4.2. SBBN Baryon Density - The Baryon Density At 20 Minutes
The universal abundance of baryons which follows from
SBBN and our adopted primordial D-abundance is:
10 =
5.6+0.6-1.2
(
B
h2 = 0.020+0.002-0.004). For the
HST Key Project recommended value for H0
(h = 0.72 ± 0.08;
Freedman et al. 2001),
the fraction of the present universe critical density
contributed by baryons is small,
B
0.04. In
Figure 10 is shown a comparison among the
various determinations of the present mass/energy density
(as a fraction of the critical density), baryonic as well
as non-baryonic. It is clear from Figure 10
that the present universe (z
1)
baryon density inferred from SBBN far exceeds that inferred from
emission/absorption observations
(Persic & Salucci
1992,
Fukugita, Hogan &
Peebles 1998).
The gap between the upper bound to
luminous baryons and the BBN band is the "dark baryon
problem": at present, most of the baryons in the
universe are dark. Evidence that although dark, the
baryons are, indeed, present comes from the absorption observed in the
Ly
forest at redshifts
z 2 - 3 (see,
e.g.
Weinberg et al. 1997).
The gap between the BBN band and the band labelled by
M
is the "dark matter problem": the mass density inferred
from the structure and movements of the galaxies and
galaxy clusters far exceeds the SBBN baryon contribution.
Most of the mass in the universe must be nonbaryonic.
Finally, the gap from the top of the
M
band to = 1 is the
"dark energy problem".
 |
Figure 10. The various contributions to the
present universal mass/energy density, as a fraction of the critical
density
( |