A new study suggests that a NASA telescope may have made the first observation of the elusive dark matter, the invisible and mysterious substance that makes up most of the matter in the universe. But scientists, including the study authors, caution that more research is needed to understand this finding.
NASA’s Fermi Gamma-ray Space Telescope, which studies high-energy wavelengths of light known as gamma rays, has discovered luminescence at the center of the Milky Way that may be associated with particles associated with dark matter, according to research published Tuesday (Nov. 25) in the Journal of Cosmology and Astroparticles.
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But the study cautions that independent confirmation of this signal will need to come not only from the Milky Way, but also from “other celestial bodies or regions” with similar properties. Similarly, theoretical physicist Sean Tulin, assistant professor of physics and astronomy at York University in Toronto, told Live Science that he would like to see an independent analysis of the study, as it is not the first time such a claim has been made using the Fermi telescope.
A notable example is the “galactic center surplus,” an unexplained source of gamma-ray light discovered by Fermidata in 2009. After nearly two decades of further research, scientists continue to debate whether the excess is due to dark matter or more conventional astronomical sources, such as rapidly spinning stars known as pulsars.
WIMP in space
Dark matter is a non-luminous substance that is thought to make up the majority of matter in the universe. So far, it has only been tracked by its gravitational influence on other objects. For example, astronomer Fritz Zwicky, in a seminal 1933 paper, stated that distant galaxies were moving toward each other faster than predicted based on visible matter seen through telescopes. The gravitational pull of dark matter was pinpointed as a likely reason.
There are several theories about the contents of dark matter, but most astronomers today suggest that it is made of subatomic particles. Toya’s research focuses on a general particle proposal: Weakly Interacting Large Particles (WIMPs).
WIMPs fall outside the widely used Standard Model of particle physics, which (most of the time) does a good job of showing how the constituents of matter interact. However, CERN says this model does not take into account gravity or the presence of dark matter.
WIMPs are heavier than protons and rarely interact with other types of matter, the statement said. However, when two WIMPs collide with each other, these particles are destroyed and other particles, including gamma-ray photons, are energetically released during the collision.
Dark matter or not?
To look for gamma rays associated with WIMP collisions, many studies have focused on clusters of dark matter, such as the center of the Milky Way. Data from 15 years of Fermi observations show that gamma rays “have a halo-like structure toward the center of the Milky Way,” which is “consistent with the shape expected from a dark matter halo.”
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These gamma rays were extremely energetic, with photon energies of 20 gigaelectronvolts (20 billion electronvolts). The energy is “consistent with the expected release from a hypothetical WIMP extinction” and is also consistent with the frequency of WIMP extinctions, the statement said.
But Tulin pointed out that this signal only appears if you remove the background of “all sources of high-energy photons coming from the Milky Way,” including the Milky Way’s center and disk. Some of the background energy also comes from the “Fermi bubble,” two giant zones of gas and cosmic rays that loom over the Milky Way.
All studies examining the source of energy from the Milky Way must model that background noise and subtract it to “reveal the underlying signal,” Tulin said. “What you infer about the signal depends very carefully on what you subtract from the background. … If you subtract something incorrectly, you run the risk of being fooled.”
Questions about the background aside, Tulin said the signal could depend on the type of dark matter particle being discussed. “What that means is, what is the model for that dark matter particle?” he said. “What is its mass? What are its fundamental properties? What are its various interactions?”
But the standard WIMP extinction model is “perfectly reasonable” for the signals Totani observed, provided the study is observing WIMPs under the model we understand and background is correctly subtracted, Tulin said.
Despite the warning, Tulin (who had access to the research preprint in an interview with Live Science) added that the discovery “would be surprising if it was due to dark matter…Not only for the future of astronomical observations, but this type of dark matter particle could be tested and discovered in all kinds of different experiments, such as underground labs and collider.”
Still, “this study turns out to be correct only once, and no one is actually risking their life for it,” Tulin said of the new study. “We’ve seen many anomalies arise. Many anomalies disappear. Some anomalies have stuck with us and still require further investigation.”
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