What is sending millisecond radio bursts across the universe?

A pulse a few milliseconds long, bright in radio, and smeared in a very particular way: the low frequencies arrive after the high ones, because the intervening plasma slows them differently.

That smearing is the useful part. Its size measures how much ionised material the signal crossed, and for these bursts it is far more than our own galaxy can supply, which is how they were argued to be extragalactic before any host galaxy was identified.

In April 2020 a burst arrived bright enough that the CHIME telescope caught it far outside the patch of sky it was pointed at, and it came from SGR 1935+2154, a magnetar inside the Milky Way. CHIME and a second instrument, STARE2, reported it independently in Nature.

That single detection did more than years of theory. It showed that an object of a known type, in a known place, can produce a burst with the right properties, even if the one from our own galaxy was less energetic than the distant ones.

Some sources repeat, sometimes with a regular window of activity. Most have been seen only once: of 3,641 sources in the CHIME/FRB catalogue for 2018 to 2023, 83 have repeated. Those may be different objects, or the same objects caught in different states.

Host galaxies vary too: most bursts traced so far come from star-forming galaxies, where young magnetars are expected, but some come from older stellar populations where they are not. One repeater has been traced to a globular cluster belonging to the nearby galaxy M81, where the stars are old, and another to the far outskirts of a quiescent elliptical galaxy.

Even granting a magnetar, the mechanism is argued: a shock far from the star, or emission from within its magnetosphere, near the star itself. The two put the emission at very different distances, which shows in the size of the emitting region and in how the polarisation turns across a burst. For one nearby burst, detected in 2022, both now point to the magnetosphere.

This is a field where the observations are currently ahead of the theory.

A handful of events look unlike the rest, such as a burst of nearly three seconds whose nine peaks keep a strict 216.8-millisecond beat, and with a population this large and this recently discovered, the tail of the distribution is barely sampled.

The honest position is that the common ones are becoming explicable and the unusual ones are not.

The same smearing that made them puzzling makes them instruments. Each burst measures the ionised material along its path, and with enough of them at known distances the diffuse matter between galaxies can be weighed.

That material had been predicted and only partly located, and fast radio bursts have now counted it: five well-placed bursts in 2020 found the amount the early universe requires, and a larger sample in 2025 began to say where it lies. A mystery converted into a measuring device is one of the more satisfying things astronomy does.