A distant orchestra plays a symphony, but the recording is full of noise. The melody is written for two instruments, yet a third voice seems to break through the static — quiet and unfamiliar. This is how astrophysicists listen to gravitational waves, trying to catch the slightest deviations from Einstein's predictions.
In 2017, the LIGO and Virgo detectors caught a signal from the merger of two neutron stars — an event named GW170817. Along with the wave came a gamma-ray burst that astronomers tracked for weeks. This burst allowed them to precisely determine the wave's orientation in space: its polarization. Previously, this angle remained unknown, but now it became a strict constraint for any theories attempting to improve gravity.
The researchers added three new parameters to the standard signal shape. One accounted for a scalar 'breathing'—an extra pulsation that changes volume rather than shape of space. The other two modified the familiar tensor modes. It turned out that the scalar addition is barely noticeable, but the overall picture still favors simplicity: the pure Einsteinian signal explains the data better than the extended model.
The main takeaway is not that Einstein was right again. The powerful tool is the joint analysis of gravitational and electromagnetic signals. The constraint on the polarization angle obtained from the afterglow improved the precision of the non-standard parameter estimates by 30–60%. Future observations, where both the wave and light from a single event are detected, will be able to test the boldest hypotheses about the nature of gravity. Then, the third voice from the distant orchestra will either become clearer or finally dissolve into the noise.
as written by the authors arXiv:2604.24526
Gravitational wave (GW) observations provide a unique opportunity to test Einstein's General Relativity (GR) in the strong-field regime. While GR predicts only two tensor polarization modes, generic metric theories allow up to six independent modes. We perform a parameterized test of GR using the parameterized post-Einsteinian (PPE) framework applied to GW170817, incorporating for the first time the polarization angle constraints from the gamma-ray burst afterglow alongside other electromagnetic (EM) counterpart information. We extend the GR waveform by adding a scalar breathing mode and modifications to the tensor modes, introducing three non-GR parameters. We perform Bayesian inference for both quadrupole $$\ell = |m|= 2$$ and dipole $$\ell = |m|= 1$$ angular harmonics, with two frequency evolution models. For $$\ell = |m| = 2$$, we find that within the extended PPE framework the scalar amplitude deviates from zero at the $$2$$--$$3\sigma$$ level, leading to a modest preference for modified gravity. However, due to penalization of extra parameters, Bayesian model comparison still favors pure GR over the extended PPE waveform model, with a log Bayes factor of $$\Delta \log \mathcal{Z} = 4.67 \pm 0.32$$. The EM constraint on the polarization angle places very tight bounds on non-GR parameters; for instance, in the case $$\ell = |m| = 2$$, the bound on the scalar (tensor) amplitude modification parameter improves by roughly $$60\%$$ $$(30\%)$$, highlighting the impact that long-term follow up of GW events can have on tests of gravity.
The neutron star collision produced not only a gravitational wave but also a kilonova — an explosion in which, it is believed, gold and platinum with a mass of an entire Earth were formed.
In Carl Sagan's novel 'Contact', a signal from space contains extra dimensions hidden in polarization — much like the search for new modes of gravity.