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.
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.