Comments on: Warm Dark Matter?

By: Count Iblis

Count Iblis — Wed, 15 Mar 2006 15:56:24 +0000

There is a renewed interest in strongly (self) interacting dark matter. Here are 5 different ideas: Cold Dark Matter as Compact Composite Objects Composite Dark Matter with Invisible Light from Almost-Commutative Geometry Spin half fermions with mass dimension one: theory, phenomenology, and dark matte Generalized mirror matter models Has DAMA Detected Self-Interacting Dark Matter?

By: Neal

Neal — Tue, 14 Mar 2006 23:57:44 +0000

I can’t speak for the other models, but inelastic dark matter is for explaining the DAMA/CDMS conflict, not for astrophysics. I don’t think it really effects structure as it’s still pretty non-interacting…

By: Sean

Sean — Thu, 09 Feb 2006 22:38:48 +0000

Shantanu, I don’t know if you can call it a “crisis” (although people do), but there are several issues with CDM on small scales. In various ways, we don’t observe as much small-scale clumpiness as you might expect. Here’s a good review:

http://arxiv.org/abs/astro-ph/0205464

It might turn out that none of them is serious, and messy astrophysics can fix them all — or not.

By: Count Iblis

Count Iblis — Thu, 09 Feb 2006 22:00:39 +0000

Shantanu:

Can someone(Sean, Risa, Mark) summarize briefly as to what is the motivation for adding these exotic properties to cold dark matter particles.

It's all explained in this paper

By: Plato

Plato — Thu, 09 Feb 2006 21:31:47 +0000

It would mean all comparative examples of such galaxies would then never be older then the age of the universe, and we run into trouble with the arrow of time? Or, can any event that we take to the age of the universe, be a natural process?

By: Aaron Bergman

Aaron Bergman — Thu, 09 Feb 2006 19:37:41 +0000

What I know on the subject comes from this review on Paul Steinhardt's webpage.

By: Shantanu

Shantanu — Thu, 09 Feb 2006 19:34:11 +0000

I am getting lost with so many recent papers/candidates on variants of
variants of dark matter such as
darm matter with early decays : astro-ph/0507300, dark matter with scalar interactions hep-th/0407097, Dark matter with non-zero pressues, astro-ph/0512213,
dark matter with inelastic couplings, astro-ph/0208403 , non-point dark matter hep-ph/0203179,
self-interacting dark matter astro-ph/9909386 and probably many more which I may missed.
(Inspite of all this there is no laboratory evidence for any of these)

Can someone(Sean, Risa, Mark) summarize briefly as to what is the motivation for adding
these exotic properties to cold dark matter particles. Is it to address the cold dark matter crisis
at smaller scales(which I thought from one of the ary posts on this blog) is not as severe as thought before?
Of course there are tons of candidates even for vanialla cold dark matter, though the WIMP
and axion are the most popular ones.

By: Jay

Jay — Thu, 09 Feb 2006 04:18:16 +0000

There’s a conference proceeding from the group on astro-ph today:

http://arxiv.org/abs/astro-ph/0602186

Basically, they are arguing that all of the Milky Way dwarf spheroidals have masses ~ 4 x 10^7 M_sun. The masses come from a straightforward application of the Jeans theorem, with the usual (and perhaps unjustified) assumptions like isotropy. Much of the conclusion is based upon velocity dispersion measurements in the outer parts of the galaxies from small numbers of stars.

By: Sean

Sean — Thu, 09 Feb 2006 02:11:27 +0000

I think it’s just too difficult at this point to figure out what they’ve done, since there is a BBC article but no real paper. One thing to keep in mind is that “warm” dark matter often refers specifically to dark matter that was marginally relativistic when it first stopped interacting with other particles and dropped out of equilibrium — it doesn’t refer to the temperature today. In fact, you’d expect that the temperature of warm DM today would be similar to that of the CMB, if it weren’t for evolution later on (like grouping into halos etc.).

The basic point seems to be that the DM is less concentrated — more spread out — in these galaxies than you might have thought it should be. There are other indications that dark matter doesn’t clump that effectively on small scales (a lack of cusps at the center of galaxies, a lack of small satellite galaxies), but those are messy because ordinary matter is also playing a role. If it’s true that these systems have much more DM than ordinary matter, it might make them a useful testing ground for these questions.

By: Kea

Kea — Wed, 08 Feb 2006 22:15:01 +0000

“…then my understanding is that this idea fails, for example, to agree with nucleosynthesis constraints.”

Mannheim in a 1999 paper addresses the original 1993 Knox and Kosowsky issue with nucleosynthesis. Yes, it appears to be a problem, but not necessarily unresolvable.