Astronomers at the University of Toronto have spotted some of the most elusive stuff in our universe by taking a deep look at the cosmic web, the network of filaments and knots that trace the large-scale distribution of galaxies. Even though galaxies produce most of the visible light in the universe, they contain fewer than 10 percent of all the atoms in the cosmos.
Most of the rest are in the universe’s cosmic web in the form of a gas that is so diffuse that there is no more than about one atom per cubic foot of space — far emptier than the best vacuum ever achieved on Earth. Cosmologist Adam Hincks, an assistant professor cross-appointed to the David A. Dunlap department of astronomy and astrophysics and St. Michael’s College, said:
“Because the gas is so thin, it’s extremely hard to see. For years, astronomers referred to this as the ‘missing baryon problem.’ They expected to see lots of atoms — which we refer to as baryons — but only found a fraction of them when we added up all the glowing matter they could spot.”
In recent years, however, astronomers have finally started discovering these elusive atoms. In Toronto, Hincks — who’s also the inaugural holder at St. Michael’s of the Sutton Family Chair in Science, Christianity, and Cultures — led an international team of scientists that detected the diffuse, hot gas in a roughly 40 million light-year-long filament between two clusters of galaxies. Hincks and his collaborators used archival data from the Planck satellite and more recent data from the Atacama Cosmology Telescope (ACT), in northern Chile, which look at the Cosmic Microwave Background (CMB), the oldest light in the universe.
By observing how the CMB light was scattered by the filament’s gas, they determined that the gas in the filament has the mass of about 50 billion suns — about 50 times more mass than our own Milky Way Galaxy. Although evidence for the filamentary gas in this system had been previously found with the Planck data, the larger ACT instrument sharpened the image considerably, making the distinction between the galaxy clusters and the filament much clearer. The research is described in a paper published earlier this year in the Monthly Notices of the Royal Astronomical Society. Hincks’ co-authors at U of T include Martine Lokken, a Ph.D. student in U of T’s astronomy and astrophysics department, and J. Richard Bond, a professor at the Canadian Institute of Theoretical Astrophysics (CITA).
Discovering the missing baryons in the universe
While the research led by Hincks homed in on the missing baryons in a particular set of galaxies, Lokken has been uncovering how this gas is distributed in an ensemble of special regions of the cosmic web. Lokken, who is supervised by Bond and Renée Hložek, an associate professor in the Dunlap Institute for Astronomy & Astrophysics, used data from the Dark Energy Survey to identify nearly 1,000 galaxy clusters that live in regions of the universe likely to be permeated by filament gas that is denser and hotter than the average. Lokken then combined their extended gas signal in the Planck and ACT data. She found evidence not only for gas in the clusters themselves, but also in filamentary patterns extending away from the clusters. These are expected to contain a great deal of the diffuse gas that was described in the paper by Hincks. Lokken said:
“Our work demonstrates a new way to study gas in the cosmic web. Accounting for all the so-called ‘missing baryons’ is one of the most important tasks that we as cosmologists need to tackle. Our directional studies of cosmic gas are a brand new way to probe this problem and other questions about the origins of our universe.”
Lokken’s work recently appeared in an article in the Astrophysical Journal co-authored by Bond, Hložek, and Hincks, among other researchers. These new results continue the strong tradition of University of Toronto cosmologists leading work in understanding the large-scale structure of the universe. Bond was among those who laid the groundwork for our understanding of the cosmic web starting in the late 1980s — a topic in which he and other Toronto researchers have been active to this day. In 2021, Hložek’s contributions to cosmology in Canada were recognized by the Harvey B. Richer Gold Medal for Early Career Research in Astronomy.
Provided by Chris Sasaki, University of Toronto [Note: Materials may be edited for content and length.]
