Astronomers have been perplexed by a high-energy gamma-ray glow at the centre of the Milky Way known as the Galactic Center GeV Excess (GCE). One proposed explanation for this glow has been the annihilation of dark matter, but new research challenges this idea. Astrophysicists have conducted extensive modelling, including recent developments in simulating the galactic bulge and other gamma-ray sources, and have ruled out dark matter annihilation as the source of the GCE. This finding narrows down the properties of dark matter and provides stronger constraints for future investigations.
A leading candidate among particle physicists for dark matter has been a thermal, weakly interacting, and weak-scale particle for several decades. However, this research eliminates that candidate even for high-mass particles. The GCE was first observed over a decade ago by the Fermi Gamma-ray Space Telescope. Gamma rays, the highest-energy electromagnetic waves in the universe, are produced by intense objects such as black holes, pulsars, and supernovae. After subtracting all known gamma-ray sources, scientists found an unexplained gamma-ray glow at the heart of the Milky Way.
When an unaccounted phenomenon is observed in space, it is logical to associate it with other unexplained phenomena like dark matter. Dark matter is an invisible mass that affects gravity in the universe and can be indirectly detected through its gravitational effects. Although direct detection of dark matter remains elusive, dark matter may produce visible radiation. The GCE has been suggested to result from annihilating Weakly Interacting Massive Particles (WIMPs), a dark matter particles. If WIMPs collide, they would annihilate each other and generate gamma-ray photons.
However, previous studies have found no evidence of WIMP collisions, and the current research builds upon this by eliminating dark matter candidates within a specific mass range. The astrophysicists developed various gamma-ray modelling scenarios for the galactic centre and its bulge, considering multiple sources such as star formation, cosmic-ray interactions, and neutron stars. After accounting for these sources, there was little room left for WIMP annihilation, leading the researchers to conclude that no significant excess in the galactic centre is attributable to dark matter annihilation.
Moreover, the distribution of gamma rays in the galactic centre is inconsistent with dark matter annihilation. Instead of a smooth distribution, the gamma-ray photons are clustered like point sources like stars. Additionally, the distribution of stars in the bulge does not support the presence of other dark matter. The findings strongly favour an astrophysical origin for the GCE, suggesting that dark matter in the galactic centre may be of a hypothetical, massive, and weakly interactive type that differs from the commonly searched mass range.
While this research narrows down the possibilities for dark matter, it does not undermine the multiple lines of evidence supporting its existence in the galaxy. Scientists will need to explore alternative explanations and think beyond the conventional framework to uncover the nature of dark matter.
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