Semianalytic Calculation of Gravitational Wave Spectrum Nonlinearly Induced from Primordial Curvature Perturbations

TL;DR

This paper presents an analytical method to calculate the spectrum of gravitational waves induced by primordial curvature perturbations, simplifying the process and deriving explicit formulas in certain cases.

Contribution

It introduces a semi-analytic approach to compute the induced GW spectrum, highlighting a universal integral component and providing fully analytic solutions for simple scenarios.

Findings

Derived a universal integral form for the induced GW spectrum

Simplified numerical calculations of the spectrum

Obtained explicit analytic formulas in specific cases

Abstract

Whether or not the primordial gravitational wave (GW) produced during inflation is sufficiently strong to be observable, GWs are necessarily produced from the primordial curvature perturbations in the second order of perturbation. The induced GWs can be enhanced by curvature perturbations enhanced at small scales or by the presence of matter-dominated stages of the cosmological history. We analytically calculate the integral in the expression of the power spectrum of the induced GWs which is a universal part independent of the spectrum of the primordial curvature perturbations. This makes the subsequent numerical integrals significantly easy. In simple cases, we derive fully analytic formulas for the induced GW spectrum.