not available yet, there is a replay of the live blog. Furthermore, Pavel Kroupa's presentation slides can be downloaded as a pdf file.  (More)

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Marcel S. Pawlowski | 18. November 2010, 10:28

Starting at 3pm local time (Germany), Andreas (AHW) and Marcel (8minutesold) will try to live blog from the debate about Dark Matter between Simon White and Pavel Kroupa in Bonn. As there is no live video stream available, this seems to be the only live coverage of the event. You can follow the report in the field below and even send in a comment or two. (More)

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Pavel Kroupa | 01. November 2010, 11:25

"The subject of this months Bethe Colloquium concerns a question at the interface of cosmology, astrophysics and elementary particle physics: the possible existence of Dark Matter. The existence of Dark Matter is the most prominent proposal to account for the discrepancy between measurements of the mass of galaxies, clusters of galaxies and the entire universe, and measurements based on the mass of the visible matter. So far the existence of Dark Matter is inferred from gravitational effects on visible matter and background radiation and not through direct detection. In his talk Professor S. White introduces the dark matter paradigm and explains its virtues. An alternative proposal to explain the observed discrepancies is introduced by Professor P. Kroupa. He argues that these effects could be due to a modification of the laws of gravity without the need of Dark Matter. The talks are followed by a discussion."

 

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Marcel S. Pawlowski | 23. September 2010, 10:00

After the succsessful AG Meeting, next week we will be at another conference, titled "New Directions in Modern Cosmology", to which Prof. Pavel Kroupa was invited. It will take place at the Lorentz Center in Leiden (in the Netherlands) from September 27 through October 1st. As the title suggests, the workshop is about the increasing amount of observational challenges of LCDM cosmology. From the website:

This workshop concentrates on the discussion of recent cosmological observations which present challenges to the standard LCDM model. These observations include: the large scale flows, the sizes and amplitude of galaxy large scale structures, the systematic effects biasing the analysis of CMB data and the lack of large-angle correlations, the anisotropy of the Hubble flow, the evolution of galaxy size, and the failure to find the sub-halo building blocks left over from the primordial fluctuation spectrum.  Last and not least, it is disturbing that in the LCDM model 95% of the Universe have not been observed 'directly'.

While each of these observations can be seen as an anomaly that the model would possibly explain, the bulk of them calls for a more careful analysis of the model foundations, particularly the amount and role of dark substances.

 (More)

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Pavel Kroupa | 19. September 2010, 20:30

At the conference "Zooming in: The Cosmos at High Resolution" Prof. Matthias Bartelmann from Heidelberg University gave a didactively beautiful presentation about the standard cosmological model for the general public.

Here is his argument why it is believed that this model is an excellent description of the universe: (More)

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Marcel S. Pawlowski | 10. September 2010, 12:10

This coming week we will have the annual meeting of the "Astronomische Gesellschaft" (the German astronomical society) in Bonn. The conference with the topic "Zoomin in: The Cosmos at High Resolution"  starts on Monday (September 13) and will last until Friday. You can find the schedule here. In the afternoons there are a number of splinter meetings on different astronomical topics. For those participating in the meeting and interested in the Dark Matter Crisis, we would like to point out some of our contributions. Of course we will also be around during the week, so feel free to approach us for discussions. (More)

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Pavel Kroupa | 08. September 2010, 10:00

What is the Fritz Zwicky Paradox?

In our previous contribution we gave three historical examples of previous failures of Newtonian mechanics or dynamics. These failures implied quantum mechanics, special and general relativity. While not evident at start, each of these break-throughs lead, many decades later, to very major technological advances with industries worth trillions of dollars today. In the present era physics is experiencing the fourth failure. But how does the failure arise? (More)

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Pavel Kroupa | 01. September 2010, 10:00

The radical conclusion that Cold- or Warm-Darm-Matter cosmology ought to be discarded as a viable description of physical reality would imply, as a strict logical process, that this physical reality must be non-Newtonian in a certain physical regime which is found on galactic scales and beyond. This goes hand-in-hand with discarding Newtonian dynamics in these regimes.

Indeed, changing dynamics away from Newtonian dynamics is actually already a very well established tradition in physics and results from the desire to understand how objects with "mass" move about and influence each other in our four-dimensional "space-time" world. (More)

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Pavel Kroupa | 25. August 2010, 10:00

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 3x2.75eV 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 status" 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.

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Pavel Kroupa | 18. August 2010, 10:00

On August 9th, 2010  Prof. Abraham Loeb from 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. (More)

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Marcel S. Pawlowski | 11. August 2010, 10:00

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.

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Anton Ippendorf | 04. August 2010, 10:00

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.

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Marcel S. Pawlowski | 30. July 2010, 14:46

Whenever a discussion about the problems of the Cold Dark Matter Hypothesis and possible alternatives like Modified Newtonian Gravity (MOND) emerges, one argument you can be sure to hear soon is “But the Bullet Cluster ...”. It is the same whether you discuss with scientists or other people interested in astronomy. But can the Bullet Cluster be considered as a proof of Cold or Warm Dark Matter? No, because that conclusion rests on further assumptions and is in itself not logically valid. Furthermore, the problems of the Dark Matter Hypothesis are independent of the Bullet Cluster, making it a false argument in many discussions. Even worse, the collision velocity of the Bullet Cluster seems to be incompatible with the concordance cosmology. At the same time, alternative gravity theories, while often said to fail in explaining galaxy clusters, can account for them rather naturally.

In this first post in a series of three we will discuss the Bullet Custer as a "smoking gun" for Dark Matter. The other two parts about "The Bullet Cluster and galaxy clusters in modified-gravity theories" and "The Train Wreck Cluster - an 'anti-Bullet-Cluster': disproof of Cold or Warm Dark Matter?" will follow in a few days.

 

What is the “Bullet Cluster”?

It all started in 2006 with a paper titled “A direct empirical proof of the existence of dark matter” and a press release with the no-less lurid headline “NASA Finds Direct Proof of Dark Matter”. Right in the title the authors claimed that their discovery would immediately settle the question whether there is Dark Matter (DM) or not. Naturally, the spectacular announcement was adopted by the majority of the media and could well be one of the most successful press releases in astronomy. Since then, the picture of the Bullet Cluster (BC) has been shown countless times:

Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.  

 

What do we see in this picture? There are two clusters of galaxies (left and right). Overlaid are blue and pink colors. The two pink clumps in the middle shows where x-ray observations find the hot gas, the Bullet Cluster got it's name from the bullet-like shape of the gas on the right. The gas usually sits in the center of a galaxy cluster, but is here shifted to the point between the clusters. Some time ago the two galaxy clusters have passed through each other, making their gas collide. As gas interacts electromagnetically, it is slowed down when it collides, like two streams of air that can not pass through each other unhindered in opposite directions. That is why the gas is a bit behind the galaxies. Those do only interact through gravity and therefore pass each other unhindered without being slowed-down like the gas. This is maybe similar to two swarms of flies that can fly through each other.

The blue blobs were derived in a bit more complicated manner using the gravitational lens effect. To put it simple, since Einstein we know that matter deforms space-time. This leads to a bending of light rays when they come close to a large amount of matter, like a galaxy cluster. Thus, when the light of distant galaxies passes a massive cluster of galaxies before it reaches us, we will see a deformed image of the distant galaxies. The effect can be calculated and astronomers are able to trace it back. From the distorted shapes of distant galaxies behind a galaxy cluster they can infer the distribution of mass in that galaxy cluster. The heavier a galaxy cluster is, the more it bends the light and the more it distorts background galaxies. For the Bullet Cluster, the blue blobs in the picture above show the distribution of mass as inferred from the gravitational lensing effect assuming General Relativity to be valid. One can see that it follows the distribution of galaxies, not the gas.

If the hot gas would be the most massive part of a galaxy cluster, the mass found through gravitational lensing would have to be centered on it. But as this is not the case, it is said that the majority of matter in the galaxy cluster has to be close to the galaxies. Because the visible mass in the galaxies is not enough to account for the velocity dispersion of a galaxy cluster (assuming Newtonian Dynamics, i.e. General Relativity), it is conjectured that there is Dark Matter, which by definition only interacts through gravity, too. Thus distributions of DM can pass through each other just like the galaxies and the majority of mass should be found close to the galaxies in such a cluster collision.

This is why the Bullet cluster is often said to be the “smoking gun” of the Standard Cosmological Model. It behaves just like it is expected. But is it really that simple? Does this proof the existence of Dark Matter? No, it doesn't.

 

Observations and Interpretations

In the most-often given description of the Bullet Cluster, what is observation and what is interpretation get mixed up. The observations tell us that the hot gas component and the lensing mass have an offset. One good conclusion from this is: The visible, hot gas can not make up the majority of mass in the system. But a wrong conclusion is: The Cold Dark Matter Hypothesis is right.

While we can argue that the majority of mass has to be close to the galaxies, we can not immediately conclude that it has to be in the form of Dark Matter as a new type of particles. Actually, we can only say that the majority of gravity, or even more specific, the major bending of space-time, happens close to the galaxies. Whether the reason is missing mass or a different law of gravity is not that easy to distinguish (there is gravitational lensing in modified gravities, too). The whole, most-mentioned conclusion is therefore based on one important, but never mentioned assumption: that gravity is best described by Newtons law. In addition to that, it supposes that other, known forms of dark matter (e.g. neutrinos) can not be the reason. Without those assumptions, the case of the Bullet-Cluster is not decided at all.

We see that the “direct proof for the existence of Dark Matter”, is an indirect hint at best, in that it is based on untested assumptions and does not even look at other possible predictions of alternative gravities. But there are more problems to come.

 

A Proof of Dark Matter?

Even if the cluster can be explained in the standard or concordance cosmological LCDM framework, this does not proof the theory. Because there can be no proof of a scientific theory. For the BC to be a proof of a scientific theory, it would have to rule out each and every alternative explanation, even those of which we can not even think of today. This, of course, is impossible. This fact is well known in the philosophy of science and I guess most scientists know this. Scientific inference does not function without this elementary fact.

Furthermore, there are different possible explanations for the BC. There even are different possible forms of Dark Matter. Not only the currently favored Cold Dark Matter, on which the Concordance Cosmology Model rests, but a model with Hot Dark Matter (where the DM-particles are fast/relativistic because they would be of low mass, like neutrinos) could explain the BC as well. So, please don't state that the Bullet Cluster has proven the LCDM-model right. It has not. And it can not.

 

The Bullet Cluster, a problem for Dark Matter?

In fact, the Bullet Cluster might not only not be a proof of the DM hypothesis but it actually appears to be a major problem for the concordance model. Mastropietro and Burkert (2008) have found that the two colliding clusters need to have a relative velocity of about 3000 km/s to produce the observed X-ray gas properties. This result was compared to a cosmological simulation named MICE. Such cosmological large-volume simulations show the formation of structure in the universe and are often said to be another important success of the Concordance Cosmological Model. In the MICE simulation, Lee and Komatsu (2010) determined the probability that the Bullet Cluster's velocity could be found in the concordance cosmological model. It is roughly one in ten billion! They ...

“... conclude that the existence of [the Bullet Cluster] is incompatible with the prediction of the ΛCDM model ...”. 

This is a paradoxical situation: While the structure formation simulations are used to argue in favor of Dark Matter because they fit so well, and the Bullet Cluster is used in favor of Dark Matter as a “direct proof” or “smoking gun”, putting them both together leads to an incompatibility.

 

Does the Bullet Cluster matter at all?

So far we have shown that the Bullet Cluster can not be understood as proof for the Dark Matter Hypothesis. We took the argument seriously. But in doing so, we have repeated the same mistake as many people who bring up the BC when they try to dismiss our work on testing the Dark Matter Hypothesis. Why that? Well, usually the discussion follows these lines:

• “Testing the predictions of the Cold Dark Matter Hypothesis on galaxy scales, we have found several serious problems.”
• “But don't you know about the Bullet Cluster? It is the proof that there is Dark Matter!”

Put that way, it is easy to spot the mistake: Even if the Bullet Cluster could only be explained with Dark Matter, the problems on small scales persist. The BC does not tell us anything about the Local Group of galaxies, the two arguments are completely independent. There is a serious problem with the DM Hypothesis and even a thousand Bullet Clusters would not make it go away. In fact this is similar to the hypothetical scenario that someone, for example at the LHC, would find “The Dark Matter Particle”. Even in that case, the current problems of the model would not go away. Rather, this would point at a much more serious issue with our understanding of physics.

In trying to understand the universe, we as good scientists should thus look for alternative explanations that account for all independent observations and discuss these without ideological pre-conceptions.

Solutions to the Bullet cluster in modified Newtonian dynamics (MOND) have indeed been shown to exist (Angus, Famaey & Zhao 2006). The authors conclude

In multicentred models, the convergence map does not always reflect the projected matter in the lens plane in MOND. This cautions simple interpretations of the analysis of weak lensing in the bullet cluster 1E 0657−56 (Clowe et al. 2004; see fig. 7).

Similarly for Modified Gravity (MOG):  Brownstein and Moffat (2007) write

The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +/- 3.9keV, in good agreement with the experimental value T = 14.8+2.0-1.7keV. Excellent fits to the 2D convergence κ-map data are obtained without non-baryonic dark matter..." and they uncover a significant disagremenet with the dark-matter based analysis (the baryon fraction is to high in a dark-matter model).

 

Stay tuned, there is more to be said about galaxy cluster and the Bullet Cluster in modified gravity theories in our next post.

by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (30.07.2010): "But the Bullet Cluster ... - Proof of Cold or Warm Dark Matter in galaxy clusters is but a myth" in "The Dark Matter Crisis - the rise and fall of a cosmological hypothesis" on SciLogs. See the overview of topics in  The Dark Matter Crisis.

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Pavel Kroupa | 28. July 2010, 12:00

In an interesting comment Daniel Fischer (22.07.2010, 17:00) points out the contribution by Ethan Siegel. In essence Ethan Siegel writes "saying that, 'since the naive predictions we can make are inadequate, the entire idea of dark matter needs to be thrown out' is absurd."

 "What is absurd is to presume that a theory that is adequate on one scale will inevitably succeed on another scale.  That LCDM is consistent with large scale measurements is no guarantee that it will work on smaller scales.  The widespread presumption among cosmologists seems to be that it must surely work out, though this is hardly a scientific attitude.

The failure of dark matter to explain galaxy scale phenomena is not just a failure of detail.  A single effective force (modified gravity) suffices to explain (and indeed,
predicted a priori) many aspects of galaxy dynamics.  To explain this with dark matter is like asserting that really the solar system operates on an inverse cube law; there just happens to be dark matter arranged just so as to always make it look like an inverse square law.

It is hard to imagine a more bizarre requirement for the distribution of dark matter." - Prof. Dr. Stacy McGaugh (his MOND papges).

It is noteworthy that the community seems to stress aparent successes of dark matter but when failures occur these are often put down as occuring in a regime not testable.  A good example is the current statement by Daniel Fisher in contrast to the statements found at the beginning of Section 2.4.

Logics:

We see that Ethan Siegel's argument is wrong logics. The dark matter hypothesis was introduced to specifically solve the "small-scale" problem of flat rotation curves of disk galaxies. To later argue that the dark matter hypothesis cannot be tested on these scales is throwing out logics and the very basics of how scientific inference works. Actually, in our research paper the problems found are not only on small scales but extend to scales of a million pc.

A Gedanken experiment helps to clarify the problem: Take one observed normal disk galaxy which you know to be in equilibrium (i.e. is not disturbed by another galaxy). Call this galaxy 1. It has a flat rotation curve. Put in the correct distribution of dark matter to explain the rotation curve. Now observe another disk galaxy which you know to be in equilibrium (galaxy 2). Measure the distribution of normal matter. Ask the astronomer to predict the shape of the rotation curve of galaxy 2 based on the observation of galaxy 1. Then measure the rotation curve of galaxy 2. The predicted and observed rotation curve will almost certainly be different.

Now, repeat the exercise in MOND (no dark matter, but a gravitational theory modified according to Milgrom). The astronomer will be able to exactly predict the rotation curve of galaxy 2 based purely on the observed distribution of normal matter in galaxy 2 and one universally valid number (a universal acceleration scale, a_0) obtained from the fit to galaxy 1. In fact, the astronomer can predict the shapes of all rotation curves using this one number a_0.

Therefore, Milgrom's theory is far more superior in describing galactic dynamics than the dark matter plus Newtonian hypothesis. What is more, the dark-matter hypothesis does not allow us to understand rotation curves. And this does not depend on unknown small-scale effects, as we are talking about scales of 10,000 to 50,000 pc.

Some explicit references:

In McGaugh & de Blok (1998a) the authors state: "Interpreting the data in terms of dark matter leads to troublesome fine-tuning problems. Different observations require contradictory amounts of dark matter. Structure formation theories are as yet far from able to explain the observations."

The companion paper, McGaugh &  de Blok (1998b) finds: "One hypothesis, Modified Newtonian Dynamics (MOND), is consistent with the data. Indeed, it accurately predicts the observed behavior. We find no evidence on any scale that clearly contradicts MOND and much that supports it."

Within the dark-matter theory galaxies would be embedded in dark-matter halos with well-known properties. That they do not match observed roation curves is documented convincingly by Kuzio de Naray et al. (2009): "The shape of the modeled NFW rotation curves does not reproduce the data".

The observed similarity of galaxies being in  contradiction to the expected large variation of galaxies if dark-matter theory were correct is documented by Disney et al. (2008).

In fact, galaxies tell us that the dark matter (if it were to exist) arranges itself according to the distribution of the normal matter, although the normal matter makes only a small fraction of the mass of a galaxy. To explain this insurmountable fine-tuning problem one would need to invoke a dark force coupling dark matter to baryons in a hitherto not understood way (Kroupa et al. 2010), or simply accept that galaxies are made up only of normal matter without dark matter and that gravity is non-Newtonian.

A lesson from history:

In this vain we might remember history: Once upon a time, not too long ago in fact, very clever people knew that heavenly bodies were either at rest or moving about other bodies on perfect circles (the planets and Sun moving around the Earth). So important and widely accepted was this idea that when the observations (of planetary motions) failed to fit the predicted motions, a circle on a circle was introduced to describe the motion of the planets. And if this was still not good enough, then yet another circular motion about a centre which moved on a circle about a centre which moved on a large circle about the Earth was added. But, in this theory of epicyclic motion an astronomer was not able to predict the motions of a new planet.

And here too, one could have argued that "just because there are minor deviations from the calculated motion of the planet and just because a Kepler and a Copernicus thought that the actual motion was on an ellipse about the Sun (!) it is surely absurd to throw out the whole large and divine picture of perfect motions of heavenly bodies, which was so successful overall."

Note also that two major mental changes had to be accepted: The centre of motion is not the Earth but the Sun and the motions are elliptical. 

In our case (Kroupa et al. 2010) two mental changes are also required: the satellite galaxies of the Milky Way are not dark-matter sub-structures but tidal-dwarf galaxies, and dynamics is non-Newtonian (e.g. Milgrom's MOND or Moffat's MOG).

And, although the Kepler laws and Copernican idea proved endlessly more successful in describing planetary motions, they were quite useless in describing the dynamics of star clusters because the deeper (Newtonian) gravitational theory had not been discovered yet. But it was absolutely evident that the epicyclic ansatz was out.

Today we would be saying that modified gravity is endlessly more successful in describing rotation curves of galaxies and galactic dynamics in general, and that cold or warm dark matter is out. But we do not yet have the full underlying gravitational theory which is likely to unite matter and space time as a single entity. 

Epicyclic additions

The concordance cosmological model arose by adding dark matter to the Theory of General Relativity, then adding inflation and then dark energy to get a model which has not been able to account correctly for the way galaxies work nor how they evolve and arrange themselves in space time. Furthermore,  an extra dark force is required as a further addition.

by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (28.07.2010): "Is it absurd to throw out the idea of Cold or Warm Dark Matter?" in "The Dark Matter Crisis - the rise and fall of a cosmological hypothesis" on SciLogs. See the overview of topics in  The Dark Matter Crisis.

 

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Marcel S. Pawlowski | 27. July 2010, 13:59

The new issue of Spektrum der Wissenschaft (SdW) is in the shops today. Our (Pavel Kroupa and Marcel Pawlowski) article on dwarf galaxies and dark matter is even mentioned on the cover and in the editorial. In Germany and Austria, you can buy the magazine for 7.40 Euros. Alternatively, you can download the free PDF of our article "Das kosmologische Standardmodell auf dem Prüfstand" from the SdW website and read it right away.

When SdW asked us to start this blog, we agreed. The main reason was to make a direct discussion about the issues we raise in the text possible. We will post additional texts (in English) on this blog, which might lead to interesting debates. In case you would like to comment on the article in SdW, feel free to give your statements or ask questions here.

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