How Does Dark Matter Avoid Gravitational Collapse?

[moving finger]

Does anyone know of any work that has been done on the possible theoretical physics/mechanics of Dark Matter, for example if Dark Matter cannot lose energy by radiation then it presumably cannot undergo gravitational collapse in the same was as baryonic matter? A layperson's guide would be nice!

Thanks

MF

 

[SpaceTiger]

Does anyone know of any work that has been done on the possible theoretical physics/mechanics of Dark Matter, for example if Dark Matter cannot lose energy by radiation then it presumably cannot undergo gravitational collapse in the same was as baryonic matter? A layperson's guide would be nice!

Hmm, I don't know if such a thing exists, unfortunately. The aspects in which this has been explored are largely in the context of the growth of structure in the universe, and everything I've seen on that is pretty technical (for example, Liddle & Lyth). I can briefly summarize the ideas.

At very large scales, when the matter is far too diffuse to be radiating, then cold dark matter and normal matter act in pretty much the same way. That is, the overdensities collapse and can be treated with linear perturbation theory. When the density in a region exceeds the background density (approximated by the critical density) by of order unity, then it becomes "nonlinear", meaning it can no longer be simply treated by linear perturbation theory. At this point, we often use the "Zeldovich Approximation", a simple mathematical trick that allows us to explore the evolution of slightly non-linear structures.

At the present time (and at most times in the universe) there are scales on which large scale structure is linear and scales on which it's non-linear. The extent to which the universe is clumped on different scales is described by the power spectrum. Overdensities of a very large size are very slow to grow, so they will still be linear in the present day, while overdensities of very small size (like clusters), have long since undergone non-linear collapse. This is the fundamental idea behind "bottom-up" structure formation, that smaller things form first and combine to form larger things.

Everything I've described up to now applies to both cold dark and normal matter. Once the overdensities collapses to galaxy cluster scales, however, the normal matter begins to interact with itself in ways other than gravitationally. If it weren't for this evolution of the baryons, then the distribution of cold dark matter could probably be described fairly simply, but it is unfortunately (or fortunately, depending on your point of view) coupled gravitationally to the normal matter. This complicates things to the point where I don't think I could give a simple conceptual explanation. However, the clumping and evolution of small haloes (a term used to describe dark matter overdensities) is treated approximately by the Press-Schechter theory.

Anyway, if you're interested in any of the above topics, you can try google searches on some of the key concepts (like the power spectrum, press-schechter, zeldovich, etc.). I can refer you to various books on this sort of cosmology, but I wouldn't really recommend any of them for the amateur. The one I mentioned at the beginning is probably the best for the pedagogy, but it would still take a lot of work to understand.

 

[moving finger]

That's great, thank you!