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 locations: towards three quasars, and in the ISM of our Galaxy.

• The extraction of D/H from quasar spectra is extremely direct, except for corrections for contamination by H, which make D/H look too large.

• 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 unlikely.

• 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 _b .