Dimming starlight may signal passage of dark compact objects

The detection of dark matter, an elusive form of matter believed to account for most of the universe’s mass, remains a long-standing goal within the physics research community. As this type of matter does not emit, reflect or absorb light, it cannot be observed using conventional telescopes and experimental methods.

Physicists have thus been trying to predict what it may consist of and proposing alternative approaches that could enable its detection. Dark compact objects are a class of dense and invisible structures that could be made up of dark matter, but that have never been directly observed so far.

Researchers at Queen’s University and the Arthur B. McDonald Canadian Astroparticle Physics Research Institute recently introduced a new possible method for detecting dark compact objects by probing their interactions with photons (i.e., light particles). Their newly proposed approach, outlined in a paper published in Physical Review Letters, is based on the idea that as dark compact objects pass between the Earth and a distant star, they will dim the light emitted by this star.

“Leo Kim, Melissa Diamond, and I were talking about our recent work on dissipative dark matter forming compact objects,” Joseph Bramante, co-author of the paper, told Phys.org. “One model we had considered was a compact object formed by dark matter cooling through the emission of dark photons.

“Part of our work needed to figure out whether these dark photons would escape the dark matter objects (and thus permit cooling) or instead scatter with the dark matter and be dispersed. It occurred to us that this ‘dark photon’ scattering process might also apply to visible photons.”

After reviewing previous literature, Bramante and his colleagues concluded that, so far, nobody had explored the extent to which star surveys such as OGLE could pick up stellar dimming resulting from the passing of dark compact objects. This realization ultimately inspired them to write their paper, in the hope of contributing to the future detection of these mysterious cosmological objects.

“Searches have already been conducted for dark matter compact objects using an effect called microlensing, where starlight is focused around the compact objects and amplified,” explained Bramante. “This would appear as a brightening of stars in stellar surveys like OGLE. The effect we identified is similar—visible light can be scattered or absorbed by the dark matter compact objects, causing stars to appear to dim.”

As part of their study, Bramante, Kim and Diamond re-analyzed data collected as part of two major star survey efforts worldwide, namely the EROS-2 and OGLE projects. Their analyses showed that searching for signs of dark compact objects by analyzing star survey data is in fact possible.

“Our study demonstrates a new fruitful approach to search for dark matter, which could be employed as part of stellar surveys like OGLE that looked for dark matter compact objects through stellar brightening caused by microlensing,” said Bramante. “In the future, these efforts could also look for stellar dimming.”

This research team’s recent efforts and proposed approach could potentially contribute to the future detection of dark compact objects. Meanwhile, Bramante and his colleagues plan to continue studying these objects, with the hope of further refining their proposed technique.

“We now plan to continue studying the cosmological formation and structure of dark matter compact objects,” added Bramante. “An improved understanding of these phenomena will allow us to more accurately determine what the ‘stellar dimming’ signature would look like.”

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