Open questionCosmology · Universe

The gravitational-wave hum

Pulsar timing arrays now detect a low-frequency background of gravitational waves. The leading suspect is pairs of supermassive black holes, but the source is not settled.

Open question · nanohertz gravitational waves

Signal
A nanohertz gravitational-wave background
Detectors
Pulsar timing arrays such as NANOGrav, EPTA, PPTA, InPTA, CPTA
Leading source
Merging supermassive black hole binaries
Announced
2023, with evidence growing since
Wavelengths
Far longer than those LIGO detects

A cosmic-scale clock

NASA/JPL-Caltech
Artist concept of a pulsar, a rotating neutron star
An artist's concept of a pulsar, the rotating dead star whose timing reveals the gravitational-wave hum. Image: NASA/JPL-Caltech.

Listening through pulsars

Millisecond pulsars spin with clock-like regularity. Timing arrays watch dozens of them for years and look for tiny, correlated drifts in arrival times. Those drifts trace a background of gravitational waves rolling across the whole sky.

The wavelengths involved are light-years long, so the signal is a slow rumble rather than a sharp chirp.

Where the hum comes from

The leading explanation is the combined signal of supermassive black hole binaries in distant galaxies. Other candidates include cosmic strings, a first-order phase transition, or effects from the early universe. The data are not yet precise enough to pick one.

Why it matters

The hum opens a window on the heaviest binaries in the universe and on events no telescope sees directly. As arrays add pulsars and accumulate data, the background should resolve into individual sources and reveal which explanation is right.

Key publications

YearPaperVenueLink
2023Agazie et al. (NANOGrav), The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave BackgroundApJL 951, L8DOI
2023EPTA Collaboration, The second data release from the European Pulsar Timing ArrayA&A 678, A48DOI
2023Reardon et al., Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar Timing ArrayApJL 951, L6DOI

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

  • The hum has wavelengths measured in light-years.

  • It was found by timing dead stars to billionths of a second.

  • Several independent teams saw a consistent signal.

Recommended videos

  • Gravitational Wave Background: Scientists Use Pulsar Timing Arrays Science Magazine
  • Gravitational Wave Background Discovered? Space Matters

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