Direct test of the FLRW metric from strongly lensed gravitational wave observations

TL;DR

This paper proposes a novel, model-independent method to test the FLRW metric using strongly lensed gravitational wave events with electromagnetic counterparts, leveraging future GW detectors for precise cosmological measurements.

Contribution

It introduces a new strategy to validate the FLRW metric through galactic-scale lensing systems involving gravitational waves and electromagnetic signals, applicable with upcoming GW observatories.

Findings

Future GW detectors can accurately measure the Universe's curvature.

The method can detect deviations from the FLRW metric.

Combined analysis of multiple systems enhances robustness.

Abstract

The assumptions of large-scale homogeneity and isotropy underly the familiar Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) metric that appears to be an accurate description of our Universe. In this paper, we propose a new strategy of testing the validity of the FLRW metric, based on the galactic-scale lensing systems where strongly lensed gravitational waves and their electromagnetic counterparts can be simultaneously detected. Each strong lensing system creates opportunity to infer the curvature parameter of the Universe. Consequently, combined analysis of many such systems will provide a model-independent tool to test the validity of the FLRW metric. Our study demonstrates that the third-generation ground based GW detectors, like the Einstein Telescope (ET) and space-based detectors, like the Big Bang Observer (BBO), are promising concerning determination of the curvature parameter or possible detection of deviation from the FLRW metric. Such accurate measurements of the FLRW metric can become a milestone in precision GW cosmology.