New research suggests that if dark matter is made up of “dark photons,” it would not have heated the early universe the way scientists long assumed, a finding that could mark a significant shift in the search for the universe’s most elusive substance.
Dark matter remains stubbornly hard to detect because, despite outweighing ordinary matter, the stuff that makes up stars, planets, moons, and people, by a ratio of roughly five to one, it doesn’t interact with light at all. Because electrons, protons, and neutrons do interact with electromagnetic radiation, physicists have long searched for particles beyond the Standard Model that might explain dark matter’s invisible presence, producing a long list of hypothetical candidates.
One such candidate is the dark photon, essentially a dark-universe counterpart to the ordinary photon, carrying a force other than electromagnetism. Previously, scientists believed dark photons would have converted into ordinary photons while still embedded in the dense plasma that filled the early universe, a process that would have further heated an already extremely hot environment and left detectable traces behind. That expectation sharply narrowed the range of conditions under which dark photons could plausibly exist, to the point where many cosmological observations appeared to rule them out entirely.
New computer simulations suggest that dismissal may have come too soon. According to the research, the conversion of dark photons into ordinary photons would have shut off before it could generate meaningful heating, reopening search parameters that had previously been excluded.
“These exclusions were saying the strength of dark matter had to be 10^8 times weaker than it actually can be,” said team member Anson Hook of the University of Maryland. “This paper opens up a lot of new possibilities to look for dark matter.”
The research team first suspected something was off when they noticed the amount of energy involved in the standard picture seemed implausibly large. The issue, they came to realize, was that the conversion process had always been modeled as linear, meaning energy was assumed to convert steadily and gradually into plasma.
“The treatment for the last 15 years is a linear treatment. If you use that approximation, you can compute the amount of energy transfer, and it’s very large,” said Junwu Huang of the Perimeter Institute. “And I realized it’s not possible.”
Running their own simulations, Huang and colleagues found that the linear model missed something crucial. “What we realized is that, as you are converting energy into the Standard Model plasma, the plasma actually goes crazy,” Huang said. “There are a lot of nonlinearities in the system, and these nonlinearities basically shut off the energy conversion after a tiny amount of energy is converted.”
The findings significantly widen the range of conditions under which dark photons could exist, expanding the search space considerably, and could also influence the broader hunt for other hypothetical particles beyond the Standard Model. “By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something,” said team member Mohamad Shalaby, also of the Perimeter Institute.
The research was published August 13 in the journal Physical Review Letters.
