In the 1960s, scientists became acutely aware of a problem with the Universe's "mass budget." Based on the observed rotational curves of galaxies, they determined that about 85% of the Universe's mass was invisible, leading to the theory of Dark Matter. Scientists have also been aware for some time that much of the "normal" or baryonic matter (that which we can see) in the Universe was also unaccounted for. This has prompted multiple efforts to probe the Universe for this "missing" mass, using everything from X-ray emissions and ultraviolet observations of distant quasars to find hints of where it might be hiding.

In a new landmark study, astronomers at the Harvard & Smithsonian Center for Astrophysics (CfA) and Caltech announced the detection of the most distant fast radio burst (FRB) on record. Using this phenomenon as a guide, the team conducted the first detailed measurement of ordinary matter distribution across the cosmos. Their results show that more than three-quarters of the Universe's baryonic matter exists between galaxies (aka. the intergalactic medium) as hot, diffuse clouds of gas previously invisible to telescopes. This research helps resolve a longstanding mystery in cosmology and is a major step forward in understanding how matter interacts and behaves in the Universe.

The study was led by Liam Connor, a Canadian astrophysicist and radio astronomer with the CfA and the Cahill Center for Astronomy and Astrophysics at the California Institute of Technology. He was joined by colleagues from the CfA, Caltech's Owens Valley Radio Observatory, and the Observatories of the Carnegie Institution for Science. The paper that describes their findings, "A gas-rich cosmic web revealed by the partitioning of the missing baryons," recently appeared in Nature Astronomy.

Artist's impression of an extragalactic FRB. Credit: ESO/M. Kornmesser

Fast Radio Bursts (FRBs) are bright explosions of radio waves that typically last for mere milliseconds, though events lasting a few seconds have been recorded. While the origin of these bursts is still subject to debate, the general consensus is that they are associated with compact objects (neutron stars and black holes). Recently, scientists demonstrated that FRBs from distant galaxies could be used to measure baryonic matter throughout the Universe. But until now, astronomers could not find the location of the most distant FRBs, which would allow them to explore the distribution of matter on cosmic scales.

"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," said Conner in a CfA press release. "Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM." As part of their research, a team analyzed 60 FRBs ranging in distance from 11.74 million light-years (FRB20200120E in the M81 galaxy) to the most distant FRB on record - FRB 20230521B, located ~9.1 billion light-years away. By measuring how much each FRB signal slowed as it passed through the intergalactic medium (IGM), Connor and his team could track the gas as it travelled to reach Earth. Said Conner:

The decades-old 'missing baryon problem' was never about whether the matter existed. It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web." In other words, scientists now know the home address of the "missing" matter. FRBs act as cosmic flashlights. 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.

According to their results, approximately 76% of the Universe's baryonic matter lies in the IGM, about 15% is located in galaxy halos, and the remainder consists of stars, cold galactic gas, and other objects. These measurements align with predictions based on advanced cosmological simulations, confirming what was theoretical until now. Locating the missing matter in the Universe also has the potential to address other cosmological questions. These include how galaxies form, how matter coalesces in the Universe, and how light travels across vast cosmological distances. Vikram Ravi, an assistant professor of astronomy at Caltech and co-author on the paper, is also the co-Principal Investigator of Caltech's Deep Synoptic Array-110 (DSA-110):

It's a triumph of modern astronomy. 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. We're entering a golden age. 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.