On 17 August 2017, the LIGO and Virgo observatories detected yet another gravitational wave. So far, unremarkable. But within seconds of recording this signal — which originated from a neutron-star merger — a gamma-ray burst was independently observed. This combination places tight constraints on the velocity of gravitational waves. And, as a result, theoretical models of dark energy predicting a modification of the gravitational wave speed were suddenly called into question. Could this be the end of the most popular dark energy models?
Perhaps not, say Claudia de Rham and Scott Melville. They pointed out a crucial detail: the speed of the gravitational wave could, in principle, depend on the frequency scale of its measurement. This means that dark energy models could still predict a deviation from the speed of light at low frequencies, but not at higher scales. Future experiments involving LISA or the Einstein Telescope will either confirm or rule out certain dark energy models — but their time isn’t up just yet.
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Reichert, S. Dark energy in doubt. Nature Phys 15, 4 (2019). https://doi.org/10.1038/s41567-018-0398-z