We present new upper and lower bounds to the primordial abundances of
deuterium and helium-3 based on observational data from the solar system and
the interstellar medium. Independent of any model for the primordial production
of the elements we find (at the 95\% C.L.): 1.5Γ10β5β€(D/H)Pββ€10.0Γ10β5 and (3He/H)Pββ€2.6Γ10β5. When combined with
the predictions of standard big bang nucleosynthesis, these constraints lead to
a 95\% C.L. bound on the primordial abundance of deuterium: (D/H)bestβ=(3.5β1.8+2.7β)Γ10β5. Measurements of deuterium absorption in the
spectra of high redshift QSOs will directly test this prediction. The
implications of this prediction for the primordial abundances of helium-4 and
lithium-7 are discussed, as well as those for the universal density of baryons.Comment: Revised version of paper to reflect comments of the referee and reply
to suggestions of Copi, Schramm, and Turner regarding the overall analysis
and treatment of chemical evolution of D and He-3. Best-fit D/H abundance
changes from (2.3 + 3.0 - 1.0)x10^{-5} to (3.5 +2.7 - 1.8) x10^{-5}. See also
hep-ph/950531