Scientists find universe's missing matter while watching fast radio bursts shine through 'cosmic fog'

Half of the universe's ordinary matter was missing — until now.

Astronomers have used mysterious but powerful explosions of energy called fast radio bursts (FRBs) to detect the universe's missing "normal" matter for the first time.

This previously missing stuff isn't dark matter, the mysterious substance that accounts for around 85% of the material universe but remains invisible because it doesn't interact with light. Instead, it is ordinary matter made out of atoms (composed of baryons) that does interact with light but has until now just been too dark to see.

Though this puzzle might not quite get as much attention as the dark matter conundrum — at least we knew what this missing matter is, while the nature of dark matter is unknown — but its AWOL status has been a frustrating problem in cosmology nonetheless. The missing baryonic matter problem has persisted because it is spread incredibly thinly through halos that surround galaxies and in diffuse clouds that drift in the space between galaxies.

Now, a team of astronomers discovered and accounted for this missing everyday matter by using FRBs to illuminate wispy structures lying between us and the distant sources of these brief but powerful bursts of radio waves.

"The FRBs 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," study team leader Liam Connor, a researcher at the Center for Astrophysics, Harvard & Smithsonian (CfA), said in a statement.

FRBs are FAB searchlights for missing matter

FRBs are pulses of radio waves that often last for mere milliseconds, but in this brief time they can emit as much energy as the sun radiates in 30 years. Their origins remain something of a mystery. That's because the short duration of these flashes and the fact that most occur only once make them notoriously hard to trace back to their source.

Yet for some time, their potential to help "weigh" the matter between galaxies has been evident to astronomers. Though thousands of FRBs have been discovered, not all were suitable for this purpose. That's because, to act as a gauge of the matter between the FRB and Earth, the energy burst has to have a localized point of origin with a known distance from our planet. Thus far, astronomers have only managed to perform this localization for about 100 FRBs.

Connor and colleagues, including California Institute of Technology (Caltech) assistant professor Vikram Ravi, utilized 69 FRBs from sources at distances of between 11.7 million to about 9.1 billion light-years away. The FRB from this maximum distance, FRB 20230521B, is the most distant FRB source ever discovered.

Of the 69 FRBs used by the team, 39 were discovered by a network of 110 radio telescopes located at Caltech's Owen Valley Radio Observatory (OVRO) called the Deep Synoptic Array (DSA). The DSA was built with the specific mission of spotting and localizing FRBs to their home galaxies.

Once this had been done, instruments at Hawaii's W. M. Keck Observatory and at the Palomar Observatory near San Diego were used the measure the distance between Earth and these FRB-source galaxies.

Many of the remaining FRBs were discovered by the Australian Square Kilometre Array Pathfinder (ASKAP), a network of radio telescopes in Western Australia that has excelled in the detection and localization of FRBs since it began operations.

As FRBs pass through matter, the light that comprises them is split into different wavelengths. This is just like what happens when sunlight passes through a prism and creates a rainbow diffraction pattern.

The angle of the separation of these different wavelengths can be used to determine how much matter lies in the clouds or structures that the FRBs pass through.

"It's like we're seeing the shadow of all the baryons, with FRBs as the backlight," Ravi explained. "If you see a person in front of you, you can find out a lot about them. But if you just see their shadow, you still know that they're there and roughly how big they are."

The team's results allowed them to determine that approximately 76% of the universe's normal matter lurks in the space between galaxies, known as the intergalactic medium. They found a further 15% is locked up in the vast diffuse haloes around galaxies. The remaining 9% seems to be concentrated within the galaxies, taking the form of stars and cold galactic gas.

The distribution calculated by the team is in agreement with predictions delivered by advanced simulations of the universe and its evolution, but it represents the first observational evidence of this.

The team's results could lead to a better understanding of how galaxies grow. For Ravi, however, this is just the first step toward FRBs becoming a vital tool in cosmology, aiding our understanding of the universe.

The next step in this development may well be Caltech's planned radio telescope, DSA-2000. This radio array, set to be constructed in the Nevada desert, could spot and localize as many as 10,000 FRBs every year.

This should both boost our understanding of these powerful blasts of radio waves and increase their usefulness as probes of the universe's baryonic matter content.

The team's research was published on Monday (June 16) in the journal Nature Astronomy.