Searching for anisotropic stochastic gravitational-wave backgrounds with constellations of space-based interferometers

TL;DR

This paper investigates whether space-based interferometer constellations like LISA and DECIGO can detect anisotropies in the stochastic gravitational-wave background, finding that direct measurement is difficult but cross-correlation with CMB data offers potential insights.

Contribution

It introduces the idea of using space-based detector constellations to probe SGWB anisotropies and explores cross-correlation with CMB as a feasible method.

Findings

Measuring SGWB anisotropies directly is nearly impossible.

Cross-correlation with CMB can marginally constrain SGWB anisotropies.

DECIGO can probe the cross-correlation from merging binaries.

Abstract

Many recent works have shown that the angular resolution of ground-based detectors is too poor to characterize the anisotropies of the stochastic gravitational-wave background (SGWB). For this reason, we asked ourselves if a constellation of space-based instruments could be more suitable. We consider the Laser Interferometer Space Antenna (LISA), a constellation of multiple LISA-like clusters, and the Deci-hertz Interferometer Gravitational-wave Observatory (DECIGO). Specifically, we test whether these detector constellations can probe the anisotropies of the SGWB. For this scope, we considered the SGWB produced by two astrophysical sources: merging compact binaries and a recently proposed scenario for massive black-hole seed formation through multiple mergers of stellar remnants. We find that measuring the angular power spectrum of the SGWB anisotropies is almost unattainable. However, it turns out that it could be possible to probe the SGWB anisotropies through cross-correlation with the CMB fluctuations. In particular, we find that a constellation of two LISA-like detectors and CMB-S4 can marginally constrain the cross-correlation between the CMB lensing convergence and the SGWB produced by the black hole seed formation process. Moreover, we find that DECIGO can probe the cross-correlation between the CMB lensing and the SGWB from merging compact binaries.