This fast radio burst traces to 3 billion years after the Big Bang

A fast radio burst, or FRB, is a sudden, extremely bright pulse of radio waves from deep space. Each one lasts only milliseconds but can release as much energy as the sun puts out in three days.

The first was spotted in 2007, and thousands have been recorded since. Most show up once and never repeat, which makes them hard to study. Nobody has proven what produces them.

As lead author Manisha Caleb of the University of Sydney put it, scientists have theories about the objects behind FRBs but no conclusive proof.

The burst, now called FRB 20240304B, was picked up on March 4, 2024, by the MeerTRAP team using the MeerKAT radio telescope array in South Africa. Its different radio frequencies arrived at slightly different times, a sign that it had passed through a huge amount of thin gas in space. The more the signal is smeared out, the farther it has traveled, and this one pointed to a record distance.

To confirm that, astronomers needed to see the galaxy it came from. They knew exactly where to look, but even the largest telescopes on the ground saw nothing there. So they turned to Webb, which sees in infrared light and can pick out very faint, distant galaxies.

Webb’s Near Infrared Camera spotted a galaxy in the right place. Its Near Infrared Spectrograph then measured the galaxy’s redshift, how much its light has been stretched by the expansion of the universe, at 2.148. That places it about 3 billion years after the Big Bang, in a universe that is now about 13.8 billion years old.

The bigger surprise was the galaxy itself. Most galaxies known to host FRBs are large and busy forming stars. This one was a dwarf, about 1,000 times less massive than the team expected.

Caleb said the team thought it would be a big, well-formed galaxy full of stars and instead found a little dwarf that was still actively making new ones. Co-author Ben Stappers of the University of Manchester said the host stands out from every other galaxy in their sample.

The galaxy lived during “cosmic noon,” the stretch of history when the universe was forming stars at its fastest rate. Its star-forming rate suggests most of its stars may have formed within only 30 million years, which is very young by cosmic standards. It also contained few heavy elements, another mark of youth.

There are two leading ideas about what causes fast radio bursts.

• Neutron star mergers. Neutron stars are the super-dense cores left when giant stars die. Two of them circling each other can take billions of years to collide, so bursts from mergers should come mostly from older galaxies.
• Magnetars. These are young neutron stars with enormously strong magnetic fields, born in supernova explosions. A magnetar could release an FRB soon after it forms, through something like a “starquake” on its surface, so bursts could appear in young galaxies.

A young dwarf galaxy, packed with recently formed stars, fits the magnetar picture far better. Caleb said the team’s work makes it very unlikely that this burst came from a merger. Earlier research had already tied at least one FRB to a magnetar, but that did not mean all of them have the same source. This result adds weight to the magnetar side, though it does not close the case for every burst.

Fast radio bursts double as probes of the universe. As a burst travels, it picks up an imprint of everything it passes through. J. Xavier Prochaska, a UC Santa Cruz astronomer on the team, compared one to a cosmic flashlight that lights up the path along the way.

FRB 20240304B carried the marks of two big structures: a galaxy cluster nobody knew about, around 3.5 billion light-years away, and the much closer Virgo Cluster, about 54 million light-years from Earth. That helps astronomers map matter that is otherwise invisible.

Co-author Themiya Nanayakkara said the team has now shown this method works across about 80 percent of cosmic history. For the rest of us, that means better maps of where the universe’s “missing” ordinary matter is hiding, a long-running gap in our picture of the cosmos.

The researchers estimate MeerKAT can find and pinpoint several bursts a year from more than halfway back to the start of the universe. They have also won more Webb time to track down the homes of other ancient FRBs, and new radio telescopes coming online should speed up the search. Webb is run by NASA with the European Space Agency and the Canadian Space Agency.