The poltergeist mechanism -- Enhancement of scalar-induced gravitational waves with early matter-dominated era

TL;DR

This paper explains how an early matter-dominated era can significantly amplify gravitational waves through the poltergeist mechanism, providing insights into the Universe's thermal history and potential observational signatures.

Contribution

It introduces and elucidates the poltergeist mechanism, demonstrating how a sudden transition from an early matter-dominated era to a pressure-dominated era enhances scalar-induced gravitational waves.

Findings

Enhanced gravitational wave signals during the eMD to pressure era transition

Growth of density perturbations in early matter-dominated era

Potential observational signatures of early Universe thermal history

Abstract

Gravitational waves induced by primordial density perturbations provide a powerful probe of the Universe's thermal history, which may include an early matter-dominated (eMD) era predicted by well-motivated particle-physics models. The induced GWs can be significantly enhanced when the Universe undergoes a sudden transition from an eMD era to an era with pressure, such as a radiation or kination era. This enhancement arises from the growth of density perturbations during the eMD era and their rapid oscillations during the era with pressure. This phenomenon is called the poltergeist mechanism. In this review, we explain the essence of the poltergeist mechanism and explore concrete scenarios in which such an enhancement can occur.