By Perimeter Institute
A new paper published in Physical Review Letters reveals that a leading dark matter candidate—the hypothetical “dark photon”—would not have heated the early universe as previously thought. The finding opens a vast region for experimental searches and could change the hunt for dark matter.
The discovery is the result of cross-disciplinary research between Perimeter Institute researchers Junwu Huang and Mohamad Shalaby in collaboration with Anson Hook at the University of Maryland.
Physicists have long assumed that dark photons, if they exist, would have converted into ordinary light in the hot gas of charged particles (plasma) that filled the early cosmos, heating the plasma further to leave detectable traces of dark photons. If this were true, then a vast range of parameter space for dark photons would have been excluded by cosmological measurements.
New computer simulations, however, show that this conversion process shuts itself off before significant heating can occur, meaning these excluded ranges could hold the key to dark photons.
“These exclusions were saying the strength of dark matter had to be 108 weaker than it actually can be,” says Hook. “This paper opens up a lot of new possibilities to look for dark matter.”
Rethinking dark photons, and the hunt for dark matter
In general, researchers treated the conversion from dark photon to normal light as linear, and that the energy would slowly convert into plasma. But the energy required was suspiciously large. “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,” says Huang. “And I realized it’s not possible.”
The discovery took Huang and Hook back to old textbooks to refamiliarize themselves with plasma physics. This eventually led them to Shalaby, a postdoctoral fellow who specializes in plasma physics. Shalaby ran simulations which proved the standard story of linear conversion was incomplete.
Instead, the simulations demonstrated that as soon as dark photon energy begins to flow into the plasma, the system becomes violently nonlinear. “What we realized is that, as you are converting energy into the standard model plasma, the plasma actually goes crazy,” Huang explains. “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.”
According to the new analysis, the conventional cosmological constraint on dark photons is invalid across roughly ten orders of magnitude in mass, from about 10⁻¹⁵ electron volts (eV) up to 10⁻⁶ eV—frequencies corresponding roughly from kilohertz to gigahertz radio bands. “By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something,” explains Shalaby.
This represents a significant range previously excluded. And it could be expanded to search for other elusive particles.
A new probe into the unknown
The discovery is not limited to dark photons. Applying nonlinear effects to other particles could mean rethinking how they behave in other environments as well. “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.”
The paper is the result of important collaborations across physics disciplines, a core principle at Perimeter Institute. By encouraging research that includes experts from different fields, physicists can tackle assumptions and push new and exciting developments in how we understand the universe.
“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.”