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'Hidden' photons could shed light on mysterious dark matter

A new method could help scientists shed light on the universe’s most mysterious substance by narrowing down the hunt for a particular dark matter candidate — hidden "dark photons."

Dark matter comprises around 85% of the matter content of the universe, yet because it doesn’t interact with light or does so only very weakly, it remains effectively invisible. The fact that dark matter doesn’t seem to interact electromagnetically means scientists know it can’t be made up of the atoms that comprise the "normal" matter that makes up stars, planets and our bodies.

The mystery of dark matter is such a pressing problem for scientists because it means the matter we see comprises just 15% of the stuff, not including energy, in the cosmos. This has led to the search for potential dark matter candidates, such as so-called "hidden" or "dark" photons.

These dark photons would differ from ordinary photons, which are massless particles that make up light, as dark photons are theorized to possess mass. The mass of dark photons would be tiny, however, at around twenty orders of magnitude less than the mass of an electron. It is this ultralight nature that makes dark photons a good candidate for dark matter and would also make them incredibly tough to detect.

Dark photons were initially suggested as a dark matter candidate because, theoretically, they would weakly interact with ordinary photons, meaning they could have played a role in heating up the early universe. This action would explain why the cosmic web, a large-scale structure in the universe linking together galaxies, was hotter than predicted when observed by the Hubble Space Telescope.

Now, researchers from the California Institute of Technology (Caltech) have come up with a new detection method for dark photons. And while this new strategy hasn’t yet turned up any of the hypothetical particles, it has placed constraints on their characteristics, which will aid future searches.

"The sensitivity of a hidden photon dark matter experiment depends on the strength of the dark matter signal compared to the smallest signal you can detect," team member Nikita Klimovich, a researcher in Oxford University's Department of Physics, told

"For hidden photon searches, the amplitude of the dark matter signal scales with the area of the metal dish used, while the minimal detectable signal level is largely determined by the noise level [the interference] of the amplifiers used to read out the antenna," Klimovich added.

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