4.1. Comparison Of BBN With Non-BBN Baryon Density Estimates
Having established the internal consistency of primordial nucleosynthesis in the standard model, it is necessary to proceed to the next key test. Does the nucleon abundance inferred from processes which occurred during the first thousand seconds of the evolution of the Universe agree with estimates/bounds to the nucleon density in the present Universe?
It is a daunting task to attempt to inventory the baryons in the Universe.
Since many (most?) baryons may be ``dark'', such approaches can best set
lower bounds to the present ratio of baryons-to-photons. One such
estimate [22]
suggests a very weak lower bound on
of:
10
0.25, entirely consistent with the BBN
estimates above. Others
[23]
have used more subjective (although cautious) estimates of
the uncertainties, finding a much higher lower bound to the global budget
of baryons: 10
1.5, which is still consistent with the
``low-'' range
identified using the high-D results.
A possible challenge to the
``low-'' case comes
from an analysis
[24]
which employed observational constraints on the Hubble parameter,
the age of the Universe, the structure-formation ``shape'' parameter, and
the X-ray cluster gas fraction to provide non-BBN constraints on the present
density of baryons, finding that
10
5 may be favored over
10
2. Even so, a significant
low-, high-D range still
survives.