The Speed of Gravity and the Fate of Dark Energy

Abstract

On August 17${}^{\mathrm{th}}$ 2017, observatories worldwide made a landmark detection: gravitational waves and light from a binary neutron star merger. This event revolutionized our understanding of astrophysics, cosmology, and gravitation. In this proceeding of the 2025 International Congress of Basic Science, we describe how it transformed our view of cosmic acceleration (dark energy). The near-simultaneous arrival of light and gravitational waves shows that their speeds agree to within one part in $10^{15}$, excluding large classes of modified gravity theories and interactions between dark energy and matter.

Comments: Frontiers of Science Award, Proceedings of the International Congress of Basic Science, 2025