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Did the Universe Reheat After Recombination?
J. Colin Hill, Boris Bolliet
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A key assumption of the standard cosmological model is that the temperature of the cosmic microwave background (CMB) radiation scales with cosmological redshift z as T_(CMB)(z) ∝(1+z) at all times after recombination at z_⋆ ≃1090. However, this assumption has only been precisely tested at z ≲3. Here, we consider cosmological models with post-recombination reheating (PRR), in which the CMB monopole temperature abruptly increases due to energy injection after last scattering. Such a scenario can potentially resolve tensions between inferences of the current cosmic expansion rate (the Hubble constant, H₀). We consider an explicit model in which a metastable sub-component of dark matter (DM) decays to Standard Model photons, whose spectral energy distribution is assumed to be close to that of the CMB blackbody. A fit to Planck CMB anisotropy, COBE/FIRAS CMB monopole, and SH0ES distance-ladder measurements yields H₀ = 71.2 ±1.1 km/s/Mpc, matter fluctuation amplitude S₈ = 0.774 ±0.018, and CMB temperature increase δT_(CMB) = 0.109^(+0.033)_(-0.044) K, which is sourced by DM decay at z ≳10. However, matter density constraints from baryon acoustic oscillation and supernovae data highly constrain this scenario, with a joint fit to all datasets yielding H₀ = 68.69 ±0.35 km/s/Mpc, S₈ = 0.8035 ±0.0081, and δT_(CMB) < 0.0342 K (95% CL upper limit). These bounds can be weakened if additional dark relativistic species are present in the early universe, yielding higher H₀. We conclude that current data disfavor models with significant PRR solely through its impact on background and linear-theory observables, completely independent of CMB spectral distortion constraints. However, a small amount of such energy injection could play a role in restoring cosmological concordance.
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