The Universe is huge. Because it is expanding and that expansion is accelerating, estimates have it stretching up to 93 billion light-years across, though the visible universe is only a mere 13.8 billion light-years across. This means most of the Universe will remain invisible, as its distant light has not had enough time to reach Earth. The Universe has very low density precisely because of its size. On average, a cubic meter of space contains only 5.9 atoms. The density of matter in the Universe, however, is only one atom per every four cubic meters of space. Matter itself is mostly empty space too; a typical atom is 100,000 times larger than its nucleus, and the nucleus contains 99.9 percent of an atom’s mass.
Researchers now believe they have found more than half of the Universe’s missing baryonic matter.
Missing Baryonic Matter
Although the mass of the Universe is known, astronomers have only been able to observe 10 percent of the amount of baryonic matter their models predicted. That’s because matter can only be seen if it’s illuminated or casting a shadow. Even though they couldn’t see it, models predicted 90 percent more matter than observed. Some of the missing matter lies in diffuse halos around galaxies, but this only accounts for a small percentage of the missing material.
At the University of Edinburgh in the U.K., and the University of British Columbia, independent teams of researchers claim to have found between 50 and 70 percent of the Universe’s missing baryonic matter by studying the Sunyaev-Zel’dovich Effect, which is a distortion of the Cosmic Microwave Background. The Sunyaev-Zel’dovich Effect occurs when photons from the CMB scatter upon interacting with particles of matter. Photons ionize the intergalactic gas, which creates distortions in the CMB’s spectrum. From these distortions, the amount of matter present can be inferred. In the past, the Sunyaev-Zel’dovich Effect has enabled astronomers to identify clusters of galaxies.
In her report, lead researcher Anna de Graaff of the University of Edinburgh states that “cosmological simulations predict that the ‘missing baryons’ are spread throughout filamentary structures in the cosmic web.” Galaxies are linked together by filaments of dark matter, which gravitationally attract intergalactic gas. Filaments link galaxies together to create what astronomers call the Cosmic Web, which is a network of 24,000 galaxies with over 100,000 filaments connecting them.
At first glance, the structure looks eerily like neurons in the human brain. As light passes through the matter in the filaments, the CMB gets distorted. Using the Sunyaev-Zel’dovich Effect, de Graaff concluded that the amount of matter in the filaments “can account for ∼30% of the total baryon content of the Universe.” De Graaff’s team estimates the density of matter around the filaments is six times greater than the average universal density.
Operating independently from their colleagues in Scotland, Hideki Tanimura and her team at the University of British Columbia achieved similar results. Also relying on the Sunyaev-Zel’dovich Effect, Tanimura stated that “to our knowledge, this is the first detection of filamentary gas at over-densities typical of cosmological large-scale structure.” Because the density of matter in the filaments is higher than the cosmological average, the team concluded that this accounts for some of the Universe’s missing baryonic matter. This research confirms previous predictions that galactic filaments contain more mass than dark matter alone, though the mysterious substance accounts for 90 percent. The team predicts the density of the newly found matter to be three times the universal average.
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