Nonlinear Gravitational Wave Memory : Universal Low-Frequency Background

TL;DR

This paper investigates the universal nonlinear memory effect in gravitational waves, calculating its spectrum across various cosmological and astrophysical backgrounds to aid future detection and understanding of the universe's thermal history.

Contribution

It provides the first comprehensive calculation of the stochastic nonlinear gravitational wave memory spectrum from diverse sources, linking it to the universe's thermal state and detection prospects.

Findings

Universal low-frequency nonlinear memory exists in all gravitational waves.

The complete frequency spectrum of stochastic backgrounds is derived.

Detection strategies for probing the universe's thermal state are discussed.

Abstract

A universal contribution exists in the infrared (low frequency) regime of all gravitational waves, which results from nonlinear memory. Nonlinear memory is sourced by linear order gravitational waves and exists for any gravitational-wave background. We calculate the stochastic nonlinear memory signal of various stochastic backgrounds of cosmological (scalar induced, reheating, phase transition, topological defect, turbulence) and astrophysical (binary mergers of stellar-mass, intermediate mass, supermassive, and primordial black holes) origins. These results allow us to derive the complete frequency spectrum of cosmological and astrophysical SGWB. We calculate how to probe the thermal state of the universe, i.e. the equation of the state, via the memory spectrum's slope and also discuss the detection prospects at various frequency bands with future experiments.