Backreaction of super-Hubble cosmological perturbations beyond perturbation theory

Robert Brandenberger,Gian Paolo Vacca,Giovanni Marozzi,Leila L Graef

doi:10.1103/physrevd.98.103523

Abstract

We discuss the effect of super-Hubble cosmological fluctuations on the locally measured Hubble expansion rate. We consider a large bare cosmological constant in the early universe in the presence of scalar field matter (the dominant matter component), which would lead to a scale-invariant primordial spectrum of cosmological fluctuations. Using the leading order gradient expansion we show that the expansion rate measured by a (secondary) clock field which is not comoving with the dominant matter component obtains a negative contribution from infrared fluctuations, a contribution whose absolute value increases in time. This is the same effect which a decreasing cosmological constant would produce. This supports the conclusion that infrared fluctuations lead to a dynamical relaxation of the cosmological constant. Our analysis does not make use of any perturbative expansion in the amplitude of the inhomogeneities.

Highlights

The cosmological constant problem is a key challenge for fundamental physics
Using the leading order gradient expansion we show that the expansion rate measured by a clock field which is not comoving with the dominant matter component obtains a negative contribution from infrared fluctuations, a contribution whose absolute value increases in time
This is the same effect which a decreasing cosmological constant would produce. This supports the conclusion that infrared fluctuations lead to a dynamical relaxation of the cosmological constant

Summary

INTRODUCTION

The cosmological constant problem (see [1] for reviews) is a key challenge for fundamental physics. We show that the back-reaction effect of super-Hubble cosmological fluctuations on the local expansion rate persists beyond perturbation theory and that, given fluctuations in the clock field relative to those of the dominant matter field, the locally measured Hubble expansion rate obtains a negative contribution, a contribution whose amplitude grows in time. This supports the claim that (quasi) de Sitter space-time is unstable, and that it will lead to a dynamical relaxation of the cosmological constant.

LOCAL HUBBLE EXPANSION RATE

EVALUATION OF THE LOCAL HUBBLE EXPANSION RATE

DISCUSSION