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Detecting the dark sector through scalar-induced gravitational waves (2407.04220v2)

Published 5 Jul 2024 in astro-ph.CO, gr-qc, and hep-ph

Abstract: We investigate the evolution of cosmological scalar perturbations in the case that the background radiation is weakly coupled to a light scalar field $\phi$. The light scalar $\phi$ is a homogeneous background field with a large initial value. In the radiation-dominated Universe, the coupling term introduces an effective mass to $\phi$ and the background ultra-relativistic particles. The oscillations of $\phi$ result in the periodic change of the equation of state parameter and the sound speed, which provides a novel mechanism to amplify subhorizon scalar perturbations through parametric resonance. The amplification of scalar perturbations leads to a stochastic gravitational-waves background~(SGWB) expected to be observed by multiband gravitational wave observers. The observation of the SGWB helps to determine the initial value of $\phi$ and the coupling strength of the interaction. This mechanism is generally applicable to the interactions that introduce an effective mass, and we take the interaction between $\phi$ and electrons as a concrete example to illustrate the result. We find that under the condition that the coupling coefficient $\lambda=10{-16}$ and the initial value $\phi_i=10{18}$ GeV, the resulting SGWB spectrum is expected to be observed by the future observers including LISA, $Taiji$, DECIGO and BBO.

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