Scientists have located the universe’s long-missing ordinary matter—made of protons and neutrons—by tracking how powerful radio wave bursts, called fast radio bursts (FRBs), are dispersed as they travel through space.
Ordinary matter, unlike the elusive dark matter, makes up everything from stars to people but only accounts for about 15 percent of the universe’s total matter. For decades, researchers knew half of this ordinary matter was missing—until now.
"So the question we've been grappling with was: Where is it hiding? The answer appears to be: in a diffuse wispy cosmic web, well away from galaxies," said Liam Connor, astronomy professor at Harvard and lead author of the study published in Nature Astronomy.
The missing matter was found in the intergalactic medium—thinly spread gas in the vast space between galaxies. Researchers also discovered a smaller portion residing in halos around galaxies, including the Milky Way.
"People, planets and stars are made of baryons. Dark matter, on the other hand, is a mysterious substance that makes up the bulk of the matter in the universe. We do not know what new particle or substance makes up dark matter. We know exactly what the ordinary matter is, we just didn't know where it was," Connor explained.
Much of this matter was displaced from galaxies by violent events such as supernova explosions or black holes ejecting gas. “If the universe were a more boring place... every proton and neutron would be a part of a star. But that's not what happens,” said Connor.
To detect the matter, researchers used 69 FRBs—short bursts of radio waves from up to 9.1 billion light-years away. As the waves travel through space, they spread out depending on the matter they pass through, helping scientists locate that matter.
Of the 69 FRBs, 39 were detected at Caltech’s Owens Valley Radio Observatory using the Deep Synoptic Array, while 30 came from other telescopes.
The findings show that about 76 percent of ordinary matter lies between galaxies, 15 percent in halos around galaxies, and only 9 percent within galaxies themselves.
"We can now move on to even more important mysteries regarding the ordinary matter in the universe," Connor said. "And beyond that: what is the nature of dark matter and why is it so difficult to measure directly?"
Courtesy: Reuters
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