Half of Universe’s Normal Matter Was ‘Missing’; Astronomers Have Found It – ryan

Astronomers may have found the long-missing half of the universe’s regular matter—and it appears to have been right under our noses all this time.

Models of how the universe has evolved since the Big Bang allow us to predict how much normal matter (as opposed to dark matter) there should be in the observable universe.

The problem, however, is that these predictions don’t match what we can actually see out in the cosmos, creating a long-standing puzzle for scientists.

In fact, our tallies of stars and gas account for only half the expected normal matter—with the other half, equaling 7.5 percent of the universe’s total matter, seemingly “missing.”

In a new study, however, astronomer Boryana Hadzhiyska of the University of California, Berkeley and colleagues believe they have accounted for this overlooked material, which they say exists in nearly invisible ionized hydrogen gas, much of which is spread out in puffy halos around galaxies.

The findings, the researchers say, suggest that the supermassive black holes at the heart of many galaxies may spew gas up to five times further than was previously realized—and may also be active at more points in their life than just their infancy.

“We think that, once we go further away from the galaxy, we recover all of the missing gas,” Hadzhiyska said in a statement. “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.”

However, paper co-author and Lawrence Berkeley National Laboratory astrophysicist Simone Ferraro added that “the measurements are certainly consistent with finding all the gas.”

In their study, the researchers estimated the distribution of ionized hydrogen around galaxies—normally too cold and spread out to be observed—by stacking images of some 7 million luminous red galaxies lying within 8 billion light-years of our Milky Way.

The images were all collected by the Dark Energy Spectroscopic Instrument (DESI) on the Mayall four-meter Telescope at Kitt Peak National Observatory in Tucson, Arizona.

They then looked for evidence of the so-called “Sunyaev–Zel’dovich effect,” in which the cosmic microwave background (“CMB”—the afterglow of the first light from the Big Bang) appears dimmed or brightened as electrons in the “hidden” area scatter the radiation.

“The cosmic microwave background is in the back of everything we see in the universe. So, you can use that as a backlight to see where the gas is,” Ferraro said.

Record-breaking measurements of the CMB were made by the Atacama Cosmology Telescope (ACT) in Chile before it was decommissioned in 2022.

According to the team, their discovery has wide implications. For example, the way that we now know gas is being expelled from galaxies challenges the assumption that the distribution of gas follows that of dark matter.

Factoring this into cosmological models, they explained, could help resolve some outstanding questions about how “clumpy” the universe is.

“There are a huge number of people interested in using our measurements to do a very thorough analysis that includes this gas,” Hadzhiyska said. “People in astronomy care a lot about it for understanding galaxy formation and evolution.”

Additionally, he said, studying the Sunyaev–Zel’dovich effect may also provide a new window on the early universe, offering potential insights into the large-scale structure of the universe.

The full findings of the study will be published in the journal Physics Research Letters.

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Reference

Hadzhiyska, B., Ferraro, S., Guachalla, B. R., Schaan, E., Aguilar, J., Battaglia, N., Bond, J. R., Brooks, D., Calabrese, E., Choi, S. K., Claybaugh, T., Coulton, W. R., Dawson, K., Devlin, M., Dey, B., Doel, P., Duivenvoorden, A. J., Dunkley, J., Farren, G. S., … Zou, H. (2024). Evidence for large baryonic feedback at low and intermediate redshifts from kinematic Sunyaev-Zel’dovich observations with ACT and DESI photometric galaxies. arXiv.