Generation of fluctuations during inflation: comparison of stochastic and field-theoretic approaches

TL;DR

This paper demonstrates the equivalence of stochastic and field-theoretic methods in calculating scalar field fluctuations during inflation, highlighting their consistency and implications for inflationary dynamics and particle production.

Contribution

It proves the exact match between stochastic and field-theoretic results for inflationary scalar fluctuations in the leading slow-roll approximation, including for inflaton and test fields.

Findings

Both approaches yield identical results for fluctuation amplitudes.

The stochastic method reproduces multi-loop diagram results from field theory.

Implications for particle production and the moduli problem are discussed.

Abstract

We prove that the stochastic and standard field-theoretical approaches produce exactly the same results for the amount of light massive scalar field fluctuations generated during inflation in the leading order of the slow-roll approximation. This is true both in the case for which this field is a test one and inflation is driven by another field, and the case for which the field plays the role of inflaton itself. In the latter case, in order to calculate the average of the mean square of the gauge-invariant inflaton fluctuation, the logarithm of the scale factor $a$ has to be used as the time variable in the Fokker-Planck equation in the stochastic approach. The implications of particle production during inflation for the second stage of inflation and for the moduli problem are also discussed. The case of a massless self-interacting test scalar field in a de Sitter background with a zero initial renormalized mean square is also considered in order to show how the stochastic approach can easily produce results corresponding to diagrams with an arbitrary number of scalar field loops in the field-theoretical approach (explicit results up to 4 loops inclusive are presented).