Back-Reaction of Long-Wavelength Cosmological Fluctuations as Measured by a Clock Field

TL;DR

This paper investigates how long-wavelength cosmological fluctuations influence the local expansion rate, revealing a decrease in expansion when measured against a clock field, within Einstein gravity with perfect fluid matter.

Contribution

It introduces a method to measure back-reaction effects using a clock field, highlighting the impact of infrared fluctuations on local cosmological expansion.

Findings

Infrared fluctuations decrease the average expansion rate.

Back-reaction effects are quantified using a clock field.

Results are specific to Einstein gravity with perfect fluid matter.

Abstract

We consider the back-reaction of cosmological fluctuations on the local expansion rate averaged over a space-like hypersurface of constant value of a clock field. We show that in the infrared limit, the fluctuations lead to a decrease in the average expansion rate, measured at a fixed value of the clock field, compared to what would be obtained in a homogeneous universe. We work in the context of Einstein gravity coupled to perfect fluid matter.