Computer simulations published on August 13 in Physical Review Letters show that hypothetical dark photons
would have halted their conversion into ordinary light in the early universe before causing significant cosmic heating, reopening a vast parameter space for dark matter research.
Reopening the Search for Dark Photon Dark Matter
For decades, cosmologists believed that a leading dark matter candidate known as the dark photon had already been ruled out by observations. The core assumption was straightforward: in the dense plasma of the early universe, these hypothetical particles would inevitably transform into ordinary light. That conversion process would have pumped immense heat into the cosmic soup, leaving detectable traces that current cosmological measurements do not see.
New computer simulations, however, indicate that previous models relied on an incomplete approximation. Researchers from the Perimeter Institute and the University of Maryland demonstrated that the transformation of dark photons into standard photons shuts itself off well before significant heating can occur.
This shift brings massive ranges of previously excluded parameter space back into play for experimental physicists. 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 at the University of Maryland in a statement. This paper opens up a lot of new possibilities to look for dark matter.
Why Linear Approximations Failed in Early Universe Plasma
The breakthrough came when researchers began questioning the mathematics behind energy transfer in the early cosmos. For roughly 15 years, physicists treated the conversion from dark photons to normal light as a linear process, where energy transfers gradually and uniformly into the surrounding plasma.
Junwu Huang of the Perimeter Institute noted that calculating the total energy transfer using that standard approximation yielded suspiciously large values. 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,
Huang said. And I realized it’s not possible.
That realization sent the team back to fundamental plasma physics textbooks and led them to collaborate with Mohamad Shalaby, a postdoctoral fellow specializing in plasma dynamics at the Perimeter Institute. Running advanced simulations, the team discovered that introducing dark photon energy into the Standard Model plasma triggers violent nonlinear effects.
“What we realized is that, as you are converting energy into the Standard Model plasma, the plasma actually goes crazy. 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.”
Junwu Huang, Perimeter Institute researcher, via Space.com
Expanding Parameter Space and Future Experimental Searches
By correcting how early-universe plasma is modeled, the new research invalidates conventional cosmological constraints across roughly ten orders of magnitude in mass. The newly opened search window spans from about 10⁻¹⁵ electron volts up to 10⁻⁶ electron volts, frequencies that roughly correspond to the kilohertz to gigahertz radio bands.
Physicists point out that this correction will directly guide upcoming laboratory design and observational campaigns. By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something,
said Mohamad Shalaby of the Perimeter Institute.
Furthermore, researchers emphasize that the implications extend beyond dark photons alone. Because nonlinear plasma effects alter how exotic particles interact with their environments, scientists may need to reexamine other astrophysical systems, including the extreme magnetospheres surrounding neutron stars and white dwarfs.