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  • Letter
  • Open Access

Inflation without an inflaton

Daniele Bertacca1,2,3, Raul Jimenez4,5, Sabino Matarrese1,2,3,6, and Angelo Ricciardone7,8

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Phys. Rev. Research 7, L032010 – Published 8 July, 2025

DOI: https://doi.org/10.1103/vfny-pgc2

Abstract

We propose a novel scenario in which scalar perturbations, which seed the large-scale structure of the universe, are generated without relying on a scalar field (the inflaton). In this framework, inflation is driven by a de Sitter space time, where tensor metric fluctuations (i.e., gravitational waves) naturally arise from quantum vacuum oscillations, and scalar fluctuations are generated via second-order tensor effects. We compute the power spectrum of such scalar fluctuations and show it to be consistent with near scale invariance. We derive the necessary conditions under which scalar perturbations become significant and much larger than the tensor modes, and we identify a natural mechanism to end inflation via a transition to a radiation-dominated phase. Our proposed mechanism could remove the need for a model-dependent scenario: the choice of a scalar field, as the inflaton, to drive inflation.

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Phys. Rev. Research 7, L032010– Published 8 July, 2025
  • Received 21 December 2024
  • Accepted 11 June 2025
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