Caltech says that, thanks to an NSF grant, they’ve found where all the dark matter has been hiding. That’s up to 50 percent of matter in the universe, the stuff that doesn’t show up in telescopes scanning the stars, which make their own light. Most of the darker stuff has been waiting in the fog between galaxies:
In a new study in Nature Astronomy, a team of astronomers at Caltech and the Center for Astrophysics | Harvard & Smithsonian (CfA) has, for the first time, directly detected and accounted for all the missing matter. To do this, the team used brief, bright radio flashes in the distant cosmos, called fast radio bursts (FRBs), to illuminate the matter lying between the FRBs and us.
“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, who performed much of the work while a Caltech research assistant professor working with Vikram Ravi, assistant professor of astronomy at Caltech.
The study looked at a total of 69 FRBs located at distances ranging from about 11.74 million to about 9.1 billion light-years away. The object 9.1 billion light-years away, named FRB 20230521B, now holds the record for the most distant FRB ever recorded. While more than a thousand FRBs have been detected, only about a hundred have been pinpointed to specific host galaxies; in other words, their origins and distances from Earth are known. These localized FRBs were needed for the current study.
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As radio-frequency light travels from the FRBs to Earth, the light becomes spread out into different wavelengths like a prism turns sunlight into a rainbow. The degree of this spreading, or dispersion, depends on how much matter is in the path of the light.
“It’s like we’re seeing the shadow of all the baryons, with FRBs as the backlight,” says Ravi. “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.”
The results revealed that 76 percent of the universe’s normal matter lies in the space between galaxies, also known as the intergalactic medium. About 15 percent resides in galaxy halos, and the remainder is concentrated within galaxies—in stars or in cold galactic gas. This distribution lines up with predictions from advanced cosmological simulations but has never been observationally confirmed until now.
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You can read more of the dark matter analysis here, in Nature Astronomy.