A recent study published in Physical Review Letters indicates that a prominent dark matter candidate, the theoretical “dark photon”, did not warm the early universe as previously believed. This revelation opens up a large area for experimental investigation and may reshape the search for dark matter.
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This finding stems from interdisciplinary collaboration among Perimeter Institute researchers Junwu Huang and Mohamad Shalaby, along with Anson Hook from the University of Maryland.
Physicists had long presumed that, if dark photons existed, they would have transformed into ordinary light within the hot, charged particle gas (plasma) that filled the primordial cosmos. This process would have further heated the plasma, leaving behind detectable traces of dark photons. If this assumption were correct, cosmic measurements would have already ruled out a substantial portion of the dark photon parameter space.
However, new computer simulations demonstrate that this conversion mechanism ceases before significant warming can occur, implying these previously excluded ranges might now be crucial for dark photons.
These exclusions were saying the strength of dark matter had to be 108 weaker than it actually can be. This paper opens up a lot of new possibilities to look for dark matter.
Anson Hook, University of Maryland
Rethinking Dark Photons, and the Hunt for Dark Matter
Scientists considered the transformation of dark photons into ordinary light to be a linear process, with energy gradually transferring into plasma. However, the necessary energy seemed unusually high.
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. And I realized it’s not possible.
Junwu Huang, Perimeter Institute
This finding prompted Huang and Hook to revisit plasma physics textbooks. Their research ultimately connected them with Shalaby, a postdoctoral researcher specializing in plasma physics. Shalaby's simulations demonstrated that the conventional understanding of linear conversion was insufficient.
Rather, the simulations revealed that the system turns intensely nonlinear almost immediately once dark photon energy starts entering the plasma.
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
Based on this novel analysis, the established cosmological restriction on dark photons is incorrect across approximately ten orders of magnitude in mass, spanning from around 10-¹5 electron volts (eV) to 10-6 eV – frequencies roughly equivalent to kilohertz through gigahertz radio bands.
Shalaby clarifies, “By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something.”
This covers a substantial range that was previously ruled out. Furthermore, this approach could be extended to search for other hard-to-detect particles.
A New Probe Into the Unknown
This discovery is not solely confined to dark photons. Introducing nonlinear phenomena to other particles might require re-evaluating their behavior in diverse settings.
“This is a test case in cosmology. A lot of astrophysical systems have also been used to look for similar effects, and we need to rethink all of them,” Huang says. “Linear approximation, which is easy to compute, might have nothing to do with how a neutron star magnetosphere [or] a white dwarf magnetosphere actually behave.”
This research stems from significant teamwork spanning various physics fields, a fundamental principle at Perimeter Institute. By promoting studies that involve specialists from varied domains, physicists can challenge assumptions and drive fresh, stimulating advances in cosmic understanding.
“It's truly interdisciplinary. It's the interaction between plasma physics and particle physics,” says Shalaby. “And this will directly impact people who do experiments.”
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Journal Reference:
Hook, A., et al. (2026) No Cosmological Constraints on Dark Photon Dark Matter from Resonant Conversion: Impact of Nonlinear Plasma Dynamics. Physical Review Letters. DOI:10.1103/98cx-7t43. https://journals.aps.org/prl/abstract/10.1103/98cx-7t43.