A 10-Billion-Year-Old Flash Just Doubled the Deepest Signal in Astronomy

In 2024, a radio telescope in South Africa's Northern Cape caught a flash that lasted less than the blink of an eye. By the time that flash reached Earth, it had been traveling for more than 10 billion years, across most of the history of the universe.

This week, astronomers announced that they have traced that signal, designated FRB 20240304B, further back in time than any fast radio burst ever recorded. A study led by the University of Sydney's Manisha Caleb and Themiya Nanayakkara, published in the journal Science, identifies the burst's host galaxy and measures its distance at a redshift of 2.148, from an era when the universe was only about 3 billion years old. The new record more than doubles the previous one. A millisecond of radio light has become the deepest probe of the cosmos astronomers have ever caught.

The galaxy nobody could see

The burst was detected by the MeerTRAP project at the MeerKAT radio telescope, a 64-dish array that watches the sky for the briefest radio transients. The radio data hinted that this one was extraordinarily distant, but radio alone cannot settle the question. To confirm a distance, astronomers must find the galaxy the burst came from and measure its redshift, the stretching of its light by the expansion of the universe.

That turned out to be harder than anyone expected. The team turned to UC Santa Cruz astronomer J. Xavier Prochaska, who has been hunting FRB host galaxies from the Keck Observatory on Maunakea for years. His team spent a full hour with the LRIS instrument on the Keck I telescope, staring precisely at the burst's location. Even one of the most powerful ground-based telescopes on Earth saw nothing. The galaxy, if it was there at all, was too faint to be found from the ground.

The answer was to leave the ground behind. NASA's James Webb Space Telescope operates above the atmosphere and at extremely cold temperatures, which strips away the background interference that drowns out faint sources. Webb's Near Infrared Camera found the galaxy at the burst's location, and its Near-Infrared Spectrograph measured the telltale emission lines of oxygen and nitrogen, fixing the redshift at 2.148. The vast majority of fast radio bursts detected to date come from billions of years later in cosmic history. This one arrived from the universe's youth.

A thousand times too small

The host galaxy was a surprise in a second way. Most galaxies known to host fast radio bursts are massive star-forming systems. FRB 20240304B's host is roughly 1,000 times less massive than those, a small, metal-poor galaxy in the grip of an intense burst of star formation. It existed during what astronomers call cosmic noon, the period when star formation across the universe was at its peak, and its stellar population is exceptionally young: most of its stars may have formed within just 30 million years.

The flash came from exactly the kind of galaxy FRB hunters least expected: tiny, young, and burning through its star formation in a cosmic instant.

That unexpected address is also the discovery's sharpest clue about what makes fast radio bursts in the first place. Two main theories compete. One says FRBs come from the merger of two old neutron stars spiraling together, a process that takes billions of years and should therefore show up in old, evolved galaxies. The other says they come from young magnetars, highly magnetized neutron stars left behind when massive stars explode as supernovas, which could crackle with starquakes soon after forming. A young, furiously star-forming galaxy fits the magnetar story far better.

The result also suggests something subtle about timing. The host is young and already forming stars, yet the burst it produced implies a delay between when galaxies begin making stars and when they become capable of generating FRBs. Some time has to pass, but not billions of years. That narrows the window in which the burst engine, whatever it is, can be built.

The flashlight and the fog

Setting a distance record is only half the story. A fast radio burst is also, in the team's phrase, almost like a cosmic flashlight: as its signal travels across most of cosmic history, it carries an imprint of everything it passes through. Astronomers can read that imprint to study structures that are otherwise nearly impossible to observe directly, the vast reservoirs of gas and matter stretched between galaxies that make up the cosmic web.

The record burst's signal carries the signature of two such structures. One is a previously unknown galaxy cluster, spotted at a redshift of 0.3, about 3.5 billion light-years from Earth, that nobody knew was there. The other is the nearby Virgo Cluster, just 54 million light-years away. Neither would have been found this way without the burst lighting them up from behind.

This is the method's real power. The space between galaxies is filled with thin, almost undetectable gas that holds a large fraction of the universe's ordinary matter. Traditional telescopes struggle to see it. A fast radio burst from the early universe crosses all of it, and its signal arrives carrying a map. The more distant the flash, the longer the path and the richer the map.

How far back can they go

The previous distance record was set only in 2023, by astronomers from the same university, which gives a sense of how fast this frontier is moving. And the frontier is nowhere near exhausted. The researchers estimate that MeerKAT alone may be able to detect and localize several FRBs per year at redshifts greater than 1.0, meaning bursts from more than halfway back to the start of the universe. As new radio facilities come online, the number of distant bursts should grow rapidly.

The team is explicit about where this leads. In principle, sufficiently powerful bursts could be detectable from the very early universe, close to the first generations of stars. The next step, in the words of MeerTRAP principal investigator Ben Stappers of the University of Manchester, is to push the frontier further and see how close to the beginning they can get.

A decade ago, fast radio bursts were an unsolved anomaly: strange flashes with no known source and no clear use. The record-breaking burst of 2024 has given them a job description. Each new one is a clue about the violent objects that make them, and a flashlight for the invisible architecture of the cosmos. The deepest signal in astronomy just got a new champion. It almost certainly will not hold the title for long.