Why is there no DM halo around the solar system?

[nikkkom]

It's not true that DM doesn't interact at all; it's that it only interacts gravitationally.

It seems at least theoretically possible that if some effect causes it to focus into a narrower stream, then the self-gravitational attraction of that stream would refocus it weakly again later

I'm googling "DM density"...

http://arxiv.org/abs/1205.4033

"we find that the data imply a local dark-matter density of 0.008 +/- 0.003 Msun/pc^3 = 0.3 +/- 0.1 GeV/cm3"

This is way, way too low for gravitational self-interaction.

 

[Buzz Bloom]

I suspect that you'd require very special (physically implausible) conditions to focus the flow sufficiently accurately for the self-gravity to have enough effect to be noticeable.

Hi Jonathan:

I am not sure I understand the implications of self-gravity here. Are you saying that if a hair structure were to form (as apparently predicted by the Prezeau paper nikkkom cited in post #47) with the DM moving out of the space occupied by the hair as new DM moved into this space, that the DM in the hair would gravitationally influence the later DM being focused towards the hair in such a way that the hair structure would cease to exist?

Regards,
Buzz

 

[Jonathan Scott]

I'm googling "DM density"...

http://arxiv.org/abs/1205.4033

"we find that the data imply a local dark-matter density of 0.008 +/- 0.003 Msun/pc^3 = 0.3 +/- 0.1 GeV/cm3"

This is way, way too low for gravitational self-interaction.

But if you assume an unrealistically ideal uniform flow being focused in an unrealistically ideal mathematical way, you can achieve far higher densities. I'm not saying that's realistic physics, but it might account for the temporary "hair" formation in this simulation. However, I wouldn't expect it to be stable even in theory, so the flow would disperse more randomly afterwards.