A frame from the Illustris simulation shows a huge galaxy cluster at the center. The red, orange and white colors show hot gas, while the blue and purple filaments represent a cosmic structure of dark matter. Credit: Illustris Collaboration
The game is underway! Astronomers may have found some of the universe’s missing matter, thanks to a team’s cosmic detective work.
The case has been open for more than 20 years. In the 1990s and early 2000s, scientists investigated the contents of the universe using observations of the cosmic microwave background radiation and models of the Big Bang. They found that normal matter (made of familiar particles like protons and neutrons, collectively called baryons) makes up about 4-5% of the total energy density of the cosmos. (The rest of the cosmos is made up of two mysterious components: dark matter and dark energy.)
The problem: Astronomers have only been able to locate about half of those baryons. Adding up all the baryons visible in stars, galaxies and gas clouds, the other half has not been explained.
In the years since, scientists have gradually debunked this mystery, dubbed the missing baryon problem. In recent years, researchers studying fast radio bursts – bursts of radio waves from space that last only a few milliseconds – have used their data to confirm the total number of expected baryons. Their results are consistent with the cosmic microwave background, which holds clues to the total amount of normal matter in the universe. But the number was even greater than what we saw: it was not clear where exactly the missing baryons were located.
Theoretical models of the cosmic web – a network of galaxies, gas and dark matter spread throughout the universe – predict that these baryons could be hidden in material called the warm-hot intergalactic medium (WHIM), spread along tendrils of gas connecting clusters of galaxies. But detecting this “ghost” matter is extremely difficult, as the gas in the WHIM is extremely thinly distributed, averaging just 10 particles per cubic meter (1 cubic meter equals about 35 cubic feet)..
Our galaxy complicates things further. The WHIM emits so-called soft (low-energy) X-rays, which are absorbed by the foreground gas and dust of the Milky Way through which we have to look. And the gas in the WHIM is extremely weak, so telescopes need high sensitivity and long exposure times to collect enough photons to study it.
Clues from the cosmic fog
In a study published today on Astronomy and astrophysics, Scientists have published the most detailed view of WHIM to date and have made significant progress in unraveling the mystery of the missing baryons. The team combined X-ray observations from the extensive ROentgen Survey with an Imaging Telescope Array (eROSITA) to precisely measure gas in nearly 8,000 gas filaments in the WHIM, some of which extend as far as 65 million light-years.
The team also measured the temperature of the gas in the WHIM. At about 10 million degrees Fahrenheit (5.6 million degrees Celsius), it is so hot that it must be made of charged particles because the heat has stripped the atoms of their electrons. This is important because it affects how the gas absorbs or emits light, which in turn helps astronomers estimate how much gas is present.
Then they measured the density of the gas. Coupling temperature and density allowed them to approximate the total amount of baryonic matter in the WHIM. And the team calculates that there could be enough to account for 20% of the missing baryons in the universe, although the uncertainty is large. Ongoing multiwavelength investigations are expected to significantly improve the precision of this estimate within this decade.
“This is one of the big questions in astrophysics and cosmology, along with mysteries like dark energy and dark matter,” says Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics (MPE), co-author of the paper. “People have been looking for these baryons for a long time, so it’s very exciting to find a significant part of them.”
An ongoing investigation
Yet the case is far from closed.
“The team relied on single average values for several parameters, including temperature and the abundance of heavy elements,” says Michael Shull, a professor of astrophysics and planetary sciences at the University of Colorado-Boulder, who is not a author of the study. He has studied the missing baryon problem for over 15 years. The heavy element content of WHIM acts as a tracer for the total amount of baryons it contains. “Precisely defining how the temperature varies would help refine the measurements, as would careful geometric studies that more precisely track the spatial extent of the WHIM filaments.”
Combining observations from different wavelengths of light to get a more complete view of the WHIM is one of the next steps, Bulbul says. This would better constrain the temperature and density of the gas, resulting in a more accurate baryon count. This, in turn, will help us better understand the universe.
“Studying this gives us a way to test cosmological simulations against observations,” says Bulbul. “This will help us understand how the universe evolved to its current state and how it might continue to evolve in the future.”
Xiaoyuan Zhang, a postdoctoral researcher at MPE who led the study, says the discovery of the missing baryons will also illuminate the evolution of galaxies. “The space surrounding galaxies is not a perfect vacuum,” he says. “There is gas there, which influences things like the transformation of a galaxy’s color, shape and star formation rate.”
Scientists are slowly but steadily solving the problem of missing baryons. As our instruments and techniques improve, the remaining baryons will likely be identified, and if not, we may learn even more about the cosmological model that predicts they should exist.
