Sound Speed Resonance in the Gravitational Wave Background as a probe for non-standard early universe cosmologies

TL;DR

This paper explores how resonant phenomena in gravitational wave spectra, caused by modifications in gravity theories, can amplify primordial signals, enabling detection of early universe physics with future gravitational wave observatories.

Contribution

It introduces a novel analysis of resonance effects in gravitational wave backgrounds within extended gravity theories, linking them to early universe cosmology and detectability.

Findings

Resonance effects can significantly amplify primordial gravitational wave signals.

Characteristic peaks depend on model parameters and primordial tensor spectrum.

Resonances may make otherwise undetectable signals observable by future detectors.

Abstract

Gravitational waves constitute a powerful probe of the underlying theory of gravity. In extensions of general relativity, additional degrees of freedom, such as scalar fields in the gravitational sector, can modify their propagation through changes in the effective friction term and propagation speed. These modifications may potentially induce resonant phenomena leading to distinctive signatures in the gravitational wave spectrum. One important aspect to be investigated is whether the resonances can be strong enough to enhance the underlying background of primordial tensor modes to levels detectable by upcoming gravitational wave detectors, such as LISA or the Einstein telescope. The characteristic peaks in the SBGW spectrum depend on the parameters of the resonant model as well as on the parameters of the primordial tensor spectrum, such as $r$ and $n_{t}$. Thus these resonance effects open a powerful pathway to explore physics of the very early Universe by amplifying otherwise feeble signals to experimentally detectable levels. Here we analyze how the signals of the primordial Universe can resonate in these scenarios, bringing the early universe physics into the realm of experimental access.