Investigating cosmic histories with a stiff era through Gravitational Waves

TL;DR

This paper explores how gravitational-wave background searches can reveal cosmic histories with a stiff era, using Bayesian analysis on LIGO-Virgo-KAGRA data and forecasting future detection prospects.

Contribution

It introduces an analytical model for gravitational-wave spectra with instant and smooth transitions, constrains cosmic histories with current data, and forecasts future detection capabilities.

Findings

Current LIGO-Virgo-KAGRA data constrains stiff era parameters.

Smooth transition models differ from instant transition approximations.

Future experiments have promising detection prospects.

Abstract

We investigate the potential of gravitational-wave background searches to constrain cosmic histories characterised by a stiff equation of state, preceded by a period of matter domination. Such a scenario leads to a characteristic peak in the primordial gravitational-wave spectrum originating from cosmological inflation. Assuming instant transitions between distinct epochs, which allows an analytical treatment of the gravitational-wave spectrum, we perform a Bayesian inference analysis to derive constraints from the first three observing runs of the LIGO-Virgo-KAGRA Collaboration. Additionally, we consider a smooth transition, employing an axion-like particle physics model, and highlight the difference with the instant transition approximation. We then forecast detection prospects for such a cosmic history through future gravitational-wave experiments.