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