Physical observers, $T$-vacuum and Unruh like effect in the radiation dominated early universe

TL;DR

This paper demonstrates an Unruh-like effect in the radiation-dominated early universe, where a new vacuum state causes physical observers to detect particles, drawing an analogy with the Unruh effect in flat spacetime.

Contribution

It introduces a novel $T$-vacuum state in cosmology and shows how physical observers perceive particle creation similar to the Unruh effect, within a two-dimensional framework.

Findings

Existence of a $T$-vacuum state in the early universe.

Physical observers detect particles due to this vacuum, indicating an Unruh-like effect.

The $T$-vacuum has well-defined Hadamard behavior across spacetime.

Abstract

We report the existence of an Unruh like effect, for physical observers (cosmological and comoving observers) in the radiation dominated early universe, which is possible due to the discovery of a new vacuum state (referred here as the $T-$vacuum). Both the comoving and the cosmological observers, who are crucial in our understanding of cosmology, observe this $T-$vacuum as a particle excited state and are able to detect radiation due to particle creation. We draw a robust analogy with the Unruh effect, whereby -- (i) the physical observers here are closely analogous to the accelerated (Rindler) observers in the flat spacetime, and (ii) the $T-$vacuum plays the role of the Minkowski vacuum state which contains particles when viewed from the physical observers frame. Our analogy is further supported by a proof of well-defined (hadamard) behavior of the $T-$vacuum in the entire spacetime. All our analysis of the particle creation process is done here within a two dimensional set up.