NANOGrav (2023): Evidence for the Gravitational Wave Background

The gravitational wave background is a low hum of spacetime ripples with periods of years. In June 2023 the NANOGrav collaboration reported evidence for it after timing 67 millisecond pulsars for 15 years. The leading explanation is a population of supermassive black hole pairs slowly orbiting each other across the universe.

The question before the paper

Ground-based detectors like LIGO hear gravitational waves that oscillate tens to hundreds of times per second, from merging stellar-mass black holes and neutron stars. Pairs of supermassive black holes, millions to billions of times the Sun’s mass, orbit each other far more slowly, producing waves with periods of years and wavelengths of light-years. No detector on Earth is big enough. Could the galaxy itself serve as the detector?

What the paper found

• A common signal across pulsars. The pulsars’ timing showed a common, slowly varying signal.
• The telltale pattern. Correlations between pairs of pulsars followed the Hellings-Downs curve, the specific pattern expected if gravitational waves are passing through the space between Earth and the pulsars. That correlation was detected at about 3 to 4 sigma depending on the statistical method.
• The strength of the hum. The signal’s amplitude is around 2.4 × 10^-15 at a frequency of one cycle per year, in the nanohertz range.

How they did it

Millisecond pulsars are neutron stars that spin hundreds of times per second and sweep beams of radio waves past Earth with extraordinary regularity, making them some of the best clocks in nature. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) timed the arrival of pulses from 67 of them for 15 years. A passing gravitational wave stretches and squeezes the space between Earth and each pulsar, making pulses arrive slightly early or late. Because the effect depends on direction, pulsars in different parts of the sky are affected in a correlated way, and that correlation pattern is the signature to look for.

Why it mattered

Pulsar timing arrays turn the galaxy into a detector for gravitational waves with wavelengths of light-years, far beyond the reach of LIGO-type detectors. This was the first strong evidence for that low-frequency hum. NANOGrav announced it on June 29, 2023, together with similar evidence from the European and Indian (EPTA/InPTA), Australian (PPTA) and Chinese (CPTA) pulsar timing arrays.

It is a natural successor to the Hulse-Taylor binary pulsar, where precise pulsar timing gave the first indirect evidence for gravitational waves. The expected correlation pattern was worked out by Hellings and Downs in 1983 (doi:10.1086/183954).

What we know now

As of October 2026 the signal is strong evidence, not yet a 5-sigma detection. The leading explanation is a population of supermassive black hole binaries; more exotic sources in the early universe are also being considered. The pulsar timing arrays now collaborate as the International Pulsar Timing Array to combine their data. In space, ESA’s LISA mission, planned for launch in the mid-2030s, will cover the frequencies in between, where merging supermassive black holes are expected to be loudest.

Who wrote it

The paper is signed by the NANOGrav Collaboration, with individual authors starting with Gabriella Agazie and Akash Anumarlapudi. It has about 1,400 citations in Semantic Scholar as of October 2026.

Read the original

NANOGrav Collaboration, Agazie, G., Anumarlapudi, A., et al. (2023). “The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background.” The Astrophysical Journal Letters. doi:10.3847/2041-8213/acdac6. Free on arXiv: 2306.16213.