Squeezed quantum states of graviton and axion in the universe

TL;DR

This paper analytically investigates the squeezing of gravitons and axions during cosmic inflation, comparing methods and effects of reheating, and explores how squeezing influences quantum nonclassicality and Bell inequality violations.

Contribution

It provides a detailed analytical comparison of squeezing in gravitons and axions using two vacuum definitions and examines the impact of reheating on quantum squeezing and nonclassicality.

Findings

Squeezing of gravitons shows no qualitative difference between methods.

Axion squeezing increases with mass in instantaneous vacuum but decreases in adiabatic vacuum.

Non-instantaneous reheating enhances the squeezing of both particles.

Abstract

Particle production during cosmic expansion can be interpreted as a two-mode squeezing process of quantum states. The two-mode squeezed states consist of an infinite number of entangled particles and then enhance the nonclassicality of particles. There are two methods for estimating the degree of squeezing. One is to employ instantaneous vacuum states, and the other is to adopt adiabatic vacuum states. We analytically study the squeezing process of gravitons and axions by using the two methods. We first consider a cosmological model of inflation followed by instantaneous reheating leading to a radiation dominated era. In the case of gravitons, we find no qualitatively difference in the squeezing between the two methods. However, for the axions, it turns out that the squeezing in the instantaneous vacuum increases as the mass increases while the squeezing decreases in the adiabatic vacuum as the mass increases. We then study the effect of non-instantaneous reheating on the the squeezing and show that the squeezing is enhanced compared with the instantaneous reheating. We also illustrate how the squeezed states enhance the violation of Bell inequality and quantum noise of gravitons and axions.