Everything you can see around you is composed of “ordinary matter.” Looking at the remnant radiation of the Big Bang, the cosmic microwave background (CMB), scientists have calculated that this matter consists of only five percent of the entire universe. A significant amount of this still eluded direct detection, however, until now: A new paper published in Nature claims to have found this missing component, which has been hiding within very hot gas along colossal strands of the “cosmic web.”
Although much of this ordinary matter – composed of baryons – is detectable by cosmologists, they have repeatedly found that 2.5 of the total 5 percent is missing from their large-scale estimation calculations. In order to shed some light on this major discrepancy, a research team from the University of Geneva decided to take a closer look at how galaxies are formed.
Galaxies form when massive amounts of ordinary matter condenses and cools down, constructing stars, planets, asteroids, black holes, and so on. Some of this compressed matter must include the missing 2.5 percent, so in order to properly understand galactic formation mechanisms, this incognito ordinary matter must be found.
The distribution of ordinary matter in the universe is not uniform: There is far more in certain regions, and far less in others. The reasons for this are still somewhat debated, but a prominent argument is that something invisible within the cosmos was attracting matter to certain points, right from the very beginning of time, which produced this uneven distribution. This invisible matter is known as “dark matter.”
Careful imaging of the night sky reveals that most matter, under the action of gravity, is being concentrated into long strands: filamentary structures, forming a gigantic network throughout the universe. Composed of a skeleton of dark matter – which attracts much of the universe’s ordinary matter towards it – this “cosmic web” was first imaged in 2014.
Image credit: Abell 2744, as imaged by Hubble. NASA
Abell 2744 is a huge cluster of galaxies with a chaotic distribution pattern; more significantly, it appears to have glowing, dark matter at its core. The XMM space telescope, which is extremely sensitive to X-ray energy, was used by the researchers to peer into its dark galactic heart.
This huge cluster represents a joining up of several filaments into a “cosmic knot,” wherein the gravitational field is particularly strong; so strong, in fact, that it is dramatically accelerating and heating the matter within the filaments to temperatures of over 10 million degrees Celsius (18 million degrees Fahrenheit). This extreme temperature makes its contents detectable from Earth.
By looking at the X-ray energy given off by the extremely hot, bright gas structures within the filaments, the researchers for the first time managed to estimate part of the density of the cosmic web, and thus the amount of ordinary matter it should contain. Comparing these results to numerical models, they found that the amount of filament matter corresponded to the missing 2.5 percent.
“Now we must verify that the discovery of Abell 2744's missing baryons is applicable to the entire universe,” said Dominique Eckert, lead author of the paper, in a statement. If it is indeed applicable, then a longstanding mystery of cosmology will have been solved.