One-loop correction to primordial tensor modes during radiation era

TL;DR

This paper calculates the one-loop quantum correction to primordial tensor modes during the radiation era, revealing a growing effect that challenges perturbation theory and suggests further investigation into early universe physics.

Contribution

It provides the first computation of one-loop corrections to tensor modes in the radiation era, highlighting potential non-perturbative effects on primordial gravitational waves.

Findings

One-loop correction grows over time during radiation era.

The correction is not suppressed by small parameters.

Perturbation theory becomes unreliable due to this growth.

Abstract

The ability to infer properties of primordial inflation relies on the conservation of the superhorizon perturbations between their exit during inflation, and their re-entry during radiation era. Any considerable departure from this property would require reinterpreting the data. This is why it is important to understand how superhorizon perturbations interact with the thermal plasma driving the radiation dominated Universe. We model the plasma by free photons in a thermal state and compute the one-loop correction to the power spectrum of primordial tensor perturbations. This correction grows in time and is not suppressed by any small parameter. While one-loop result is not reliable because it invalidates perturbation theory, it signals potentially interesting effects that should be investigated further.