The separate universe and the back reaction of long wavelength fluctuations

TL;DR

This paper explores how long wavelength cosmological perturbations influence universe evolution, demonstrating that non-adiabatic modes impact the expansion, and establishing the equivalence of backreaction effects to the separate universe approach.

Contribution

It applies the renormalization group method to show the backreaction of long wavelength fluctuations is equivalent to the separate universe model and identifies the specific modes affecting expansion.

Findings

Backreaction effects are equivalent to the separate universe.

Only non-adiabatic modes influence the expansion law.

Renormalized solutions incorporate backreaction effects.

Abstract

We investigate the backreaction of cosmological long wavelength perturbations on the evolution of the Universe. By applying the renormalization group method to a Friedmann-Robertson-Walker universe with long wavelength fluctuations, we demonstrate that the renormalized solution with the backreaction effect is equivalent to that of the separate universe. Then, using the effective Friedmann equation, we show that only non-adiabatic mode of long wavelength fluctuations affects the expansion law of the spatially averaged universe.