Raising the Optical Depth to Reionization with Dark Matter Decay

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 ${\tau }_{\mathrm{r}\mathrm{e}\mathrm{i}\mathrm{o}}\simeq 0.09$, 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 $\chi \to e^{+}{e}^{-}$ 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 ${\tau }_{\mathrm{r}\mathrm{e}\mathrm{i}\mathrm{o}}$ 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 $m_{\chi }=1\mathrm{G}\mathrm{e}\mathrm{V}$, DM decay raises the marginalized optical depth from ${\tau }_{\mathrm{r}\mathrm{e}\mathrm{i}\mathrm{o}}=0.064$ 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 $m_{\chi }=2\mathrm{M}\mathrm{e}\mathrm{V}$–$7\mathrm{G}\mathrm{e}\mathrm{V}$, the central $68\mathrm{\%}$ credible intervals remain below the ${\tau }_{\mathrm{r}\mathrm{e}\mathrm{i}\mathrm{o}}\simeq 0.08$ level previously found sufficient to bring both the neutrino-mass tension and the exclusion of $\mathrm{\Lambda }$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.