Mysterious Radio Bursts Help Track the Universe’s Missing Matter

Fast radio bursts remain one of astronomy’s strangest phenomena. Now, researchers are using these fleeting signals to trace matter that is almost impossible to see.

Fast radio bursts are among the strangest signals in the universe.

They appear suddenly, blaze across radio wavelengths for only a few thousandths of a second and then disappear. Some unleash as much energy in that instant as the Sun radiates over several days.

Years after the first one was reported, scientists are still working out exactly what produces them. Magnetars—highly magnetic remnants of dead stars—appear capable of making at least some FRBs, but whether they explain them all remains unclear.

But the mystery surrounding FRBs has not stopped astronomers from putting them to another use.

Sleuthing the Universe

A study published in Nature Astronomy led by Caltech shows researchers used 114 FRBs to investigate another puzzling question from the cosmos: how ordinary matter is distributed through the universe and how galaxies have pushed some of it far from where gravity would otherwise keep it.

Everything familiar in the universe—you, me, planets, stars, gas, coffee mugs and so on—is made from what physicists call baryonic, or ordinary, matter.

Scientists know roughly how much ordinary matter should exist based partly on measurements of the early universe. For years, however, astronomers had trouble accounting for all of it close by.

Much of that apparently missing matter turned out to be extremely thin gas spread through and around galaxies and the spaces between them. But finding it is only part of the problem. Researchers also need to know what galaxies have done to it.

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Stars explode as supernovae and actively feeding supermassive black holes can launch powerful winds from the gas swirling around them. This can push enormous amounts of gas hundreds of thousands or even millions of light-years outward. Astronomers group these effects under the term “feedback.”

Over time, feedback can push vast amounts of gas far beyond the regions where gravity would otherwise keep it concentrated.

This can be a problem for astronomers who use the way matter is distributed across the universe to test some of their most important ideas about cosmology. Gravity gradually pulls matter together, forming galaxies, galaxy clusters and the vast cosmic web that connects them. Exactly how strongly matter clumps can reveal information about dark matter, dark energy and even the masses of neutrinos.

But when feedback moves ordinary gas outward, supernovae and black holes make matter look less concentrated on certain scales. If astronomers do not account for that redistribution, it can complicate attempts to use cosmic structure to probe the universe’s more fundamental physics.

In other words, ordinary gas can get in the way of understanding some of the least ordinary things in the universe.

A signal with a travel history

FRBs offer astronomers another way to measure the movement of matter.

An FRB may last only milliseconds, but its journey to Earth can take hundreds of millions or billions of years. Along the way, its radio waves pass through ionized gas containing free electrons.

Those electrons leave a fingerprint. Lower-frequency radio waves are delayed slightly more than higher-frequency waves. Astronomers measure that delay through something called the dispersion measure. The more free electrons the signal encounters, the larger the effect.

A single FRB cannot reveal exactly where every cloud of gas lies. But if astronomers know the burst’s distance, its dispersion measure tells them about the total amount of ionized material along that path.

Compare many such sightlines and patterns begin to emerge.

The researchers analyzed 114 localized FRBs, meaning astronomers had identified the galaxies where the bursts originated.

The team examined how dispersion measurements varied from one line of sight to another. Those differences revealed information about how clumpy the gas is and how strongly feedback has redistributed it.

They also constrained how much feedback has reduced the clustering of matter and estimated how much gas remains inside massive halos surrounding groups and clusters of galaxies.

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Modern astronomical surveys often collect measurements of thousands or millions of objects. Yet this comparatively small FRB sample produced constraints already competitive with established measurements from the Atacama Cosmology Telescope and eROSITA, an X-ray space telescope used to study hot gas across the sky.

For a technique based on a phenomenon astronomers still cannot fully explain, that is an impressive start.

Now a new generation of observatories, including the Vera C. Rubin Observatory and the Euclid telescope, will make increasingly precise measurements of how matter is distributed throughout the universe.