The Speed of Gravity and the Fate of Dark Energy

TL;DR

The paper discusses how the 2017 neutron star merger observation confirmed that gravitational waves and light travel at the same speed, significantly impacting theories about dark energy and cosmic acceleration.

Contribution

It provides evidence that gravitational waves and light have nearly identical speeds, constraining modified gravity theories and interactions involving dark energy.

Findings

Gravitational waves and light arrive within one part in 10^{15} of each other.

This observation excludes many modified gravity models.

The results support the standard cosmological model with dark energy.

Abstract

On August 17$^{\rm th}$ 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.