Cosmological observables, IR growth of fluctuations, and scale-dependent anisotropies

TL;DR

This paper investigates how infrared growth of fluctuations during inflation affects large-scale anisotropies and inhomogeneities, proposing IR-safe observables and analyzing their implications for late-time cosmological observations.

Contribution

It extends semiclassical inflationary methods to include IR corrections, defining IR-safe observables and analyzing their impact on large-scale anisotropies and late-time measurements.

Findings

IR effects can be absorbed into coordinate changes for local observables

IR growth enhances statistical inhomogeneities at short scales

Late-time observers may see significant IR corrections

Abstract

We extend semiclassical methods in inflationary cosmology that capture leading IR corrections to correlators. Such large IR effects can be absorbed into a coordinate change when examining sufficiently local observables, but not when comparing observations at large separation in scales, such as seen by a late-time observer. The analysis is facilitated by definition of a scale-dependent metric and physical momentum. These assist definition of "IR-safe" observables seen by a post-inflationary observer, which are contrasted to those based on the local geometry of the reheating surface. For such observables, the observer's horizon provides an effective IR cutoff. IR growth contributes to enhanced statistical inhomogeneities/anisotropies at short scales, observation of which by a present day observer might be sought in 21 cm measurements. Such IR corrections are argued to grow large for a very late-time observer.