We propose a new strategy to probe non-tensorial polarizations in the stochastic gravitational-wave (GW) background. Averaging over polarization angles, we find that three-point correlations of the GW signal vanish for tensor and vector modes, while scalar modes generically leave a nonzero imprint. This property makes the GW bispectrum a distinctive and robust diagnostic of scalar polarizations predicted in theories beyond General Relativity. We derive the corresponding response functions for ground-based interferometers, pulsar timing arrays, and astrometric observables, and we construct an optimal estimator together with simple Fisher forecasts for pulsar-timing sensitivity. As a proof of principle, we show that second-order GWs sourced by primordial magnetogenesis can be characterized by large three-point functions. Our results demonstrate that GW three-point correlations provide a novel observational window on physics beyond General Relativity.
@article{arxiv.2509.08273,
title = {New test of modified gravity with gravitational wave experiments},
author = {N. M. Jiménez Cruz and Flavio C. Sánchez and Gianmassimo Tasinato},
journal= {arXiv preprint arXiv:2509.08273},
year = {2025}
}