# Particle propagation in cosmological backgrounds

## TL;DR

This paper investigates how particles propagate in expanding cosmological backgrounds, revealing how expansion influences decay rates and long-term behavior, with implications for understanding early universe physics.

## Contribution

It introduces a detailed analysis of dissipative effects on particle decay rates in expanding universes, including contributions beyond linear order and implications for stable particles.

## Key findings

- Decay rate depends on local temperature at first order
- Expansion introduces additional decay contributions beyond linear order
- Long-term propagator behavior has implications for trans-Planckian issues

## Abstract

We study the quantum propagation of particles in cosmological backgrounds, by considering a doublet of massive scalar fields propagating in an expanding universe, possibly filled with radiation. We focus on the dissipative effects related to the expansion rate. At first order, we recover the expected result that the decay rate is determined by the local temperature. Beyond linear order, the decay rate has an additional contribution governed by the expansion parameter. This latter contribution is present even for stable particles in the vacuum. Finally, we analyze the long time behaviour of the propagator and briefly discuss applications to the trans-Planckian question.