Detecting the dark sector through scalar-induced gravitational waves

We investigate the evolution of cosmological scalar perturbations in the case that the background radiation is weakly coupled to a light scalar field φ. The light scalar φ is a homogeneous background field with a large initial value. In the radiation-dominated Universe, the coupling term introduces an effective mass to φ and the background ultra-relativistic particles. The oscillations of φ 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 φ 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 φ and electrons as a concrete example to illustrate the result. We find that under the condition that the coupling coefficient λ=10⁻¹⁶ and the initial value φᵢ=10¹⁸ GeV, the resulting SGWB spectrum is expected to be observed by the future observers including LISA, $Taiji$, DECIGO and BBO.