| Abstract: |
Many dark energy and modified gravity models can alter the sound speed of gravitational waves, which was recently tightly constrained by the LIGO/Virgo Collaborations. However, these constraints must be interpreted with great care. The effective sound speed in these theories is controlled by how new degrees of freedom couple to metric fluctuations, and can be described using a low energy Effective Field Theory. Within such a framework, the sound speed generically runs with scale, and therefore the (cosmologically relevant) speed at the Hubble scale can be very different from the (observed) speed at binary merger scales. This has the potential to save many theories previously believed to be ruled out by the GW170817 observations. |
