Universality of Primordial Anisotropies in Gravitational Wave Background

TL;DR

This paper introduces a universal formalism for describing anisotropies in the primordial gravitational wave background, revealing a common angular structure driven by fundamental cosmological principles.

Contribution

It presents a model-independent framework using the Cosmological Form Factor to unify the description of SGWB anisotropies from various primordial effects.

Findings

Universal angular structure of SGWB anisotropies

The CFF encodes modulation by long-wavelength modes

Multipole scaling is determined by fundamental principles

Abstract

We propose a model-independent formalism for describing anisotropies in the stochastic gravitational wave background (SGWB) originating from primordial perturbations. Despite their diverse physical origins -- such as Sachs-Wolfe effects, integrated Sachs-Wolfe effects, or fossil effects from primordial non-Gaussianity -- SGWB anisotropies exhibit a universal angular structure. We show that this universality arises from a single vertex function, the Cosmological Form Factor (CFF), which encodes the information on how long-wavelength modes modulate the SGWB statistics. Two fundamental principles -- statistical isotropy and locality -- uniquely determine the angular dependence of the CFF, resulting in a universal multipole scaling of the SGWB anisotropies. The CFF formalism provides a common language for classifying SGWB anisotropies and offers a powerful framework for interpreting upcoming observations.