One-loop quantum gravitational backreaction on the local Hubble rate

TL;DR

This paper calculates gauge-invariant quantum corrections to the Hubble rate caused by graviton loops in different cosmological backgrounds, revealing that quantum effects can influence the universe's expansion rate.

Contribution

It provides explicit gauge-invariant expressions for quantum gravitational corrections to the Hubble rate in various cosmological eras, including inflation.

Findings

Quantum corrections are gauge-invariant.

Corrections can be expressed via a quantum-corrected slow-roll parameter.

Quantum effects tend to make spacetime more de Sitter-like.

Abstract

We determine corrections to the Hubble rate due to graviton loops in a cosmological background spacetime of constant deceleration parameter. The corrections are gauge-invariant, based on a recent proposal for all-order gauge-invariant observables in perturbative quantum gravity. We find explicit expressions for the cases of matter- and radiation-dominated eras and slow-roll inflation with vanishing second slow-roll parameter. Interestingly, in the latter case the corrections can be described by a quantum-corrected first slow-roll parameter, which brings the spacetime closer to de Sitter space.