  We discuss bounds on the cosmological relativistic energy density as a
function of redshift, reviewing the big bang nucleosynthesis and cosmic
microwave background bounds, updating bounds from large scale structure, and
introducing a new bound from the magnitude-redshift relation of Type Ia
supernovae. We conclude that the standard and well-motivated assumption that
relativistic energy is negligible during recent epochs is not necessitated by
extant data. We then demonstrate the utility of these bounds by constraining
the mass and lifetime of a hypothetical massive big bang relic particle.
