Invisible no more: Scientists map 76% of ordinary matter lost between galaxies
Now, in a breakthrough study, astronomers from Caltech and the Center for Astrophysics | Harvard & Smithsonian (CfA) have managed to directly detect this missing matter using fast radio bursts (FRBs)—brief, powerful flashes of radio waves originating from distant galaxies.
These cosmic signals, lasting just milliseconds, serve as precise probes, lighting up the otherwise invisible intergalactic medium.
As these FRBs travel billions of light-years to reach Earth, they pass through clouds of ionized gas between galaxies. The radio waves slow down ever so slightly depending on how much matter they encounter along the way.
By measuring this delay, known as dispersion, scientists can calculate the amount of invisible matter in the FRBs' path.
"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," says Liam Connor, assistant professor at Harvard and lead author of the study.
The study analyzed 69 well-localized FRBs, each with a host galaxy and known distance. One of the FRBs studied, dubbed FRB 20230521B, is located a staggering 9.1 billion light-years away, making it the most distant fast radio burst ever recorded.
Although astronomers have detected over a thousand FRBs to date, only around a hundred have been accurately traced back to their host galaxies. This localization is crucial, as knowing both the origin and distance of an FRB is essential for using it to map the matter it passed through, making these select few key to the current study.
Of these, 39 were discovered using the Deep Synoptic Array (DSA)-110, a network of 110 radio antennas in California designed specifically to detect and pinpoint FRBs. The remaining FRBs came from global observatories, including Australia's Square Kilometre Array Pathfinder. Instruments at Hawaii's W. M. Keck Observatory and the Palomar Observatory near San Diego helped determine the distance to each FRB's host galaxy.
Their findings confirm that roughly 76 percent of ordinary matter resides in the intergalactic medium, thinly spread across space. Another 15 percent lies in gaseous halos surrounding galaxies, while only a small fraction is found inside galaxies themselves, in stars, or cold galactic gas."It's like we're seeing the shadow of all the baryons, with FRBs as the backlight," said Vikram Ravi, assistant professor of astronomy at Caltech.
"If you see a person in front of you, you can find out a lot about them. But if you just see their shadow, you still know that they're there and roughly how big they are."
This distribution of matter aligns with predictions made by advanced cosmological simulations but has never been confirmed observationally—until now.
The findings also open new avenues for probing fundamental physics. For instance, they may help determine the mass of subatomic particles called neutrinos.
While the standard model of particle physics assumes neutrinos have no mass, real-world observations suggest otherwise. Knowing their precise mass could unlock physics beyond current theories.
According to Ravi, this is just the beginning for FRBs in cosmology. A new project, Caltech's DSA-2000 radio telescope, currently in development for the Nevada desert, is expected to localize up to 10,000 FRBs each year—dramatically expanding their role in probing the universe's structure.
The study, published in Nature Astronomy, was funded by the National Science Foundation.
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