Raising the Optical Depth to Reionization with Dark Matter Decay

Abstract

DESI DR2 baryon acoustic oscillation measurements have sharpened a discrepancy with cosmic microwave background (CMB) inferences that can be recast as a preference for a reionization optical depth of $\tau_{\rm reio}\simeq 0.09$, well above the $\simeq 0.06$ inferred from large-scale CMB polarization measurements. Here we test whether electromagnetic energy injection from dark matter (DM) decaying through $\chi\rightarrow e^{+}e^{-}$ can provide this additional optical depth. Adopting a Gompertzian model for astrophysical reionization, which provides an asymmetric, simulation-calibrated description of cosmic reionization, we derive $\tau_{\rm reio}$ self-consistently from the evolution of the free-electron fraction, including the contribution from DM decay. We jointly constrain cosmological and reionization parameters and the DM decay rate using ACT+Planck CMB temperature and polarization anisotropies, CMB lensing, BAO measurements, and quasar damping-wing observations. For $m_\chi=1\,{\rm GeV}$, DM decay raises the marginalized optical depth from $\tau_{\rm reio}=0.064$ to $0.072$, but does not significantly improve the fit. The additional optical depth arises primarily from a broad ionization tail extending through the cosmic dark ages, rather than from an earlier onset of astrophysical reionization. Across $m_\chi=2\,{\rm MeV}$--$7\,{\rm GeV}$, the central $68\%$ credible intervals remain below the $\tau_{\rm reio}\simeq 0.08$ level previously found sufficient to bring both the neutrino-mass tension and the exclusion of $\Lambda$CDM below the $2\sigma$ level. The corresponding DM lifetime constraints are complementary to cosmic-ray and gas-heating bounds and competitive with previous CMB limits. Decaying DM therefore shifts the inferred optical depth in the direction required to ease the CMB--BAO discrepancy, but the allowed increase remains insufficient to alleviate it substantially.