The amount of matter present in the universe simply hasn’t added up. Astronomers accounting for all the normal matter in the universe that makes up stars, galaxies, and gasses have fallen far short of the amount they should find produced by the Big Bang 13.6 billion years ago.
About 85 percent of the universe is made up of dark matter. The remaining 15 percent is the more conventional kind — much of which builds the things we can see in space, like stars and planets. Of that, about half has been missing from our tally sheet of what makes up the universe.
Now, however, a team of 75 astronomers has finally balanced the cosmological books, so to speak. The missing matter has been hiding out as invisible ionized hydrogen gas, which forms halos around galaxies far larger and more frequently than earlier estimations, according to a report in a physics preprint server, as well as at scientific conferences. The paper will be published in the journal Physical Review Letters.
The Case of the Missing Matter
“We robustly show that the gas is much more extended than the dark matter” relative to subatomic particles called baryons, according to the paper, which explains that astronomers have essentially gotten the ratio between those two components wrong — leading to the massive matter miscount. Better telescopes and more sophisticated instruments to measure the building blocks of the universe have led the astronomers to their new conclusion.
The paper also provides some explanations of some reasons behind the miscount. For one, astronomers underestimated the power of the massive black holes at the centers of galaxies.
Read More: How Astronomers Define Where a Galaxy Ends and Interstellar Space Begins
Effects of Massive Black Holes
The black holes are “fountaining gas much farther from the galactic center than expected — about five times farther,” according to a press release. New measurements show that this diffuse and invisible ionized hydrogen gas encircles galaxies in a “halo” and is “more puffed out and extensive than astronomers thought,” the press release continued.
“We think that, once we go further away from the galaxy, we recover all of the missing gas,” Boryana Hadzhiyska, a research fellow at the University of California, Berkeley and first author of a paper, said in the press release. “To be more accurate, we have to do a careful analysis with simulations, which we haven’t done. We want to do a careful job.”
To conduct their count of the estimated amount of ionized hydrogen gas, the astronomers stacked images of about 7 million galaxies within 8 billion light-years from Earth. Then they measured the extent to which the cosmic microwaves either slightly brightened or dimmed due to the presence of the invisible hydrogen gas, which scatters the radiation — a process called the kinematic Sunyaev-Zel’dovich effect.
The researchers used those scattering microwaves as a background that frames the edge of the universe and acts as a backlight to detect the gas, according to the press release.
Read More: Here’s What Would Happen If You Walked Through a Black Hole
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
