# Enhancement of Gravitational Waves Induced by Scalar Perturbations due to a Sudden Transition from an Early Matter Era to the Radiation Era

## TL;DR

This paper investigates how a rapid transition from an early matter-dominated era to radiation domination enhances the production of gravitational waves from scalar perturbations, potentially detectable by future experiments.

## Contribution

It introduces a new mechanism for gravitational wave enhancement during a quick reheating transition, surpassing previous models in efficiency.

## Key findings

- Enhanced gravitational wave production just after reheating
- Detectability of induced GWs for specific reheating temperatures
- Mechanism effective even without small-scale scalar perturbation enhancement

## Abstract

We study gravitational waves induced from the primordial scalar perturbations at second order around the reheating of the Universe. We consider reheating scenarios in which a transition from an early matter dominated era to the radiation dominated era completes within a timescale much shorter than the Hubble time at that time. We find that an enhanced production of induced gravitational waves occurs just after the reheating transition because of fast oscillations of scalar modes well inside the Hubble horizon. This enhancement mechanism just after an early matter-dominated era is much more efficient than a previously known enhancement mechanism during an early matter era, and we show that the induced gravitational waves could be detectable by future observations if the reheating temperature $T_{\text{R}}$ is in the range $T_\text{R} \lesssim 7\times 10^{-2}$GeV or $20 \, \text{GeV} \lesssim T_\text{R} \lesssim 2 \times 10^7 \, \text{GeV}$. This is the case even if the scalar perturbations on small scales are not enhanced relative to those on large scales, probed by the observations of the cosmic microwave background.