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Q & A: why aren't WIMPs tightly clustered?

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Most recent answer: 02/20/2012
I'm trying to understand the WIMP (Weakly Interacting Massive Particle) explanation for Dark Matter. Since WIMPs are slow and cold and interact gravitationally, they should also exhibit the kinds of clumping behavior seen in baryonic matter (largely). Given the large percentage of mass they represent in a standard galaxy, shouldn't we then see large concentrations of WIMPs in most of the large bodies of our solar system, and even potentially large planetary-mass and greater bodies made up principally of WIMPs in orbit around most stars (including ours)? If that's the case why can't we see gravitational effects created by these WIMP-bodies within our own solar system, and why don't we see large inexplicable (non-baryonic) extra mass in most of the planetary bodies? I must be missing something or jumping to a false conclusion.
- Kevin Barnes (age 43)
San Diego, CA USA
That's a great question. Here's a first draft of a response, to be checked with experts. In order to settle into a cluster, a particle needs not only to be gravitationally attracted to it but also to have some way to dissipate energy. Think of a bouncing ball. If it weren't for various sorts of friction, it would just continue to bounce without ever settling down on the earth. If I understand correctly, the non-gravitational interactions of the hypothetical particles proposed to account for dark matter are just too weak to give enough dissipation to cause much clustering on scales smaller than a galaxy. A WIMP, for example, attracted to the earth might just pass right through it.

Mike W.

And our expert, Brian Fields, adds:

It also is worth noting that the dark matter represents most of the mass in a galaxy, so in practice galaxy formation is first characterized by dark matter forming the gravity potential wells, and baryons falling into these potentials.  Then the dissipative nature of baryons (e.g.,
they can cool by radiating photons) allows them to be (much) more condensed than the large dark matter halos that surround visible galaxies.

A corollary
to this is that if baryons didn't  dissipate energy, they would have the same spatial distribution as the DM.

And finally, it also is true, as the questioner hints, that baryons can have gravitational effects on dark matter.  In particular, the Sun should gravitationally capture the slowest DM particles, which should then annihilate in the Sun's center.  Searches for such annihilation events are one of the ways we looks for dark matter.


(published on 02/20/2012)

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