Comprehensive analysis of dissipative effects in the induced gravitational waves

TL;DR

This paper investigates how dissipative effects in the cosmic fluid influence induced gravitational waves, revealing characteristic spectral features and potential for probing new physics through gravitational-wave observations.

Contribution

It provides a comprehensive analysis of dissipative effects on IGWs, including their spectral signatures and implications for detecting new particles and physics beyond the Standard Model.

Findings

Dissipative effects cause a double-valley feature in the IGW spectrum.

Neutrino dissipation impacts pulsar timing array data analysis.

Features related to new particles could be detectable by gravitational-wave experiments.

Abstract

Dissipation is an intrinsic property of the cosmic fluid, leading to the damping of curvature perturbations at small scales. In this paper, we comprehensively study dissipative effects in gravitational waves induced by curvature perturbations, known as induced gravitational waves (IGWs). We find dissipative effects become especially significant at wavenumber $k \sim k_{\mathcal{H},\mathrm{dec}}$, where $k_{\mathcal{H},\mathrm{dec}}$ corresponds to the horizon scale at the decoupling of weakly-interacting particles. They can leave characteristic features on the IGW spectrum, including a notable suppression with a ``double-valley'' structure at $k \sim k_{\mathcal{H},\mathrm{dec}}$ and a modified infrared behavior without logarithmic running at $k \lesssim k_{\mathcal{H},\mathrm{dec}}$. Within the Standard Model of particle physics, dissipative effects caused by neutrinos at the nanohertz frequencies can be important in the analysis of pulsar timing array data. Furthermore, dissipation-induced features associated with possible new weakly-interacting particles can be detectable by a wide range of gravitational-wave experiments, serving as a promising probe of new physics at extremely high energy scales. As an extension, we also discuss dissipative effects in the presence of primordial non-Gaussianity and their impacts on the anisotropies of IGWs and the poltergeist mechanism. These dissipative effects not only provide a more realistic description of IGWs but also exhibit rich phenomenology and profound physical implications, opening a new window into understanding the early Universe and fundamental physics.