{
  "abstract": "The primordial scalar power spectrum provides a powerful window onto early-universe physics, including a broad class of inflationary scenarios. These models often predict a characteristic scalar spectral index $(n_s)$, nonzero running $(α_s)$, or more general deviations from a pure power-law form. We find that freeing the number of light relic species $(N_{\\rm eff})$ and the sum of the neutrino masses $(\\sum m_ν)$ broadens the $n_s$ error contours enough that key early-Universe models are no longer excluded. In particular, while both Starobinsky inflation and the Bi-thermal Big Bang model are ruled out at about the 95% confidence level in the six-parameter $Λ$CDM model, we find that in the nine-parameter $Λ{\\rm CDM} + α_s + N_{\\rm eff} + \\sum m_ν$ model, CMB data from Planck, ACT, and SPT (CMB-PAS) combined with DESI DR2 give $n_s = 0.9758 \\pm 0.0064$ and $α_s = 0.0080 \\pm 0.0064$, consistent with both models. Future CMB facilities will sharply improve constraints on $n_s$, $α_s$, and, more generally, the binned primordial power spectrum $\\mathcal{P}(k)$. For example, CMB-PAS constrains $e^{-2τ}\\mathcal{P}(k = 0.2\\,{\\rm Mpc}^{-1})$ to 0.5%, while SO-like and CMB-HD-like surveys would tighten this to 0.1%; CMB-HD would also extend this measurement to smaller, previously unprobed scales, constraining $e^{-2τ}\\mathcal{P}(k = 30\\,{\\rm Mpc}^{-1})$ to within a factor of ten. We release our forecast code and update the public CMB-HD likelihood and Fisher codes to support CLASS in addition to CAMB.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.35964",
  "author_total": 7,
  "authors": [
    {
      "name": "Zachary Cheslog"
    },
    {
      "name": "Emily Finson"
    },
    {
      "name": "Amanda MacInnis"
    },
    {
      "name": "Neelima Sehgal"
    },
    {
      "name": "Niayesh Afshordi"
    },
    {
      "name": "Simran K. Nerval"
    },
    {
      "name": "Renée Hložek"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/93gy07zxdvt57.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.35964",
  "fulltext_url": null,
  "id": "93gy07zxdvt57",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.35964",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.CO"
    ],
    [
      "arxiv",
      "hep-ph"
    ],
    [
      "arxiv",
      "hep-th"
    ],
    [
      "openalex",
      "3103|Cosmology and Gravitation Theories"
    ]
  ],
  "openalex": "W7214932758",
  "pdf_url": "https://arxiv.org/pdf/2609.35964",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-28",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/93gy07zxdvt57.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:31Z",
      "native_id": "2609.35964",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.35964"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.CO",
      "subfield": "cosmology"
    },
    {
      "field": "physics",
      "origin": "arxiv:hep-ph",
      "subfield": "high-energy"
    }
  ],
  "title": "Current and Future Constraints on the Primordial Power Spectrum",
  "title_text": "Current and Future Constraints on the Primordial Power Spectrum",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:31Z",
  "url": "https://research.draconitic.com/paper/93gy07zxdvt57",
  "venue": "arXiv",
  "versions": []
}