A neutron-star merger finds no sign that gravity’s strength changed over time

Data from GW170817 and its associated gamma-ray burst place the tightest limits yet from real gravitational-wave observations on changes in the gravitational constant.

Editorial illustration — not from the study.

Researchers developed a gravitational-wave model that allows the gravitational constant, G, to vary slowly. The model includes possible effects on the motion of the merging neutron stars and on the waves as they travel through the expanding universe.

They then combined gravitational-wave data from GW170817 with independent information from GRB 170817A about the event’s distance, location and binary orientation. The analysis found no evidence for a changing G and produced the most stringent limits to date based on real gravitational-wave observations.

What the merger showed

The analysis found no evidence for a temporal change in the gravitational constant. It constrained the fractional time derivative of G, written as dot G divided by G, to between −3.36 × 10⁻⁹ and 5.34 × 10⁻¹⁰ per year.

Why the test matters

The constancy of the gravitational constant is a central part of the strong equivalence principle and general relativity. By combining gravitational waves with light from the same cosmic event, the study tests this principle in the strong-gravity, rapidly changing conditions of a neutron-star merger and sets tighter limits than previous real gravitational-wave observations.

Evidence and limits

The result comes from a joint Bayesian analysis of one gravitational-wave event, GW170817, together with electromagnetic constraints from its associated gamma-ray burst, GRB 170817A. The researchers used a waveform model that includes possible changes in G during the binary’s dynamics and during wave propagation. Because the result is based on this single merger and on the assumptions in that model, it constrains variation of G in this event rather than ruling out every possible form of variation in all circumstances.

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