Backreaction during inflation: a physical gauge invariant formulation

TL;DR

This paper develops a gauge-invariant framework to analyze how scalar fluctuations during inflation influence the universe's expansion, revealing conditions where certain observers detect no backreaction effects.

Contribution

It introduces a gauge-invariant method to compute cosmological backreaction during inflation, clarifying observer-dependent effects on the effective Hubble rate and equation of state.

Findings

Geodetic observers experience no scalar backreaction under certain conditions.

The gauge-invariant approach ensures physical results independent of coordinate choices.

Scalar fluctuations can influence the effective dynamics of the universe during inflation.

Abstract

Within a genuinely gauge invariant approach recently developed for the computation of the cosmological backreaction, we study, in a cosmological inflationary context and with respect to various observers, the impact of scalar fluctuations on the space-time dynamics in the long wavelength limit. We stress that such a quantum backreaction effect is evaluated in a truly gauge independent way using a set of effective equations which describe the dynamics of the averaged geometry. In particular we show under what conditions the free falling (geodetic) observers do not experience any scalar-induced backreaction in the effective Hubble rate and fluid equation of state.