Science & Technology

Scientists Finally Found The Ordinary Matter Missing From The Universe

The universe is full of stuff we can't detect. Dark energy and dark matter — named for their mysterious natures — make up most of that stuff, but there's even a good amount of ordinary matter that we just can't account for. In 2017, however, two separate teams of astronomers announced that they had discovered this missing matter, and where they found it is actually pretty beautiful.

Come Out, Come Out, Wherever You Are

The protons, neutrons, and electrons that make up ordinary matter — or what they call baryonic matter — make up only about five percent of the universe. Dark energy and dark matter make up the rest.

How do we know? Astrophysicists study the afterglow left over by the Big Bang, what's known as the cosmic microwave background (CMB), to figure out how much matter was present at the beginning of the universe. Then, they study the light emitted today by stars and galaxies to figure out how massive they are, then multiply that mass by what they know about how big and dense the Universe is currently. That total falls way short of how much baryonic matter the CMB tells us should be there, by about half. Astrophysicists call this the "missing baryon problem."

Scientists think they've found bits and pieces of this baryonic matter hiding out in halos around galaxies. But as for the rest, they've long suspected that it may form filaments that bridge galaxies across space. In April of 2017, researchers at the University of Waterloo discovered that dark matter connects galaxies together in this exact way, and actually created a composite image to prove it. That made scientists even more confident that they'd find baryonic matter doing the same thing. They just had to find it.

First measurements of the thermal Sunyaev-Zel’dovich effect from the Atacama Large Millimeter Array

Walkin' Into Spiderwebs

The problem is that these filaments would be incredibly diffuse, not dense enough to block light or hot enough to announce its presence on an image. But the two research teams — one from the University of British Columbia, the other from the University of Edinburgh — had something else up their sleeves.

They relied on the Sunyaev-Zel'dovich effect, a hard-to-pronounce phenomenon that makes the leftover light particles in the CMB jump into higher energies when they pass through the gases that surround galaxy clusters. Just like the researchers who found the dark matter web, these researchers collected pairs of galaxies from the Sloan Digital Sky Survey, oriented them to match each other, and then stacked them to reinforce any patterns that might exist.

And exist they did. The University of British Columbia found baryon filaments with a density just under three times what you'd find in empty space. The University of Edinburgh calculated that the filaments they found were six times as dense. That makes sense, since the filaments each team was examining were observed at different distances, but the fact that both teams came to the same conclusion is pretty rock-solid evidence. If those filaments exist between every galaxy, that could mean that all of the missing baryonic matter is now found.

Want to learn more about the mysteries of the Universe? Check out "Chandra's Cosmos: Dark Matter, Black Holes, and Other Wonders Revealed by NASA's Premier X-Ray Observatory" by Wallace H. Tucker. The audiobook is free with a trial of Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Quick Mysteries: Missing Baryons

Written by Ashley Hamer October 23, 2017

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