Astronomers analyzed 69 precisely localized fast radio bursts as cosmic backlights to map ordinary matter, estimating in a study published on 16 June 2025 in Nature Astronomy that 76 per cent of the universe’s baryons reside as diffuse ionized gas between galaxies, while only 9 per cent remains in stars and cold gas.
Cosmologists have long faced an inventory problem when tracking the universe’s normal matter. While early-universe measurements and Big Bang nucleosynthesis calculations establish how many protons and neutrons should exist, standard telescope surveys of stars, cold gas, and hot clusters left a substantial shortfall. That missing material was never truly lost; instead, it proved exceptionally difficult to detect directly because it sits spread across the vast expanses of the cosmos.
Weighing Invisible Gas with Fast Radio Bursts
To solve this inventory puzzle, astronomers turned to fast radio bursts—brief, intense flashes of radio energy originating from distant galaxies. As these millisecond blasts travel billions of light-years across the universe, they encounter clouds of plasma, dust, and ionized gas. Free electrons within this intergalactic medium interact with the radio waves, causing lower frequencies to arrive slightly later than higher ones.
This frequency-dependent timing signature yields a dispersion measure, which is directly proportional to the column of free electrons the signal encounters along its sightline. By identifying the host galaxy of each burst and measuring its redshift, researchers can link the accumulated timing delay to a precise cosmological distance. That method allows scientists to weigh gas far too faint to capture in conventional optical photographs.
Inside the New 76/15/9 Baryon Census
In research published on 16 June 2025 in Nature Astronomy, Liam Connor and colleagues utilized a sample of 69 precisely localized bursts to partition the late-universe baryon budget. The analysis concludes that roughly 76 per cent of ordinary matter resides as extremely diffuse ionized gas within the intergalactic medium, accompanied by an uncertainty range of plus 10 and minus 11 percentage points.
The remaining ordinary matter breaks down into two additional reservoirs. About 15 per cent occupies extended galactic halos, while only 9 per cent is housed inside stars and cold galactic gas. Researchers emphasize that this partition applies exclusively to ordinary matter, or baryons, which make up only about one-sixth of the universe’s total matter-energy budget. Dark matter and dark energy constitute the remaining dominant shares.
Localization Power at Owens Valley and Beyond
A fast radio burst without a securely identified host galaxy provides limited value because its exact distance remains ambiguous. To overcome this limitation, thirty-nine of the 69 events analyzed in the study were detected and pinpointed by the Deep Synoptic Array-110 at California’s Owens Valley Radio Observatory. An additional 30 bursts came from global facilities, primarily the Australian Square Kilometre Array Pathfinder, with host redshifts confirmed through optical observations at the Keck and Palomar observatories.
According to Caltech’s account of the project, these localized bursts spanned cosmic distances ranging from approximately 11.74 million to 9.1 billion light-years, with FRB 20230521B standing out as the most distant event in the sample. This broad spatial baseline enabled astronomers to probe accumulated electrons across a wide swath of cosmic history rather than relying on a narrow local slice.
Probing Feedback, Dark Matter, and Neutrino Masses
Beyond accounting for missing atoms, researchers are looking to fast radio bursts to tackle broader cosmological questions, including the nature of dark matter, the acceleration driven by dark energy, and the mass of neutrinos. These mysterious particles, often called ghost particles, interact so infrequently that roughly 100 trillion pass through the human body every second without leaving a trace.
However, analyzing cosmic structure requires researchers to account for astrophysical feedback—energy pumped out from the hearts of galaxies by supermassive black holes that thins surrounding gas and smooths out cosmic clumpiness.
“The feedback process thins the gas around the galaxies, redistributing matter across vast distances. It smooths out clumps of matter in a way that looks astonishingly similar to what massive neutrinos do, or what dark energy or dark matter theories predict. Unless scientists can independently measure this contribution from feedback, they can’t tell these effects apart.”
By examining how signals change during their multi-billion-year journeys, scientists have begun isolating these competing effects. Team members note that while galactic feedback does smooth surrounding material, the measured effect is weaker than previously thought.
With roughly 100 total bursts examined in these foundational investigations, researchers anticipate a major boost when Caltech’s Deep Synoptic Array begins operating in Nevada in 2029.
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