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.

 

[AndyW]

I agree with your point about the tiny momentum exchange between dark matter particles. In addition, dark matter is thought to interact very weakly with other particles, making it difficult for it to clump together. This is why it is able to pass through normal matter without interacting. In comparison, normal matter interacts with other particles through electromagnetic and strong nuclear forces, allowing it to clump together more easily.

As for other high-quality exchanges, I recently came across a discussion on the nature of dark energy and its effects on the expansion of the universe. The question was about whether dark energy can be explained by a modification of gravity, rather than the existence of a mysterious energy field. The answer provided a thorough explanation of the current theories and evidence surrounding dark energy, and why modifying gravity may not be a viable explanation. It was a great example of clear and concise writing in the field of cosmology and astrophysics.