Cosmology as a weak gravitational field and the trans-Planckian problem

TL;DR

This paper demonstrates that at high momenta, cosmological expansion acts as a weak gravitational field, allowing standard quantum field theory techniques to analyze phenomena like mode-stretching and particle production, with implications for the trans-Planckian problem.

Contribution

It introduces a coordinate system where cosmological effects are treated as a weak field, simplifying the analysis of QFT in expanding backgrounds and addressing the trans-Planckian issue.

Findings

Re-derivation of mode-stretching and particle production in cosmology.

High-momentum regime treated with Minkowski space perturbation theory.

Insights into the trans-Planckian problem's implications.

Abstract

At momenta much higher than the Hubble scale, the cosmological expansion can be thought of as a weak gravitational field. We consider QFT in a particularly convenient set of coordinates that makes this manifest, so that, for those high momenta, the effects of the cosmological expansion can be dealt with using the standard tools of perturbation theory in Minkwoski space. In this way, we re-derive standard results of QFT in a cosmological background, such as mode-stretching and gravitational particle production. We discuss the implications of our results for the trans-Planckian problem.