Cosmological stimulated emission

TL;DR

This paper investigates how stimulated emission and absorption of gravitons in a squeezed vacuum state could enhance gravitational wave signals, with implications for cosmology and future detection technologies.

Contribution

It introduces a theoretical framework for graviton stimulated processes in cosmological and laboratory contexts, highlighting potential signal amplification mechanisms.

Findings

Net graviton emission depends on initial squeezing parameters.

Graviton occupation numbers can be significantly enhanced at certain frequencies.

Superhorizon modes show secular growth, indicating breakdown of perturbation theory.

Abstract

We study stimulated emission and absorption of gravitons in a squeezed vacuum state immersed in a thermal radiation bath. Employing one-loop interaction-picture perturbation theory, we track the time evolution of the graviton number operator and its expectation value in the squeezed vacuum, which characterizes the inflationary graviton state. In a Minkowski background with a thermal bath as a toy example, we demonstrate that the net graviton emission or absorption rate depends sensitively on the initial squeezing parameters. As a thought experiment, we consider LIGO/Virgo-like detectors operating in radiation at temperatures of order 0.1 GeV and find that graviton occupation numbers at frequencies of order 100 Hz can be significantly enhanced, suggesting a novel mechanism for amplifying gravitational-wave signals. Although these conditions exceed current experimental capabilities, they point toward potential future advances in detection. Extending our analysis to an expanding, radiation-dominated universe, we show that subhorizon gravitons undergo stimulated absorption, while superhorizon modes exhibit secular logarithmic growth, indicating the breakdown of perturbative methods and motivating further investigation. These findings open a new direction for exploring graviton coherence effects in realistic cosmological and laboratory settings.