sciencehabit writes: Roughly half of the "normal" matter in the universe -- the stuff that makes up stars, planets, and even us -- exists as mere wisps of material floating in intergalactic space, according to cosmologists. But astronomers had no good way to confirm that, until now. A new study has used fast radio bursts (FRBs) -- powerful millisecondslong pulses of radio waves coming from distant galaxies -- to weigh intergalactic matter, and the results match up with predictions. "Using FRBs as a probe has been an exciting prospect for a while," says astronomer Paul Scholz of the University of Toronto, who was not involved with the work. "Now that we've built up a sample of local FRBs, we're starting to be able to do this. It's certainly exciting." Over the past few decades, cosmologists have compiled an inventory of the stuff that makes up the universe. Some 68% is dark energy, a mysterious force accelerating the universe's expansion. Another 27% is clumps of dark matter that hold galaxies together. Just 5% is so-called normal matter.
Cosmologists know how much normal matter there should be; they can calculate it from how much the big bang should have produced and from the microwave ripple of this cosmic event that still echoes through space. But they can only see about half of it glowing as galaxies and dense gas clouds. The rest, a rarified, intergalactic gas of just one or two atoms in the volume of a typical office room, has been almost impossible to detect. That was until the first FRB burst on the scene in 2007. Because these sporadic blasts are so bright and short, FRBs were originally thought to come from an instrumental glitch, or a source on Earth. (Some early "FRBs" were found to come from a microwave oven at an observatory.) But as detections of FRBs piled up, astronomers realized they were coming from distant corners of the universe. Pinpointing them was difficult because of their rarity: Observers had to be pointing in the exact right direction to catch one, and they wouldn't have time to focus other scopes on the source. These days, telescopes that view large portions of the sky continuously are bagging more FRBs.
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