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We derive constraints on the injection of free-streaming dark radiation afterbig bang nucleosynthesis (BBN) by considering the decay of a massive hiddensector particle into dark radiation. Such a scenario has the potential toalleviate the Hubble tension by introducing a new energy component to theevolution of the early universe. We employ observations of the cosmic microwavebackground (CMB) from $\textit{Planck}$ 2018 and SPT-3G, measurements of theprimordial deuterium abundance, Pantheon+ Type Ia supernovae data, and baryonacoustic oscillation (BAO) measurements from BOSS DR12 to constrain these decayscenarios. Pre-recombination decays are primarily restricted by observations ofthe CMB via their impact on the effective number of relativistic species. Onthe other hand, long-lived decay scenarios in which the massive particlelifetime extends past recombination tend to decrease the late-time matterdensity inferred from the CMB and are thus subject to constraints fromPantheon+ and BAO. We find that, when marginalizing over lifetimes of $\tau_Y =[10^{-12.08}, 10^{-1.49}]$ 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 norlong-lived scenarios are successful at substantially mitigating the Hubbletension.

Comprehensive Constraints on Dark Radiation Injection After BBN