Was entropy conserved between BBN and recombination

Abstract

We test the assumption of entropy conservation between big bang nucleosynthesis and recombination by considering a massive particle that decays into a mixture of photons and other relativistic species. We employ Planck temperature and polarization anisotropies, COBE/FIRAS spectral distortion bounds, and the observed primordial deuterium abundance to constrain these decay scenarios. If between 56% and 71% of the decaying particle's energy is transferred to photons, then Neff at recombination is minimally altered, and Planck data alone allows for significant entropy injection. If photons are injected by the decay, the addition of spectral distortion bounds restricts the decay rate of the particle to be ΓY>1.91×10-6 s at 95% C.L. We find that constraints on the energy density of the decaying particle are significantly enhanced by the inclusion of bounds on the primordial deuterium abundance, allowing the particle to contribute at most 2.35% (95% C.L.) of the energy density of the Universe before decaying.