Renormalization Group Approach to Cosmological Back Reaction Problems

TL;DR

This paper applies the renormalization group method to second order cosmological perturbations, revealing how inhomogeneities influence the universe's evolution by effectively acting as curvature or radiation.

Contribution

It introduces a gauge-invariant renormalization group framework to analyze the back reaction of cosmological perturbations on universe dynamics.

Findings

Scalar and long wavelength tensor modes act as positive spatial curvature.

Short wavelength tensor modes behave like radiation fluid.

Back reaction effects can be incorporated into effective cosmological equations.

Abstract

We investigated the back reaction of cosmological perturbations on the evolution of the universe using the second order perturbation of the Einstein's equation. To incorporate the back reaction effect due to the inhomogeneity into the framework of the cosmological perturbation, we used the renormalization group method. The second order zero mode solution which appears by the non-linearities of the Einstein's equation is regarded as a secular term of the perturbative expansion, we renormalized a constant of integration contained in the background solution and absorbed the secular term to this constant. For a dust dominated universe, using the second order gauge invariant quantity, we derived the renormalization group equation which determines the effective dynamics of the Friedman-Robertson-Walker universe with the back reaction effect in a gauge invariant manner. We obtained the solution of the renormalization group equation and found that perturbations of the scalar mode and the long wavelength tensor mode works as positive spatial curvature, and the short wavelength tensor mode as radiation fluid.