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.

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.


Author: Marcel S. Pawlowski

I am a postdoc at the Department for Astronomy of Case Western Reserve University in Cleveland, OH (soon Hubble Fellow at UC Irvine). My work revolves around tidal dwarf galaxies – second-generation galaxies forming from the debris of galaxy collisions – and their use for testing models explaining the dark matter phenomenon. During my PhD studies in Bonn (in Pavel's group) my research concentrated on the phase-space distribution of the Milky Way's satellites (dwarf galaxies, globular clusters and tidal streams), their possible formation scenarios (in particular tidal dwarf galaxies) and tests of cosmological models on (cosmologically) small scales. My research interests are complemented by my interest in the philosophy of science and in science outreach. You can follow me on Twitter (@8minutesold) or find out more about me and my photography on my websites ( &

4 thoughts on “The Train Wreck Cluster – an "anti-Bullet-Cluster": disproof of Cold or Warm Dark Matter”

  1. Scientific PolemicismThis article criticises scientists for picking and choosing data or results to support their arguments. Yet the authors themselves do exactly the same. The main reason why most scientists remain sceptical about the Abell 520 “train wreck” results is that different groups analysing the *same data* obtain different mass maps (see Okabe et al. 2008). Now that’s a train wreck! The same cannot be said for the bullet cluster, where – to the best of my knowledge – all authors currently agree on the lensing mass maps. This does not mean, of course, that the bullet is right and Abell 520 is wrong – we should remain open minded about both. But I am particularly sceptical of the Abell 520 results because of a well-known problem with lensing mass reconstruction: the monopole degeneracy. This was illustrated beautifully in recent work by Liesenborgs et al. 2008 who show that the monopole degeneracy can lead to phantom peaks in the mass distribution (see their Figure 3). Their work focused on strong rather than weak lensing, but weak lensing suffers from exactly the same problems.

  2. Basic Problem of Lensing MapsDear JR,
    Thank you for pointing out this basic problem of lensing mass maps, which to our impression sound serious. We agree that one has to remain sceptical about lensing data. However, we also have to disagree with you. In this article, we do not “pick and choose data or results to support [our] argument”. We merely point out that galaxy clusters and their lensing maps are not as well understood as often stated. They thus can not verify the one or the other theory, which needs to be mentioned whenever somebody claims that the Bullet Cluster has proven the existence of Cold Dark Matter. You probably agree with that statement, too.

  3. In both cases, since the spatial distribution of proposed dark matter generally coincides with that of observed galaxies within the colliding galaxy clusters, in any case where the luminous galactic mass has been underestimated or the derived weak gravitational lensing effects have been overestimated, the presence of dark matter would be proportionately overestimated.
    Since the estimations of galaxy masses are at best a rough approximation and the weak lensing effects are derived using highly interpretive techniques, it should not be surprising that their results might not be in complete agreement, producing the opportunity for dark matter and/or modified gravitation explanations for the discrepancy which may have simply arisen due to derivation error.
    Personally, I’d prefer more definitive evidence for these extraordinary conclusions!
    You’ve got to admit, though, that this method of using brightly visible colors to represent virtual entities makes it easy for any casual viewer to convince himself of the concreteness of the evidence. It’s a PR coup!

  4. Subject was estimation errorBTW, the extended difficulty of usefully approximating the collective galactic mass of galaxy clusters may be the source of what I understand to be MOND’s difficulty when applied to clusters rather than individual galaxies.
    The old axiom is: garbage in, garbage out…

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