The direct detection of gravitational waves ( GWs ) is an invaluable new tool to probe gravity and the nature of cosmic acceleration .
A large class of scalar-tensor theories predict that GWs propagate with velocity different than the speed of light , a difference that can be \mathcal { O } ( 1 ) for many models of dark energy .
We determine the conditions behind the anomalous GW speed , namely that the scalar field spontaneously breaks Lorentz invariance and couples to the metric perturbations via the Weyl tensor .
If these conditions are realized in nature , the delay between GW and electromagnetic ( EM ) signals from distant events will run beyond human timescales , making it impossible to measure the speed of GWs using neutron star mergers or other violent events .
We present a robust strategy to exclude or confirm an anomalous speed of GWs using eclipsing binary systems , whose EM phase can be exquisitely determined .
he white dwarf binary J0651+2844 is a known example of such system that can be used to probe deviations in the GW speed as small as c _ { g } / c - 1 \gtrsim 2 \cdot 10 ^ { -12 } when LISA comes online .
This test will either eliminate many contender models for cosmic acceleration or wreck a fundamental pillar of general relativity .