5.5.5. Why we believe that the D/H is Primordial
Here we review why we believe that the low D/H is
primordial. These arguments are best made without reference to the other
nuclei made in BBN, because we wish to use the abundances of these nuclei
to test SBBN theory.
- D/H is known to be low in four widely separated
towards three quasars, and in the ISM of our Galaxy.
- The extraction of D/H from quasar spectra is
except for corrections for contamination by H, which make D/H look too
- Since contamination is common, all data are
consistent with low D/H, and no data require high D/H.
- High D/H is rare, or non-existent, because it
should be easy to see
in many existing spectra, but we have no secure examples.
- The low D/H in the quasars, pre-solar system and
in the ISM today are
all consistent with Galactic chemical evolution.
- The quasar absorption systems are large - many
kpc across, as was
the initial volume of gas which collapsed to make our Galaxy.
- The abundance of the metals in the quasar cases
are very low,
and much too low for significant (> 1%) destruction of D in stars.
- The quasar absorbers are observed at high
redshifts, when the universe is
too young for low mass stars to have evolved to a stage where they
eject copious amounts of gas.
- The ratio of the abundances of Si/C in the
absorbers is normal for
old stars in the halo of our galaxy, indicating that these elements
were made in normal stars.
- In the quasar absorbers, the temperatures and
velocities are low, which
argues against violent events immediately prior to the absorption.
- If BBN D/H were high, the hypothetical destruction
of D would have to
reduce D/H by similar large amounts in all four places.
- The above observations make local destruction of D
- There are no known processes which can make or
destroy significant D.
- Global destruction of D by photodissociation in
the early universe
requires very specific properties for a hypothetical particle, and
is limited by other measures of