Raising the Optical Depth to Reionization with Dark Matter Decay

Abstract

DESI DR2 baryon acoustic oscillation (BAO) measurements have sharpened a discrepancy with cosmic microwave background (CMB) inferences that can be recast as a preference for $\tau_{\rm reio}\simeq0.09$, above the $\simeq0.06$ inferred from large-scale CMB polarization. We test whether electromagnetic energy injection from dark matter (DM) decaying through $\chi\to e^+e^-$ can supply this additional optical depth. Adopting an asymmetric, simulation-calibrated Gompertzian model for astrophysical reionization, we derive $\tau_{\rm reio}$ self-consistently from the free-electron history including DM decay. We jointly constrain cosmological and reionization parameters and the decay rate using ACT+Planck CMB temperature and polarization anisotropies, CMB lensing, BAO, and quasar damping-wing observations. For $m_\chi=1$ GeV, DM decay raises the marginalized mean optical depth from 0.064 to 0.072 under a flat decay-rate prior, without significantly improving the fit. Reweighting the chains with log-uniform priors lowers the mean optical depth, indicating prior dependence. The additional optical depth arises primarily from a broad ionization tail through the cosmic dark ages, rather than an earlier onset of astrophysical reionization. For DM masses from 2 MeV to 7 GeV, the 68% credible intervals remain below $\tau_{\rm reio}\simeq0.08$, a benchmark previously found sufficient to bring both the neutrino-mass tension and the exclusion of $\Lambda$CDM below $2\sigma$. The DM lifetime constraints complement photon, cosmic-ray, and gas-heating bounds and are competitive with previous CMB limits. DM decay therefore shifts the inferred optical depth upward, but the posterior-favored increase remains insufficient to substantially alleviate the CMB--BAO discrepancy.