| Annu. Rev. Astron. Astrophys. 1994. 32:
531-590
Copyright © 1994 by Annual Reviews. All rights
reserved
|
3.4. Variants of the Baryonic Dark Matter Scenario
One may consider three variants of Baryonic Dark Matter (BDM) scenario,
depending on how strongly one wishes to retain homogeneous primordial
nucleosynthesis, inflation, and nonbaryonic dark matter:
- In the standard BDM scenario, one retains all three
assumptions, which requires
b
0.01
h-2 and
= 1. In this case,
we conclude that 70%
to 95% of baryons are dark but MACHOs alone can provide galactic halos
only if Rh < 50 h-1 kpc. The cluster
and critical density dark matter must
be WIMPs and, if they are cold, the halo dark matter is expected to be a
mixture of MACHOS and WIMPS. This conclusion pertains even for
< 1,
as may be required by the large baryon fraction in clusters.
- In the maximal BDM scenario, one assumes that the Universe
has a critical density of baryons
(B =
= 1), thereby
relinquishing the need for
nonbaryonic dark matter without giving up inflation
(Fowler 1990).
The scenario is inconsistent with homogeneous nucleosynthesis unless one
invokes unrealistically low values of H0. When the
upper limit on
b was
0.06 h-2 s
(Yang et al 1984),
it was possible to have
b = 1 by
invoking the only moderately extreme value H0 = 25
(Shanks 1985),
but the new upper limit would require H0 = 10, which
is probably absurd [see, however,
Harrison (1993)
for a contrary view].
- In the baryon-dominated scenario, one only assumes the
existence of the
dark matter for which there is direct dynamical evidence and attributes this
solely to baryons
(b =
0.1). In this case,
one has to give up both
inflation and homogeneous nucleosynthesis. In order to explain the observed
light element abundances, one then has to invoke some exotic astrophysical
process, such as the spallation of primordial helium by high energy photons
from accreting black holes
(Gnedin & Ostriker 1992).
The viability of this
scenario also depends on whether the isocurvature baryon-dominated model
is compatible with the CMB anisotropy constraints.
Most of the emphasis in this review is on the standard BDM scenario but, in
assessing which baryonic candidates are viable, it is important to bear
in mind the more radical proposals.