For decades, one of the universe’s most significant components remained elusive: the vast majority of ordinary matter, the stuff that makes up everything visible around us – stars, planets, and even people. While scientists had long accounted for the mysterious, unseen “dark matter” and the equally perplexing “dark energy,” roughly half of the universe’s expected ordinary matter (also known as baryonic matter) simply couldn’t be found by traditional means. This cosmic puzzle, often called the “missing baryon problem,” has finally been solved, thanks to a groundbreaking study utilizing powerful cosmic signals called Fast Radio Bursts (FRBs).
Astronomers from Caltech and the Center for Astrophysics | Harvard & Smithsonian (CfA) have, for the first time, directly detected and accounted for this long-sought missing matter. Their findings confirm a key prediction of cosmological models: that this matter isn’t hiding in stars or galaxies, but is instead dispersed as a thin, diffuse gas across the immense voids and filaments that connect galaxies, forming the universe’s large-scale structure known as the cosmic web or intergalactic medium (IGM).
Weighing the Unseen Cosmic Fog with FRBs
So, how do you find matter that’s too diffuse to see? The research team used Fast Radio Bursts as cosmic probes. FRBs are incredibly brief, yet extraordinarily powerful flashes of radio waves originating from distant galaxies. As these intense pulses travel billions of light-years through the universe towards Earth, they interact with the ionized gas – primarily free electrons associated with ordinary matter – scattered throughout the intergalactic medium.
This interaction causes a phenomenon known as “dispersion.” Different wavelengths of the radio signal travel at slightly different speeds, with longer wavelengths slowing down more than shorter ones, much like how a prism separates white light into a rainbow. By precisely measuring the degree of this “dispersion measure” for an FRB with a known distance, astronomers can directly infer the total amount of ionized gas, and thus the ordinary matter, located along the signal’s path.
Lead author Liam Connor, an assistant professor at Harvard, describes it vividly: “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.” Co-author Vikram Ravi, assistant professor of astronomy at Caltech, adds, “It’s like we’re seeing the shadow of all the baryons, with FRBs as the backlight.”
The Study: Pinpointing Matter Across Billions of Light-Years
The study, published in Nature Astronomy, analyzed 69 localized FRBs. “Localized” means astronomers were able to pinpoint the specific host galaxy and therefore the distance of the FRB source. Obtaining precise distances is crucial for accurately calculating the amount of matter traversed by the signal.
Many of these localized FRBs were detected and pinpointed using Caltech’s DSA-110 (Deep Synoptic Array-110), a network of 110 radio telescopes specifically designed for catching and localizing these transient events. Instruments at other observatories like the W. M. Keck Observatory and Palomar Observatory helped ascertain the distances to the host galaxies. The analyzed FRBs originated from distances ranging from about 11.74 million up to an astonishing 9.1 billion light-years away, with the most distant one setting a new record at the time.
Where Was the Missing Matter Hiding?
By measuring the dispersion of FRB signals traveling through vast cosmic distances, the astronomers were able to map the distribution and weigh the amount of ordinary matter spread throughout the universe.
Their findings reveal that the vast majority of ordinary matter – approximately 75 to 76 percent – resides in the space between galaxies, within the diffuse, hot gas of the intergalactic medium and the filamentary cosmic web. Another roughly 15 percent is found within the diffuse halos surrounding galaxies. Only a small fraction is concentrated within galaxies themselves, in stars and cold gas clouds – the portion of ordinary matter astronomers could previously see.
This distribution aligns remarkably well with predictions from advanced cosmological simulations, which had long theorized that energetic processes like supernovae and supermassive black holes eject gas from galaxies, dispersing it into the surrounding cosmic web. This study provides the first direct observational confirmation of this predicted distribution.
Unlocking New Cosmological Insights
Finding the missing ordinary matter is a significant milestone, resolving a long-standing puzzle in cosmology. But the implications extend further. This research validates FRBs as powerful tools for cosmological investigations, allowing astronomers to trace the otherwise invisible structure of the cosmic web.
The ability to “weigh” the matter between galaxies using FRBs can help researchers better understand how galaxies grow and evolve within this cosmic framework. It also offers new avenues for addressing other mysteries, such as constraining the mass of subatomic particles called neutrinos – tiny particles thought to be ubiquitous but incredibly difficult to measure. The standard model of physics predicts neutrinos have no mass, but observations suggest they possess a tiny amount; better understanding the distribution of baryonic matter can help refine these neutrino mass measurements and potentially point towards new physics beyond the standard model.
The astronomers believe this is just the beginning. Caltech is already planning its next-generation radio telescope, the DSA-2000 in the Nevada desert, which will be capable of detecting and localizing up to 10,000 FRBs per year. Such capabilities promise to dramatically enhance our ability to map the cosmic web, deepen our understanding of the universe’s structure and evolution, and potentially uncover even more cosmic secrets using these extraordinary radio flashes.
