Recent observations from the center of the Milky Way might just point to a new type of dark matter. This mysterious substance, which makes up about 85% of the universe, has always been hard to pin down. Now, scientists think they might be looking at dark matter’s subtle effects on cosmic chemistry.
The new candidate for dark matter is lighter than many previous ideas and has a unique property: it can self-annihilate. This means that when two dark matter particles collide, they annihilate each other, creating a negatively charged electron and its counterpart, a positron.
This annihilation process produces a surge of electrons and positrons. When these particles interact with neutral atoms in the dense gas in the Milky Way’s Central Molecular Zone (CMZ), they can strip electrons away from those atoms, resulting in a phenomenon known as ionization. This could explain the high levels of ionized gas observed in the CMZ.
Even if dark matter collisions are rare, they would be more likely at the core of galaxies, where dark matter is believed to concentrate. “We suggest that dark matter lighter than a proton could be responsible for unusual effects seen at the center of the Milky Way,” said Shyam Balaji, the team leader and a researcher at King’s College London. “Unlike other dark matter candidates that mostly reveal themselves through gravity, this one could indicate its presence by ionizing gas.”
Dark matter remains elusive because it doesn’t interact with light. This lack of interaction means scientists can’t observe it in the same way as ordinary matter, which is made of particles like electrons and protons. We suspect dark matter’s existence based on its gravitational effects, as it influences light and matter around it.
Scientists have proposed several dark matter particles over the years, each with different characteristics. The current leading candidates include axions and particles similar to them. However, Balaji’s team believes these traditional candidates do not fit the observations linked to the gas ionization in the CMZ. Their new dark matter candidate is light and can self-annihilate, allowing it to directly influence the interstellar medium.
In the CMZ, positrons that emerge from dark matter collisions interact quickly with nearby hydrogen atoms, stripping away their electrons. This process may be particularly effective in the dense regions of the galaxy’s center. “The model we propose helps explain the increased ionization observed in the CMZ,” Balaji noted. Scientific models generally suggest cosmic rays would cause this ionization, but the levels observed are too high to be explained just by cosmic rays, which usually produce detectable gamma rays as well. However, such emissions haven’t been seen, making our dark matter hypothesis more compelling.
If dark matter is indeed responsible for the CMZ’s ionization, we might be indirectly observing it by studying its chemical effects rather than through direct detection. There is also a mysterious faint gamma-ray glow from the Galactic Center that could be linked to the same processes causing ionization.
The annihilation process in this dark matter model may also account for light emissions from the CMZ, which arise when positrons and electrons combine to form a state called positronium that decays into X-rays. “The numbers align better than we anticipated,” Balaji remarked, emphasizing that this scenario fits within known scientific constraints and existing observations.
While this new dark matter candidate doesn’t yet have a catchy nickname like WIMP or MACHO, it signifies a potential shift in dark matter research. Scientists still need to gather more data to confirm its presence. For example, the upcoming COSI gamma-ray telescope, launching in 2027, could provide crucial insights into this dark matter theory by measuring specific cosmic processes.
In summary, this research opens a new door to understanding dark matter and its subtle influence on the universe. It highlights the potential to study dark matter not just through its gravitational effects but also through how it shapes the very fabric of our galaxy.
This exciting development was recently published in the journal Physical Review Letters.
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