Was Entropy Conserved between BBN and Recombination?

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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 $N_{\mathrm{eff}}$ 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 $\Gamma_Y > 1.91\times10^{-6} \text{s}^{-1}$ at $95\%$ confidence level (C.L.).