Finding cosmic anisotropy with networks of next-generation gravitational-wave detectors

TL;DR

This paper explores how networks of next-generation gravitational-wave detectors can measure cosmic anisotropy, specifically dipoles in luminosity distance, improving constraints over current methods with potential for multi-year observations.

Contribution

It demonstrates the feasibility of detecting cosmic dipoles using gravitational waves with next-generation detectors and provides projections for measurement precision and directional sensitivity.

Findings

Next-generation detector networks can constrain dipole amplitude to about 13% after one year.

The constraints improve with more detections and years of observation.

Dipole location has minimal impact on measurement accuracy for advanced networks.

Abstract

The standard cosmological model involves the assumption of isotropy and homogeneity, a principle that is generally well-motivated but is now in conflict with various anisotropies found using independent astrophysical probes. These anisotropies tend to take the form of dipoles; while some can be explained by simple kinematic effects, many others are not fully understood. Thus, generic phenomenological models are being considered, such as a dipole in the luminosity distance. We demonstrate how such a dipole could be measured using gravitational waves from binary neutron star mergers observed by six different networks of gravitational-wave detectors, ranging from upgraded LIGO detectors to anticipated next-generation ground-based observatories. We find that, for example, a network of three next-generation detectors would produce strong constraints on a dipole's amplitude ($\sim 13\%$) and location ($\sim 84$ deg$^2$) after just one year of observing. We demonstrate that the constraints scale with the number of detections, enabling projections for multiple years of observing. Our findings indicate that future observations of binary neutron star mergers would improve upon existing dipole constraints, provided that at least one next-generation detector is built. We also assess directional sensitivity of the dipole measurements by varying the dipole's location on a grid across the sky. We find that for a network of three next-generation detectors, the range of the constraints is only $\lesssim 1.2\%$ for the amplitude and $\lesssim 4\%$ for the location, indicating that the location of the dipole will not greatly impact our ability to measure its effects.