Gravitational wave fossils in nonlinear regime: halo tidal bias and intrinsic alignments from gravitational wave separate universe simulations

TL;DR

This paper explores how long-wavelength gravitational waves influence nonlinear structure formation, revealing scale-dependent biases in halo shapes and clustering through innovative simulations that incorporate GWs as anisotropic expansion effects.

Contribution

It introduces a novel method to simulate long-wavelength GWs as anisotropic expansion in the tidal separate universe framework, enabling the study of their effects on large-scale structure.

Findings

Detection of halo tidal bias induced by GWs.

Measurement of linear shape bias from halo ellipticity.

Identification of scale-dependent effects due to GW evolution.

Abstract

We investigate impacts of long-wavelength gravitational waves (GWs) on nonlinear structure formation by utilizing the tidal separate universe simulations. Based on the equivalence of a long-wavelength GW to a uniform tidal field in a local frame, we provide a way to incorporate a long-wavelength GW into the tidal separate universe simulation as an effective anisotropic expansion. This methodology enables us to study effects of GWs on large-scale structure efficiently. We measure the anisotropic imprint in the local power spectrum from the tidal separate universe simulations with GWs, which corresponds to the scalar-scalar-tensor bispectrum in squeezed limit or the so-called power spectrum response to GWs. We also detect the halo tidal bias induced by GWs from the response of the halo-matter cross-power spectrum to GWs, as well as the linear shape bias (or the linear alignment coefficient) induced by GWs from the one-point function of the halo ellipticity. In contrast to the case of the tidal field induced by scalar perturbations, we discover that the wavenumber dependence of the temporal evolution of GWs naturally causes these biases to be scale-dependent. We also find that this scale dependence is well approximated by the second-order density induced by the coupling between scalar and tensor perturbation. This highlights that the structure formation, especially the process to determine the halo shape, is nonlocal in time. Our findings lay the foundation for predicting the impact of GWs on large-scale structure.