Scientists Think Dark Matter May Be Filling Our Galaxy With Mysterious Light

Here’s what you’ll learn from reading this story:

• Several theories have attempted to explain the excess far-ultraviolet light present in the Milky Way, but none have been satisfactory.

• Now, researchers have hypothesized that nuggets of axion quark dark matter could collide with visible matter, releasing ultraviolet light.

• This work could have implications for understanding light emissions in the early universe.

Elusive but omnipresent, dark matter is thought to make up around 27% of the universe, but no molecules have been directly detected until now. It’s completely invisible. The only way to detect dark matter is to use the gravitational effects it exerts on baryonic or visible matter.

Well… that, and the ghostly glow it seems to emit.

Dark matter can take many forms. One hypothesis suggests that some of them exist in the form of axion quark nuggets (AQN), which are ultra-dense objects made of quarks and linked to axions (ultralight particles that are still hypothetical). AQNs would behave gravitationally like cold dark matter, the invisible, slow-moving matter thought to help shape galaxies. But unlike many dark matter candidates, AQNs could also leave observable traces in the form of electromagnetic radiation over a range of wavelengths when they interact with ordinary matter. And if some AQNs were made of antimatter, these encounters would be even more numerous. more dramatic, triggering annihilation events that would convert mass into shards of light. In principle, this process could help explain the unexplained excess of far-ultraviolet radiation in the Milky Way.

Michael Sekatchev (an astrophysicist at the University of California, Berkeley) wanted to test whether antimatter NQAs could explain the ultraviolet glow that scientists noticed after all known sources were eliminated. About a decade ago, NASA’s Galaxy Evolution Explorer (GALEX) used its far-ultraviolet (FUV) instrument to map the diffuse FUV background, the faint ultraviolet light that spreads across the sky and does not come from easily identifiable individual sources. In the Milky Way, most of this glow is thought to come from starlight scattered by interstellar dust. But even after accounting for the combined light of the galaxy’s hundreds of billions of stars, astronomers still found excess FUV radiation. Later work showed that this extra light emission likely came from the Milky Way itself, but its distribution suggested that it did not come from just any random part of the galaxy.

“[AQNs] have a mass greater than a few grams and a size less than a micrometer,” wrote a group of researchers (led by Sekatchev) in a study recently published in the Journal of Cosmology and Astroparticle Physics. “They would also help explain the matter-antimatter asymmetry and the similarity between the visible and dark components of the universe. [by allowing researchers to calculate] the FUV electromagnetic signature in a [region] surrounding the solar system, resulting from the interaction between AQNs and baryons.

This was further complicated by the fact that the brightness of GALEX’s observations matched previous observations made by Dynamics Explorer (also powered by NASA), which was much further from Earth. This meant that the excess FUV radiation could not come from terrestrial bodies or the solar system. Even stranger was the discovery that this radiation was evenly distributed, unlike UV light emitted by stars, which is generally not so constant. The excess Also did not match the galactic longitudes of the brightest UV-emitting stars in the Milky Way, suggesting that it was not simply unresolved starlight. Observations from the Alice UV spectrograph aboard New Horizons reinforced this conclusion: while about half of the measured FUV intensity could be attributed to known UV sources, the rest remained unexplained.

The unusually soft distribution of the FUV glow led Sekatchev to wonder whether it could be produced by the annihilation of dark matter. After finding that several existing models did not fit the GALEX data, he turned to an earlier speculative study describing an electrically neutral dark antimatter composite object whose charged constituents could still interact with ordinary matter. If such an object exists, its encounter with visible matter could trigger its annihilation and produce some of the unexplained light. Axion quark nuggets are an object of a hypothetical class that fits this description.

Sekachev and his team turned to computer simulations to determine how much FUV light would be emitted by AQNs if they constituted the distribution of dark matter already determined for parts of the Milky Way. The results matched the GALEX and New Horizons findings on which they based their research. The team also discovered that the ionizing photons produced during these matter-antimatter annihilations could help explain other long-standing astrophysical puzzles. “Recent observations from the James Webb Space Telescope reveal that faint early galaxies are prolific producers of ionizing photons,” Sekatchev said. “Can this be enough to explain the JWST result without significant change? […] remains to be demonstrated. »

Dark matter may be invisible, but if Sekachev’s team is right, it’s not entirely silent. This ghostly glow could prove to be its calling card.

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