Kudo list of well-written exchanges for the month of April :-D

[marcus]

I've never done this before. Hope the parties involved are not embarrassed by being pointed out. I feel strongly about clarity of thought and writing in Astrophysics and Cosmology. Every once and a while I see a brief exchange that is exceptionally well written from both sides. Someone asks a thoughtful question very clearly and someone else answers it very accurately and clearly. No words wasted and no confusing loose ends. Nice.

Can anybody beat this, as an exemplary Q/A exchange for this month of April 2013? It is a really good question. Why doesn't darkmatter clump?

I understand there are no friction forces to slow it down but there are other means of dissipating kinetic energy.

Every time 2 dark matter particles come close enough to interact gravitationally there should be some exchange of momentum. If there is a cloud of dark matter particles, such as in and around a galaxy, there would be constant re-distribution of momentum. Each time a particle acquires enough momentum to escape the cloud the average momentum of the particles in the cloud is reduced. This would be a sort of evaporation that continually lowers the average temperature of the cloud. Cosmic expansion would create a difference between the amount of energy gained from incoming particles and that lost with outgoing particles. The result of all this should be small, dense, bodies of dark matter.

I understand that small, dense, non-luminous bodies have been ruled out as dark matter candidates. What prevents dark matter from behaving as I have suggested?

Every time 2 dark matter particles come close enough to interact gravitationally there should be some exchange of momentum.

Sure, but that momentum exchange is tiny. Gravitational attraction between individual particles is almost negligible. It is sufficient to give clumping on the scale of galaxies (where you have many particles interacting at the same time), but not on smaller scales within the current lifetime of the universe.

If you see another highquality efficient exchange like this during April in Astrophysics or Cosmology, I hope you will add it to this thread, as a kind of Kudo list. We'll see how good we are as a group, at asking and answering this sort of questions.