Causality Implies Inflationary Back-Reaction

TL;DR

This paper challenges the common belief that super-horizon gravitons are unobservable locally during inflation, showing instead that they cause measurable effects like inflationary back-reaction and violate gravitational field equations.

Contribution

It demonstrates that super-horizon gravitons induce inflationary back-reaction and affect local measurements, contradicting the idea that they are unobservable by local observers.

Findings

Local Hubble parameter varies over time due to super-horizon gravitons

Gravitational field equations are violated when super-horizon modes are removed

Super-horizon gravitons have observable inflationary back-reaction effects

Abstract

There is a widespread belief among inflationary cosmologists that a local observer cannot sense super-horizon gravitons. The argument goes that a local observer would subsume super-horizon gravitons into a redefinition of his coordinate system. We show that adopting this view for pure gravity on de Sitter background leads to time variation in the Hubble parameter measured by a local observer. It also leads to a violation of the gravitational field equation $R = 4 \Lambda$ because that equation is obeyed by the full metric, rather than the one which has been cleansed of super-horizon modes.