{
  "abstract": "General relativity coupled to multiple scalar fields is a diffeomorphism-invariant constrained system. Consequently, a naive counting of the perturbative degrees of freedom unavoidably overestimates the true number of physical modes propagating in the theory, as gauge redundancies and constraint equations remove non-dynamical ones. While this problem has been solved for linear fluctuations, this work presents the first explicit calculation of all large-scale gauge-invariant phase-space variables in multifield inflation and at second order in perturbation theory, in a Hamiltonian language. Building upon the well-known Sasaki-Mukhanov variables, we show how to construct a finite-dimensional basis of quadratic corrections which are invariant under gauge transformations. Although our procedure is generic to any number of fields and at any scale, we restrict to super-Hubble scales for their explicit solution, which we deliver. Henceforth, we prove that it is possible to recover the usual flat-gauge and comoving-gauge fluctuations as large-scale gauge-invariant combinations, making for a robust consistency check of the gauge-fixed procedure to connect theoretical predictions above the horizon to observations. We derive the quadratic and cubic Hamiltonian of multifield inflation in a gauge independent manner, then we gauge fix our theory by going into the flat gauge, and we show perfect agreement with the literature on this topic, usually based on a Lagrangian approach. After these concrete steps, we propose a more formal proof of the existence of gauge-invariant variables at quadratic order, and we provide a sketch of the procedure that should allow to go to higher orders in perturbation theory.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2608.28337",
  "author_total": 3,
  "authors": [
    {
      "name": "Julien Grain"
    },
    {
      "name": "Hugo Holland"
    },
    {
      "name": "Lucas Pinol"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/ey0p5fzcfykyy.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2608.28337",
  "fulltext_url": null,
  "id": "ey0p5fzcfykyy",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2608.28337",
  "language": "en",
  "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",
      "gr-qc"
    ],
    [
      "arxiv",
      "hep-th"
    ],
    [
      "hal",
      "0.phys"
    ],
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    ],
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      "hal",
      "1.phys.grqc"
    ],
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    [
      "openalex",
      "3103|Cosmology and Gravitation Theories"
    ]
  ],
  "openalex": "W7204772722",
  "pdf_url": "https://arxiv.org/pdf/2608.28337",
  "pmcid": null,
  "pmid": null,
  "published": "2026-08-28",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/ey0p5fzcfykyy.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:17:22Z",
      "native_id": "2608.28337",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2608.28337"
    },
    {
      "first_seen": "2026-10-30T12:17:31Z",
      "native_id": "hal-05742547",
      "source": "hal",
      "url": "https://hal.science/hal-05742547v1"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.CO",
      "subfield": "cosmology"
    },
    {
      "field": "physics",
      "origin": "arxiv:gr-qc",
      "subfield": "gravitation"
    },
    {
      "field": "physics",
      "origin": "arxiv:hep-th",
      "subfield": "high-energy"
    }
  ],
  "title": "Explicit gauge-invariant variables in multifield inflation beyond linear order and Hamiltonian dynamics",
  "title_text": "Explicit gauge-invariant variables in multifield inflation beyond linear order and Hamiltonian dynamics",
  "type": "preprint",
  "updated_at": "2026-10-30T12:17:31Z",
  "url": "https://research.draconitic.com/paper/ey0p5fzcfykyy",
  "venue": "arXiv",
  "versions": []
}