The overwhelming majority of matter in the cosmos is dark—it remains completely unseen and can only be recognized through its gravitational influence. Regular matter—spanning protons, planets, and humans—accounts for merely 16 percent. In contrast to dark matter, regular matter emits light across a spectrum of wavelengths, making it visible. However, a significant portion of it is diffuse, dispersed thinly within halos that encircle galaxies as well as across the vast voids between them.
Owing to its dispersed character, roughly half of the regular matter in the cosmos was previously unconsidered and labeled as “missing”—until now.
In a recent investigation in Nature Astronomy, a collective of astronomers from Caltech and the Center for Astrophysics | Harvard & Smithsonian (CfA) has, for the initial time, directly identified and accounted for all the missing matter. To achieve this, the team utilized quick, bright radio signals from the distant universe, known as fast radio bursts (FRBs), to illuminate the matter situated between the FRBs and us.
“The FRBs pierce through the haze of the intergalactic medium, and by meticulously gauging how the light is slowed, we can assess that haze, even when it’s too faint to visualize,” states Liam Connor, assistant professor at Harvard and principal author of the study, who conducted much of the research while a research assistant professor at Caltech collaborating with Vikram Ravi, assistant professor of astronomy at Caltech.
The research examined a total of 69 FRBs situated at distances ranging from approximately 11.74 million to about 9.1 billion light-years. The object 9.1 billion light-years distant, designated FRB 20230521B, now holds the record for the most distant FRB ever noted. Despite more than a thousand FRBs being recorded, only about a hundred have been traced back to specific host galaxies; in other words, their origins and distances from Earth have been established. These localized FRBs were essential for the current investigation.
Out of the 69 localized FRBs in the investigation, 39 were discovered utilizing the DSA (Deep Synoptic Array)-110, a constellation of 110 radio telescopes situated at Caltech’s Owen Valley Radio Observatory (OVRO), near Bishop, California. The radio array, specifically designed to capture and pinpoint FRBs, identified the 39 objects and their galaxies of origin, while instruments at Hawaii’s W. M. Keck Observatory and the Palomar Observatory near San Diego ensured their distances. The other 30 FRBs in the study were found by telescopes worldwide, predominantly the Australian Square Kilometre Array Pathfinder.
These FRBs, while captivating in their own right, were employed in this investigation to reveal the absent regular matter; other methods had only suggested its presence. As radio-frequency light travels from the FRBs to Earth, the light disperses into various wavelengths, much like a prism transforms sunlight into a spectrum. The level of this dispersion relies on the amount of matter in the light’s path.
“It’s akin to observing the shadow of all the baryons, with FRBs acting as the backlight,” explains Ravi. “If you can see an individual before you, you can learn a great deal about them. But if you can only see their shadow, you still recognize that they exist and have a rough idea of their size.”
The findings indicated that 76 percent of the universe’s normal matter is situated in the regions between galaxies, referred to as the intergalactic medium. Approximately 15 percent resides in galaxy halos, while the remainder is concentrated within galaxies—in stars or within cold galactic gas. This distribution aligns with predictions from sophisticated cosmological simulations but has never been observationally validated until now.
The discoveries will enhance researchers’ understanding of how galaxies evolve, and they illustrate how FRBs can assist with challenges in cosmology, including determining the typical mass of subatomic particles known as neutrinos. (The mass of neutrinos varies based on the clustering of baryons.) The standard model of physics anticipates that neutrinos should possess no mass, yet observations have revealed that these particles do carry an extremely minuscule amount. Consequently, determining the exact mass of neutrinos could lead to new physics that transcends the standard model of particle physics.
According to Ravi, this is merely the beginning of utilizing FRBs in cosmology. In the future, Caltech’s DSA-2000 radio telescope in the Nevada desert, currently under development, will build upon investigations like this one. The radio array will discover and localize up to 10,000 FRBs annually, significantly enhancing their utility as probes of regular matter and enriching our overall comprehension of these extreme phenomena.
The study titled “A gas-rich cosmic web revealed by partitioning the missing baryons,” received funding from the National Science Foundation. Additional Caltech contributors include Kritti Sharma (MS ’24), Stella Koch Ocke, Jakob Faber (MS ’24), Gregg Hallinan, Charlie Harnach, Greg Hellbourg, Rick Hobbs, David Hodge, Mark Hodges, Nikita Kosogorov (MS ’24), James Lamb, Casey Law, Paul Rasmussen, Myles Sherman, Jean Somalwar, Sander Weinreb, and David Woody.