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Dark Matter Is Even More of a Mystery Than Expected
schwit1 writes: Using the Hubble and Chandra space telescopes astronomers have discovered that dark matter is not only invisible to direct observation, it is invisible to itself! Quoting: "As two galactic clusters collide, the stars, gas and dark matter interact in different ways. The clouds of gas suffer drag, slow down and often stop, whereas the stars zip past one another, unless they collide — which is rare. On studying what happens to dark matter during these collisions, the researchers realized that, like stars, the colliding clouds of dark matter have little effect on one another. Thought to be spread evenly throughout each cluster, it seems logical to assume that the clouds of dark matter would have a strong interaction — much like the colliding clouds of gas as the colliding dark matter particles should come into very close proximity. But rather than creating drag, the dark matter clouds slide through one another seamlessly." The data here is on the very edge of reality, built on too many assumptions. We know that something undetected as yet is influencing the motions of galaxies, but what exactly it is remains completely unknown. These results only make the mystery more mysterious.
Isn't this what one would expect if dark matter is WIMPs?
"99 dead duelists of Dios on the wall. 99 dead duelists of Dios! Take one's ring, pass it around..."
We're trying to explain inflation and the motions of stars orbiting galaxies not matching our naive model.... couldn't a non-linear gravity model explain all this without the dark energy/matter hocus pocus?
Sure, but that would involve even greater hocus pocus than the current theory and fails at explaining other observations. So far, trying to come up with any hypothetical explanation involving MOND has been so complex that nobody has been able to come up with one that explains even the rotation of galaxies. If you or any other person could come up with a good law of nonlinear gravity that works, even if it completely fails at any of the other observations, there's a paper in a prestigious journal and some physics cred for you.
Indeed. I've always wondered why there's an automatic assumption that matter of some sort must be causing these excess gravitational effects that are popularly associated with dark matter. I believe there's just as much merit to the idea that our understanding of gravity is simply wrong, and that it can exist absent of matter. I would think of it as space time curvature that is inherent to the universe. Matter would naturally "fall in" to these wrinkles of the fabric in space time, and perhaps that is what causes the cosmic web structure we observe in the universe today.
Thought to be spread evenly throughout each cluster, it seems logical to assume that the clouds of dark matter would have a strong interaction
It would actually be completely illogical to assume that precisely BECAUSE Dark Matter is spread evenly through each cluster. If it had a strong self interaction then, just like matter, it would bump into itself and coalesce into clumps just like that other strongly, self interacting stuff we call matter. The fact that Dark Matter has a completely different mass distribution than ordinary matter is clear evidence that it does not have a large self interaction cross-section...and we have had direct evidence of this since the Bullet Cluster was discovered.
It's always nice to have more confirmation but since another recent story on the same site was talking about the "new" possibility of invisible Higgs decays to Dark Matter particles (something we looked for 15+ years ago at the Tevatron as well as the previous Run 1 of the LHC) I have to wonder if the writers of the site have suffered extreme time dilation for the past decade or two.
There have been several theories built on that assumption, most prominently one called MOND (MOdified Newtonian Dynamics), but more recently one that builds on relativity rather than Newtonian gravity/dynamics.
But none of these theories (hypotheses?) have gained much acceptance from the physics community, as far as I know.