Early-universe plasma may have stopped dark photons from heating cosmos

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 discovery is the result of cross-disciplinary research by Perimeter Institute researchers Junwu Huang and Mohamad Shalaby in collaboration with Anson Hook at the University of Maryland. The findings suggest that dark photons would not have converted into ordinary light in the hot gas of charged particles (plasma) that filled the early cosmos, thus preventing significant heating of the plasma. The paper challenges previous assumptions about the energy conversion process and opens a vast region for experimental searches.

The research team ran simulations that demonstrated the standard story of linear conversion was incomplete. As dark photon energy begins to flow into the plasma, the system becomes violently nonlinear, shutting off the energy conversion after a tiny amount of energy is converted. This means the conventional cosmological constraint on dark photons is invalid across roughly 10 orders of magnitude in mass, from about 10⁻¹⁵ electron volts (eV) up to 10⁻⁶ eV—corresponding roughly to radio frequencies from kilohertz to gigahertz. The new analysis shows that experiments will probe new parameter spaces and potentially actually see something, representing a significant range that was previously excluded.

The discovery is not limited to dark photons. Applying nonlinear effects to other particles could mean rethinking how they behave in other environments. The paper highlights the importance of interdisciplinary collaboration, as it involves experts from different fields to tackle assumptions and push developments in how we understand the universe. The research team emphasizes that this is a test case in cosmology, with implications for other astrophysical systems. The paper is the result of important collaborations across physics disciplines, a core principle at Perimeter Institute.

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