{"about":{"site":"https://codewithpapers.app","non_affiliation":"Code with Papers and Syntology are not affiliated with, endorsed by, or sponsored by Papers with Code, Meta, or the pwc-archive mirror.","licence":"CC BY-SA 4.0","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","attribution":"https://codewithpapers.app/attribution","modified":"archive material modified by Syntology; see the attribution page"},"url":"/paper/did-the-universe-reheat-after-recombination","title":"Did the Universe Reheat After Recombination?","arxiv_id":"2304.03750","date":"2023-04-07","proceeding":null,"authors":["J. Colin Hill","Boris Bolliet"],"abstract":"A key assumption of the standard cosmological model is that the temperature of the cosmic microwave background (CMB) radiation scales with cosmological redshift $z$ as $T_{\\rm CMB}(z) \\propto (1+z)$ at all times after recombination at $z_\\star \\simeq 1090$. However, this assumption has only been precisely tested at $z \\lesssim 3$. Here, we consider cosmological models with post-recombination reheating (PRR), in which the CMB monopole temperature abruptly increases due to energy injection after last scattering. Such a scenario can potentially resolve tensions between inferences of the current cosmic expansion rate (the Hubble constant, $H_0$). We consider an explicit model in which a metastable sub-component of dark matter (DM) decays to Standard Model photons, whose spectral energy distribution is assumed to be close to that of the CMB blackbody. A fit to Planck CMB anisotropy, COBE/FIRAS CMB monopole, and SH0ES distance-ladder measurements yields $H_0 = 71.2 \\pm 1.1$ km/s/Mpc, matter fluctuation amplitude $S_8 = 0.774 \\pm 0.018$, and CMB temperature increase $\\delta T_{\\rm CMB} = 0.109^{+0.033}_{-0.044}$ K, which is sourced by DM decay at $z \\gtrsim 10$. However, matter density constraints from baryon acoustic oscillation and supernovae data highly constrain this scenario, with a joint fit to all datasets yielding $H_0 = 68.69 \\pm 0.35$ km/s/Mpc, $S_8 = 0.8035 \\pm 0.0081$, and $\\delta T_{\\rm CMB} < 0.0342$ K (95% CL upper limit). These bounds can be weakened if additional dark relativistic species are present in the early universe, yielding higher $H_0$. We conclude that current data disfavor models with significant PRR solely through its impact on background and linear-theory observables, completely independent of CMB spectral distortion constraints. However, a small amount of such energy injection could play a role in restoring cosmological concordance.","url_abs":"https://arxiv.org/abs/2304.03750v1","url_pdf":"https://arxiv.org/pdf/2304.03750v1.pdf","source":{"archive":"pwc-archive (Hugging Face), CC BY-SA 4.0","snapshot":"2025-07-28","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","row_kind":"links_only","authors_date_abstract":"arXiv metadata, CC0 1.0 (https://info.arxiv.org/help/license), from the Kaggle arXiv metadata snapshot of 2026-09-12"},"code_links":[{"paper_slug":"did-the-universe-reheat-after-recombination","repo_url":"https://github.com/CobayaSampler/cobaya","is_official":1,"mentioned_in_paper":1,"mentioned_in_github":0,"framework":"none","reach":null}],"tasks":[],"methods":[],"datasets_introduced":[],"methods_introduced":[],"results":[],"syntology":{"syntology_url":null,"atlas_url":null,"mcp":null,"developers":"https://syntology.ai/developers"},"arxiv_metadata":null,"syntology_extracted_results":null}