Unruh-DeWitt detectors in cosmological spacetimes

TL;DR

This paper investigates how Unruh-DeWitt detectors respond and thermalize in cosmological spacetimes, comparing different definitions of surface gravity and temperature in dynamical horizons using quantum field theory.

Contribution

It provides a detailed analysis of detector responses in evolving cosmological backgrounds and evaluates various surface gravity definitions in this context.

Findings

Detector thermalization aligns with KMS conditions.

Different surface gravity definitions yield varying temperature estimates.

Insights into horizon thermodynamics in cosmological spacetimes.

Abstract

We analyse the response and thermal behaviour of an Unruh-DeWitt detector as it travels through cosmological spacetimes, with special reference to the question of how to define surface gravity and temperature in dynamical spacetimes. Working within the quantum field theory on curved spacetime approximation, we consider a detector as it travels along geodesic and accelerated Kodama trajectories in de Sitter and asymptotically de Sitter FLRW spacetimes. By modelling the temperature of the detector using the detailed-balance form of the Kubo--Martin--Schwinger (KMS) conditions as it thermalises, we can better understand the thermal behaviour of the detector as it interacts with the quantum field, and use this to compare competing definitions of surface gravity and temperature that persist in the literature. These include the approaches of Hayward-Kodama, Ashtekar et al., Fodor et al., and Nielsen-Visser. While these are most often examined within the context of a dynamical black hole, here we shift focus to surface gravity on the evolving cosmological horizon.