# Detector response along null geodesics in black hole spacetimes and in a Friedmann-Lemaitre-Robertson-Walker Universe

## TL;DR

This paper investigates the response of a detector moving along null geodesics in black hole and cosmological spacetimes, revealing thermal particle detection in various vacuum states across different cosmic stages.

## Contribution

It provides a detailed analysis of detector responses along null geodesics in black hole and FLRW spacetimes, highlighting particle detection in vacuum states during different cosmic epochs.

## Key findings

- Detector registers particles in black hole and FLRW vacuums.
- Detection probabilities are thermal in all studied scenarios.
- Response occurs even for freely falling detectors.

## Abstract

We study the detector's response when moving along an ingoing null geodesic. The backgrounds are chosen to be black hole spacetimes ($(1+1)$ dimensional Schwarzschild metric and near horizon effective metric for any stationary black hole in arbitrary dimensions) as well as Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe. For black holes the trajectories are defined in Schwarzschild coordinates and the field modes are corresponding to {\it Boulware vacuum}. Whereas for FLRW case, the detector is moving along the path defined in original {\it cosmic time} and the field modes are related to {\it conformal vacuum}. The analysis is done for three stages (de-Sitter, radiation dominated and matter dominated) of the Universe. We find that, although the detector is freely falling, it registered particles in the above mentioned respective vacuums. We confirm this by different approaches. The detection probability distributions, in all situations, are thermal in nature.