The hum: a background of gravitational waves across the whole sky

Two kinds of gravitational wave

The detectors that made headlines in 2015 catch short chirps: two black holes or neutron stars spiralling together and merging in a fraction of a second. Those waves wobble hundreds of times per second.

There should also be waves that take years to complete one cycle. The expected source is pairs of supermassive black holes, millions of times heavier than the Sun, circling each other at the centres of merged galaxies. No ground detector can sense a wave that slow. Its wavelength is light-years long.

Using the galaxy as the detector

A pulsar is a collapsed star that spins and flashes with astonishing regularity, some of them better than an atomic clock over years. A passing gravitational wave stretches and squeezes the space between us and the pulsar, so the flashes arrive very slightly early or late.

One pulsar cannot tell a gravitational wave from ordinary noise. But dozens of pulsars, watched for fifteen years, should show a specific pattern: pulsars close together on the sky are affected in the same way, pulsars far apart in an opposite way. That pattern has a predicted shape, worked out in 1983, and it is hard to fake.

What was reported

In June 2023 four independent collaborations, in North America, Europe, Australia and China, published results on the same day. All saw a signal with roughly the right strength, and the North American set saw the tell-tale angular pattern with a confidence a little short of the traditional discovery threshold.

That is why the word used was "evidence", not "detection". More years of data should push it over the line or dismantle it.

What it could be

The simplest reading is the combined hum of every supermassive black-hole pair in the observable universe, which would confirm that such pairs really do get close and merge. A stranger reading is that some of the hum comes from the first fraction of a second after the Big Bang. Distinguishing the two needs a measurement of how the strength changes with frequency.

Why it matters

It is a new way to see. For the first time, the slowest gravitational waves in nature are within reach, using a method that costs telescope time rather than a new machine.