The gravitational wave background induced by nonlinear interactions between isotropic inhomogeneities and anisotropic inhomogeneous density perturbations

TL;DR

This paper investigates how nonlinear interactions between isotropic inhomogeneities and anisotropic density perturbations can generate a gravitational wave background, potentially observable through future cosmic microwave background measurements, thus constraining isotropic inhomogeneities.

Contribution

It introduces a calculation of the gravitational wave background induced by isotropic inhomogeneities and anisotropic perturbations, providing a method to observationally constrain isotropic inhomogeneities.

Findings

The gravitational wave background's energy density can be comparable to primordial inflationary waves.

Future CMB observations could detect this gravitational wave background.

Isotropic deviations of about 6% could produce observable effects.

Abstract

Usually, we assume that there is no inhomogeneity isotropic in terms of our location in the universe. This assumption has not been observationally confirmed yet in sufficient accuracy and we need to consider a method to restrict isotropic inhomogeneities more strongly. If there are isotropic inhomogeneities in the universe, the gravitational wave background is induced by nonlinear interactions between isotropic inhomogeneities and anisotropic inhomogeneous density perturbations. In this paper, we calculate the power spectrum and the relative energy density of this gravitational wave background to discuss observability. We show that, at the decoupling time, the relative energy density of this gravitational wave background is comparable to that of the primordial inflationary gravitational wave background whose tensor-to-scalar ratio is 0.001, if 6%-level isotropic deviations from homogeneity exist. Therefore, there are possibilities of observing this gravitational wave background and restricting isotropic inhomogeneities by using the future cosmic microwave background observation project.