Space
A puzzling increase of gamma rays emanating from the heart of the Milky Way may stem from dark matter particles colliding and annihilating each other.
By Leah Crane
Gamma rays appear in unexpectedly high amounts in the center of the Milky Way
The core of our galaxy exhibits unusual behavior, potentially due to dark matter. In 2009, findings from the Fermi Gamma-ray Space Telescope detected an unexpected concentration of gamma rays from the center of the Milky Way, a phenomenon now referred to as the galactic center gamma-ray excess (GCE). New simulations suggest these gamma rays might result from the annihilation of dark matter particles.
The discussion around the cause of the GCE has been ongoing since its discovery, with two prevalent theories emerging. One proposes that the gamma rays could originate from a yet-undiscovered group of pulsars, which are swiftly rotating neutron stars that emit beams of intense radiation.
The alternative hypothesis posits that it could be due to weakly interacting massive particles (WIMPs), long considered primary candidates for dark matter. These particles have minimal interaction with ordinary matter, but if two collide, they would annihilate, producing an outburst of gamma rays.
Recently, the dark matter hypothesis has faced skepticism, partly due to the lack of direct evidence in WIMP searches. “Given the absence of direct proof for dark matter despite thorough investigations, a more substantial burden of proof is necessitated for the dark matter theory,” remarks Jeff Grube from King’s College London.
Moreover, this theory has been sidelined because dark matter is expected to form a spherical distribution in the galaxy’s center, while the GCE exhibits a flattened center. However, recent simulations by Joseph Silk at Johns Hopkins University and his team suggest this might not be a hurdle after all.
These simulations incorporated a more detailed history of the Milky Way than prior studies of the GCE. “We understand that billions of years ago, our galaxy underwent mergers with several smaller galaxies, which contributed their dark matter,” states Silk. “As a result, no rational person would expect the center of the galaxy to be perfectly symmetric.”
The simulation results produced a flattened distribution of dark matter that corresponded with the GCE’s shape, reopening the possibility of a dark matter explanation. However, the mystery remains unsolved as pulsars still provide a legitimate alternative. “At best, this leaves the scenario unclear,” Grube adds.
The current gamma-ray observatories lack the capability to differentiate between the two hypotheses, but the upcoming Cherenkov Telescope Array Observatory, being constructed in the Canary Islands and Chile, is set to commence observations in 2026.
“In some sense, there’s a 50 percent chance we could have found vast quantities of dark matter, but resolving this issue will depend on a new telescope,” says Silk. If the GCE is indeed linked to dark matter, it could offer the best chance we’ve ever had to comprehend this mysterious substance that holds the universe together.
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