Fast Radio Bursts

Summary

A flash of radio waves lasting a few thousandths of a second, carrying as much energy as the Sun emits in days, arriving from a galaxy billions of light-years away.

Thousands of them have now been catalogued. For a decade nobody had any idea what they were, and the speculation was extravagant, and some of it was about aliens.

They are, at least in part, magnetars: neutron stars with magnetic fields of a violence that is difficult to write down.

And then something happened that the registry finds more remarkable than the bursts. Astronomers took this unexplained phenomenon and used it as an instrument, and with it they found half the ordinary matter in the universe, which had been missing.

What is documented

The discovery. In 2007, Duncan Lorimer and a student, David Narkevic, found a burst in archival data taken at Parkes in 2001. It became known as the Lorimer burst.

The properties. Milliseconds long. Extremely bright. And dispersed.

Dispersion, which is the key to everything in this record. Radio waves travelling through space pass through ionised gas, and that gas is dispersive: it slows the lower frequencies more than the higher ones.

So a burst that leaves its source as a single instantaneous flash arrives smeared out, with the high frequencies first and the low frequencies trailing behind. (This is the same physics as AR-0046, on a cosmological scale.)

The amount of smearing tells you how much ionised matter the signal passed through on its way to you.

The dispersion says they are far away. The measured dispersions of fast radio bursts are far too large to be explained by material within our own galaxy. They have come from outside it, from very great distances.

The catalogue. Thousands are now known. The CHIME telescope in Canada, which stares at a large strip of sky continuously, has found the bulk of them.

Repeaters. Some bursts repeat. The first known repeater, FRB 121102, was identified in 2016. Many are now known. Others have never been seen twice. This suggests, though it does not prove, that there may be more than one kind of source.

The magnetar. In April 2020, a magnetar within our own galaxy, SGR 1935+2154, produced a burst that was detected simultaneously by CHIME and by STARE2.

It was the first fast radio burst ever detected from inside the Milky Way, and the first for which the source could be identified with confidence.

A magnetar is a neutron star with a magnetic field of the order of a hundred trillion times the Earth's. They are among the most extreme objects in the universe, and one of them made a fast radio burst, and we watched it happen.

The caveat. The galactic burst was substantially less energetic than a typical cosmological fast radio burst, by a factor of around a thousand. Magnetars can do this. Whether magnetars can do the brightest ones is not settled.

Leading explanations

Magnetars, for at least some of them. Established by direct observation.

The 2020 detection is the strongest single result in the field. It converted the question from "what could possibly do this" into "can the thing we have now seen doing it also do the rest."

The environments differ, and that is a real puzzle. Bursts that have been localised to their host galaxies do not come from a single kind of place. FRB 121102 sits in a dwarf galaxy in a region of intense star formation, alongside a persistent radio source. Others have been traced to large, quiet spiral galaxies, in regions where nothing much is happening.

That is not what a single population of identical objects looks like.

And then they became a tool. This is the part that deserves the record.

For decades, cosmology had a missing baryon problem. The theory said the universe should contain a certain amount of ordinary matter, the stuff that atoms are made of, and when astronomers added up everything they could see, roughly half of it was not there.

It was believed to be spread through intergalactic space as a thin, hot, diffuse gas, too tenuous to detect by any ordinary means.

But a fast radio burst passes through it. And the dispersion of the burst measures exactly how much ionised matter it went through.

Macquart and colleagues published in Nature in 2020. Using the dispersion measures of localised fast radio bursts, together with the known distances to their host galaxies, they weighed the intergalactic medium.

The missing matter was there. The census came out right.

Current status

Partially explained. Magnetars are established as a source of at least some bursts. The full population is not accounted for, the repeaters and non-repeaters may be different things, and the host galaxies are too varied for a single simple story.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.