Particle production in time-dependent gravitational fields: the expanding mass shell

TL;DR

This paper analyzes particle production caused by an expanding mass shell's gravitational field, revealing a non-thermal spectrum dependent on expansion parameters, and connects it to Hawking radiation through time reversal.

Contribution

It introduces a method to compute particle production in a horizonless, expanding mass shell, extending understanding beyond collapsing scenarios.

Findings

Particle production depends on expansion velocity, radius, and mass.

The energy spectrum is non-thermal for an expanding shell.

Time reversal reproduces Hawking's thermal spectrum in a specific limit.

Abstract

We compute the production of particles from the gravitational field of an expanding mass shell. Contrary to the situation of Hawking radiation and the production of cosmological perturbations during cosmological inflation, the example of an expanding mass shell has no horizon and no singularity. We apply the method of `ray-tracing', first introduced by Hawking, and calculate the energy spectrum of the produced particles. The result depends on three parameters: the expansion velocity of the mass shell, its radius, and its mass. Contrary to the situation of a collapsing mass shell, the energy spectrum is non-thermal. Invoking time reversal we reproduce Hawking's thermal spectrum in a certain limit.