{
  "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 $τ_{\\rm reio}\\simeq 0.09$, 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 $χ\\rightarrow e^{+}e^{-}$ 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 $τ_{\\rm reio}$ 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 $m_χ=1\\,{\\rm GeV}$, DM decay raises the marginalized optical depth from $τ_{\\rm reio}=0.064$ 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 $m_χ=2\\,{\\rm MeV}$--$7\\,{\\rm GeV}$, the central $68\\%$ credible intervals remain below the $τ_{\\rm reio}\\simeq 0.08$ level previously found sufficient to bring both the neutrino-mass tension and the exclusion of $Λ$CDM 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.39923",
  "author_total": 5,
  "authors": [
    {
      "name": "Yu-Ning Wang"
    },
    {
      "name": "Paulo Montero-Camacho"
    },
    {
      "name": "Ziwei Wang"
    },
    {
      "name": "Yin Li"
    },
    {
      "name": "Yue-Lin Sming Tsai"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/b42evwp3n5y66.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.39923",
  "fulltext_url": null,
  "id": "b42evwp3n5y66",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.39923",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.CO"
    ],
    [
      "openalex",
      "3103|Cosmology and Gravitation Theories"
    ]
  ],
  "openalex": "W7215098264",
  "pdf_url": "https://arxiv.org/pdf/2609.39923",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-30",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/b42evwp3n5y66.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:49Z",
      "native_id": "2609.39923",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.39923"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.CO",
      "subfield": "cosmology"
    }
  ],
  "title": "Raising the Optical Depth to Reionization with Dark Matter Decay",
  "title_text": "Raising the Optical Depth to Reionization with Dark Matter Decay",
  "type": "preprint",
  "updated_at": "2026-11-05T15:20:17Z",
  "url": "https://research.draconitic.com/paper/b42evwp3n5y66",
  "venue": "arXiv",
  "versions": []
}