Comprehensive constraints on dark radiation injection after BBN

We derive constraints on the injection of free-streaming dark radiation after big bang nucleosynthesis by considering the decay of a massive hidden sector particle into dark radiation. Such a scenario has the potential to alleviate the Hubble tension by introducing a new energy component to the evolution of the early Universe. We employ observations of the cosmic microwave background (CMB) from Planck 2018 and South Pole Telescope 2018, measurements of the primordial deuterium abundance, $\mathrm{Pantheon}+$ type Ia supernovae data, and baryon acoustic oscillation measurements from BOSS DR12 to constrain these decay scenarios. Prerecombination decays are primarily restricted by observations of the CMB via their impact on the effective number of relativistic species. On the other hand, long-lived decay scenarios in which the massive particle lifetime extends past recombination tend to decrease the late-time matter density inferred from the CMB and are thus subject to constraints from $\mathrm{Pantheon}+$ and baryon acoustic oscillations. We find that, when marginalizing over lifetimes of ${\tau }_Y=[{10}^{-12.08},{10}^{-1.49}]\mathrm{Gyr}$, the decaying particle is limited at $2\sigma$ to only contribute a maximum of 3% of the energy density of the Universe. With limits on these decays being so stringent, neither short-lived nor long-lived scenarios are successful at substantially mitigating the Hubble tension.