Intrinsic limits on the detection of the anisotropies of the Stochastic Gravitational Wave Background

TL;DR

This paper demonstrates that the intrinsic time variation of the isotropic component of the Stochastic Gravitational Wave Background imposes fundamental limits on measuring its anisotropies, even with ideal detectors and negligible noise.

Contribution

It introduces a method to calculate the intrinsic sample limit on SGWB anisotropy detection and applies it to various detector configurations.

Findings

Multipole anisotropies at   can only be measured to ^{-5} - ^{-4} over 5 years.

Measurement sensitivity improves with the square root of observation time.

Intrinsic limits pose a serious challenge for detecting cosmological SGWB anisotropies.

Abstract

For any given network of detectors, and for any given integration time, even in the idealized limit of negligible instrumental noise, the intrinsic time variation of the isotropic component of the Stochastic Gravitational Wave Background (SGWB) induces a limit on how accurately the anisotropies in the SGWB can be measured. We show here how this sample limit can be calculated and apply this to three separate configurations of ground-based detectors placed at existing and planned sites. Our results show that in the idealized, best-case scenario individual multipoles of the anisotropies at $\ell \leq 8$ can only be measured to $\sim 10^{-5} - 10^{-4}$ level over 5 years of observation as a fraction of the isotropic component. As the sensitivity improves as the square root of the observation time, this poses a very serious challenge for the measurement of the anisotropies of SGWB of cosmological origin, even in the case of idealised detectors with arbitrarily low instrumental noise.