For decades, cosmologists have known that roughly 17% of the universe’s matter is ordinary baryonic matter, the stuff of stars, planets, and people. The problem has been finding it. Direct surveys of stars and gas have consistently come up short, accounting for only about half the predicted total.
A new study using fast radio bursts as cosmic probes has traced the rest. It was hiding in plain sight all along: in diffuse plasma stretching between and around galaxies, pushed to far larger scales than standard models predicted.
FRBs as Cosmic Flashlights
Fast radio bursts are millisecond pulses of radio energy from extragalactic sources. As they travel across the universe, free electrons in ionized gas stretch and smear the signal, a phenomenon called dispersion. The more matter an FRB passes through, the more dispersed it becomes.
A team led by Haochen Wang and Kiyoshi Masui at MIT used this principle to map the invisible matter. They cross-correlated dispersion measurements from 2,873 FRBs in the CHIME/FRB Catalog 2 with the positions of roughly 6 million galaxies from the Dark Energy Spectroscopic Instrument Legacy Imaging Survey.
The logic is straightforward: if missing baryons exist as warm diffuse plasma in the intergalactic and circumgalactic medium, FRBs passing through galaxy-rich regions should show systematically higher dispersion.
Where the Matter Is
The correlation was clear. Where more galaxies clustered, FRBs encountered more diffuse plasma, confirming that the missing baryons are distributed through the intergalactic medium and the halos surrounding galaxies.
But the distribution was wider than expected. Galaxies appear to be ejecting matter to much greater distances than current models predict, pushed outward by energetic feedback from supernovae and active supermassive black holes.
“We’re finding missing matter that is pushed out to larger scales,” Masui said. “These measurements indicate that star activity, and activity from black holes, is stronger and much more violent than predicted.”
The results are qualitatively consistent with Sunyaev-Zeldovich studies that detected gas filaments extending beyond galaxy pairs and found evidence for enhanced feedback in baryon density profiles.
A New Tool for Cosmology
The study, published in Physical Review Letters, demonstrates that FRBs are a powerful probe of baryonic structure formation, one that will only improve as surveys expand. CHIME continues to catalog hundreds of new FRBs each year, and next-generation radio arrays promise even greater sensitivity to the diffuse intergalactic medium that contains most of the universe’s ordinary matter.
The implication is that galaxy formation is a more violent and disruptive process than standard models account for, with stars and black holes pushing gas far beyond the visible boundaries of galaxies.