Evrim Yazgin
Cosmos science journalist
Normal matter makes up about a quarter of all the matter in the universe but it turns out that a good portion of it has been hiding from astronomers for decades.
New research published in Nature Astronomy shows how astronomers have found this “missing” matter.
Observations of the gravitational forces in galaxies were used to measure the amount of normal, or visible, matter. The rest is dark matter.
At least half of the visible matter – also called baryonic matter, composed mostly of protons – was unaccounted for according to astronomical observations. Astronomers had tried X-ray emission and ultraviolet observations of distant quasars (the bright material around supermassive black holes) to try and account for the missing matter.
Astronomers looked in the spaces between galaxies, hoping to find hints of thin, warm gas that would otherwise be invisible to most telescopes.
The new research uses fast radio bursts (FRBs) to expand this search. FRBs themselves are unusual and not well understood. They are short, bright bursts of radio waves from distant galaxies. The source of FRBs is not yet known.
In 2020, astronomers showed that FRBs could help find baryonic matter in the universe. But only now have astronomers been able to use this data to pinpoint the location of the matter.
The new study analysed radio signals from 60 FRBs ranging from just 12 million light-years to more than 9 billion light-years away. In the process, they identified FRB 20230521B – the most distant FRB recorded.
Studying the FRBs allowed the astronomers to show the missing baryonic matter is in the space between galaxies, or the intergalactic medium.
“The decades-old ‘missing baryon problem’ was never about whether the matter existed,” says lead author Liam Connor, an astronomer at Harvard-Smithsonian Center for Astrophysics in the US. “It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web.”
“FRBs act as cosmic flashlights,” Connor adds. “They shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it’s too faint to see.”
The results show that about 76% of baryonic matter is in the intergalactic medium, 15% is in the halos around galaxies and the rest is inside stars or cold galactic gas.
“It’s a triumph of modern astronomy,” says co-author Vikram Ravi, an assistant professor at Caltech. “We’re beginning to see the universe’s structure and composition in a whole new light, thanks to FRBs. These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies.”
“Baryons are pulled into galaxies by gravity, but supermassive black holes and exploding stars can blow them back out – like a cosmic thermostat cooling things down if the temperature gets too high,” says Connor. “Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM.”
The authors say that unlocking the mysteries of the missing baryonic matter will help uncover deeper insights into the structure of the universe, how light travels and how galaxies form.
“We’re entering a golden age,” said Ravi. “Next-generation radio telescopes like the DSA-2000 and the Canadian Hydrogen Observatory and Radio-transient Detector will detect thousands of FRBs, allowing us to map the cosmic web in incredible detail.”
