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 τreio≃ 0.09, well above the ≃ 0.06 inferred from large-scale CMB polarization measurements. Here we test whether electromagnetic energy injection from dark matter (DM) decaying through χ→ 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 τreio 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 mχ=1 GeV, DM decay raises the marginalized optical depth from τreio=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 mχ=2 MeV--7 GeV, the central 68% credible intervals remain below the τreio≃ 0.08 level previously found sufficient to bring both the neutrino-mass tension and the exclusion of ΛCDM below the 2σ 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.
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BibTeX
@misc{wang2026raising,
title = {{Raising the Optical Depth to Reionization with Dark Matter Decay}},
author = {Yu-Ning Wang and Paulo Montero-Camacho and Ziwei Wang and Yin Li and Yue-Lin Sming Tsai},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.39923},
eprint = {2609.39923},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.39923},
}
RIS
TY - UNPB
TI - Raising the Optical Depth to Reionization with Dark Matter Decay
AU - Yu-Ning Wang
AU - Paulo Montero-Camacho
AU - Ziwei Wang
AU - Yin Li
AU - Yue-Lin Sming Tsai
PY - 2026
DA - 2026/09/30
PB - arXiv
DO - 10.48550/arxiv.2609.39923
UR - https://arxiv.org/abs/2609.39923
AB - 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 τ_reio≃ 0.09, well above the ≃ 0.06 inferred from large-scale CMB polarization measurements. Here we test whether electromagnetic energy injection from dark matter (DM) decaying through χ→ 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 τ_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_χ=1 GeV, DM decay raises the marginalized optical depth from τ_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_χ=2 MeV--7 GeV, the central 68% credible intervals remain below the τ_reio≃ 0.08 level previously found sufficient to bring both the neutrino-mass tension and the exclusion of ΛCDM below the 2σ 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.
ER -