The limits of lattice inflation: a cautionary tale

arXiv:2606.14853 · astro-ph.CO, gr-qc, hep-ph, hep-th · Submitted 2026-06-12 · Read on arXiv

astro-ph.CO, gr-qc, hep-ph, hep-th

Submitted: 2026-06-12

Updated: 2026-08-27

Comments: 19 pages, 12 figures, 9 appendices; v2: improvements in response to community feedback, especially clarifying that small $ε$ provides no criterion for the validity of the FLRW approximation

Code: https://github.com/bengladwyn/SupplementalMaterials2606

License: http://creativecommons.org/licenses/by/4.0/

The gist: Cosmological lattice simulations have become important tools for studying non-perturbative dynamics in the early Universe.

Terminology

Abstract

Cosmological lattice simulations have become important tools for studying non-perturbative dynamics in the early Universe. Many widely used codes, however, approximate the gravitational background by an exact Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime and neglect metric perturbations. We show that, during inflation, this approximation prevents the freezing of superhorizon modes. During slow roll, the curvature power spectrum decays as H 4, while the deviation becomes substantially stronger during ultra-slow roll. As a result, inflationary observables can be significantly distorted. In contrast, reheating studies appear to be considerably less sensitive to the omission of metric perturbations. We propose a practical criterion for assessing the validity of FLRW simulations based on the inclusion of first-order metric perturbations, and implement it in CosmoLattice.

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