4.1. Deuterium - The Baryometer Of Choice
Recall that D is produced (in an astrophysically interesting
abundance) ONLY during BBN. The predicted primordial
abundance is sensitive to the baryon density (D/H
-1.6).
Furthermore, during post-BBN evolution, as
gas is cycled through stars, deuterium is ONLY destroyed,
so that for the "true" abundance of D anywhere, at any time,
(D/H)P 
(D/H)TRUE. That's the
good news. The bad news is that the spectra of
HI and DI
are identical, except for the wavelength/velocity shift in
their spectral lines. As a result, the true D-abundance may
differ from that inferred from the observations if some
of the presumed DI is actually an
HI interloper masquerading
as DI: (D/H)TRUE
(D/H)OBS.
Because of these opposing effects, the connection between
(D/H)OBS and (D/H)P is not predetermined;
the data themselves which must tell us how to relate the two.
With this caveat in mind, it will be assumed that the value
of (D/H)P identified above is a fair estimate of the
primordial D abundance. Using it, the SBBN-predicted baryon
abundance may be determined. The result of this comparison
is shown in Figure 9 where, approximately, the
overlap between
the SBBN-predicted band and that from the data fix the allowed
range of .
 |
Figure 9. The diagonal band is the
SBBN-predicted deuterium abundance (by number relative to hydrogen) as
a function of the nucleon-to-photon ratio
|
range
in the adopted primordial deuterium abundance. The vertical
band is, approximately, the corresponding SBBN-predicted