Nearly half of the universe’s ordinary matter was uncharted, until now
Much of it resides in strings of hot gas between galaxies
A vast cosmic web of matter exists in space, linking clusters of galaxies. But much of the ordinary matter in those branches is sparse and difficult to detect, which is why it has been called “missing" matter.
Illustris Simulation/Illustris Collaboration
Nearly half of the universe’s ordinary matter has been hiding — until now.
Bursts of radio waves have illuminated the whereabouts of all ordinary matter, revealing its distribution between, around and within galaxies, researchers report June 16 in Nature Astronomy. And X-rays have uncovered details about a once hidden string of gas linking four galaxy clusters, another team reports in the June Astronomy and Astrophysics.
“The two papers are very complementary,” says astrophysicist Jason Hessels of McGill University in Montreal, who was not involved in either study. While one takes a statistical approach to fill in the matter gap, the other measures a specific slice of it.
Ordinary matter makes up everything that can be observed, such as planets and people. It’s made of standard particles like protons and neutrons, collectively called baryons. But it accounts for just about 15 percent of all matter in the universe. The rest is the mysterious dark matter.
Almost half of ordinary matter is scattered like fine mist, so it’s hard to detect. “This gas is missing in the sense that theoretical models expect that it should be … in certain places,” Hessels says. “But how much of it is where and actually showing that that is true is very difficult because the gas is very, very diffuse.”
Millisecond-long flashes called fast radio bursts, or FRBs, can spotlight that sparse matter. An FRB from a faraway galaxy separates into distinct wavelengths as it travels through intergalactic matter, like how a prism splits visible light into a rainbow. The matter slows down lower-frequency waves more than higher-frequency ones, and the delay grows as the FRB travels through more of the medium.
“This is a really small effect,” says astrophysicist Liam Connor of Harvard University. But “over billions of years of traveling through this … matter, it becomes a detectable effect here on Earth.”
Connor and colleagues examined nearly 70 FRBs, including one whose light took about 9 billion light-years to reach Earth, the most distant FRB on record. How each flash dispersed on its way to Earth revealed the amount of matter between the planet and the FRB’s source. Comparing those calculations with a cosmological simulation showed where to find all the universe’s ordinary matter: 76 percent snakes between galaxies, 15 percent surrounds galaxies in halos and the rest lies within galaxies, in stars and cold gas.
While this work quantifies the missing matter — matching predictions — it doesn’t describe its appearance or other properties, says astrophysicist Konstantinos Migkas of Leiden University in the Netherlands.
He and his colleagues found those details by studying faint X-rays emitted by a recently discovered filament of gas where some missing matter probably lurks. The researchers first filtered out background signals from other sources, such as black holes and galaxy clusters. The team then tallied the photons, or particles of X-ray light, released across different energy ranges. These and other techniques revealed that the gas filament spans 23 million light-years, exceeds 10 million degrees Celsius and contains about 10 protons per cubic meter.
The work shows how the missing matter looks and behaves, Migkas says. Moreover, “the properties match what we expected” from the standard model of cosmology.
Future research might be able to combine these two ways of studying the hard-to-detect matter, says astronomer Shami Chatterjee of Cornell University, who was not involved in the new studies.
“Imagine if … we could find some FRBs that were behind this particular filament,” he says. “We could compare whether this filament produced a measurable difference in the [intergalactic medium] on and off the filament.”
Connor agrees that this is just a starting point for astronomers.
“The reason people cared about the missing matter in the first place is because of all the profound impact it has on some of the most sought-after questions in astrophysics, like how do galaxies form?” he says. With insights from the new studies, “now the fun really begins.”
