Black holes: best candidate for dark matter?

[Loren Booda]

Black holes are constructed from ordinary matter. Their charge to mass ratio, a measure of photonic interaction, is comparatively small. They have been indirectly (gravitationally) observed. They are common in the halos of galaxies where older, Population II stars have hereto collapsed, and where the telltale circular rotation indicates. Potential galactic black holes, formed in the early universe, could well occupy the halo.

 

[Garth]

Hi Loren!
Could DM be BHs? This is a qualified yes, they are a favourite candidate of mine!

A population of IMBHs (Intermediate Mass Black Holes) of mass 10² Mass_Solar - 10⁴ Mass_Solar might not have been detected and could fit the bill of DM.

However two major problems.
1. They would have formed from baryonic material and the standard model BBN can produce only 4% closure density whereas we need another 23% for DM.

2. Their formation would most likely not be 100% efficient and so a lot of unbound mass would still be around.

Do BHs then fit the DM bill?

They could go a long way to making up the 23% if the BBN restriction was lifted. This cannot happen in ordinary BBN, however if the expansion of the universe happened in a strictly linear fashion then it would fit the bill. The “Freely Coasting” Cosmology model requires about 20% or so baryonic density to produce the correct element abundances. (All except deuterium which has to be made from spallation).

The BHs would mostly then be the end product of the PopIII stars widely believed to have pre-existed galactic formation.

They would probably leave behind about half their mass, which might now be in the form of WHIM (Warm-Hot Intergalactic Medium). Enough WHIM (with very large error bars) has been detected to allow for this: Measured Cosmological Mass Density in the WHIM: the Solution to the ’Missing Baryons’ Problem, their upper limit of WHIM is:
$\Omega_b$^WHIM > 4.3 × 10^0.47 % = 12.6%
and their lower WHIM limit is:
$\Omega_b$^WHIM > 1.3 × 10^0.32 % = 2.7%?

Which is only just consistent (by stretching a point) with the standard model of about $\Omega_b$ = 0.04 of which $\Omega_b$(missing) = 2.1 +0.5/−0.4) % (Nicastro et al., A Warm-Hot Intergalactic Medium Location for the Missing Cosmic Baryons, Nature, accepted for publication (2004):). That is, the lower limit on the WHIM observed is slightly more than the standard model can allow.

But also note this is consistent with a much higher $\Omega_{WHIM}$ allowed by the Freely Coasting Model BBN $\Omega_b$ ~ 0.2

In this scenario half the DM is now WHIM and half IMBHs ~ 10³ Mass_Solar.

Garth

 

[SpaceTiger]

Black holes are constructed from ordinary matter.

Actually, if this were always true, it would make them a terrible dark matter candidate (except for Garth, who needs more ordinary matter for his theory). In standard $\Lambda CDM$ and its close relatives, the dark matter needs to be non-baryonic in order to satisfy the constraints we have from nucleosynthesis and the CMB. Since the dark matter is thought to be weakly-interacting, it's hard to imagine how it could have collected into black holes.

Black holes don't, however, need to form from matter. Many believe that there could be a sizable population of primordial black holes; that is, black holes that formed in the early universe. Since the early universe is radiation-dominated, this means that primordial black holes would likely have formed primarily from radiation.

Even given the possibility of primordial black holes, however, I think few (if any) astronomers would put their money on black holes as the dark matter. We've already ruled out a large part of parameter space ($M_{PBH}<10^{15} g$, $M_{PBH}>10^4 M_{sun}$, $M\sim M_{sun}$) and most theoretical calculations suggest that PBHs would form with a bottom-heavy mass distribution. Furthermore, accretion onto these things might cause problems for reionization.

If nucleosynthesis and WMAP interpretations were wrong, one can still ask whether or not the dark matter BHs could have formed from Pop III stars. In conventional theories of structure formation (and even stellar evolution), this would be an extremely difficult task. Remember that 90% of all matter in the universe is dark matter -- that means you'd need 90% of the universe to have formed heavy Pop III stars at a very early epoch. Given that we see virtually no low-mass remnants of such an extreme star formation event, that seems very unlikely.