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Dark matter could be composed of ultralight dark photons that have heated our universe. This is a new scenario proposed in a study published recently in the journal Physical Review Letters. This hypothesis, say the authors, is in line with observations made by the Cosmic Origin Spectrograph (COS) aboard the Hubble Space Telescope, which takes measurements of the “cosmic web”, the complex, tenuous network of filaments that fills the space between galaxies.

The data collected by COS suggest that the intergalactic cosmic filaments are hotter than predicted by standard structure-forming model hydrodynamics simulations. “Since dark photons would be able to convert into low-frequency photons and heat the cosmic structures,” the scientists explain, “they could very well explain the experimental information.” The study was carried out by researchers from the International School for Advanced Studies (SISSA, in Italy) in collaboration with researchers from the universities of Tel Aviv (Israel), Nottingham (UK) and New York (USA).

“Dark photons are hypothetical new particles that are the force carriers for a new force in the dark sector, just as the photon is the force carrier for electromagnetism,” explained study authors James S. Bolton (University of Nottingham), Andrea Caputo (CERN, France, and Tel Aviv University), Hongwan Liu (New York University) and Matteo Viel (SISSA). “Unlike the photon, however, they can have mass. In particular, the ultralight dark photon – with a mass as small as 20 orders of magnitude smaller than that of the electron – is a good candidate for dark matter.”

Dark photons and regular photons are also expected to mix together as the different types of neutrinos, allowing the ultralight dark matter dark photons to convert into low-frequency photons. These photons will heat the cosmic web, but unlike other heating mechanisms based on astrophysical processes, such as star formation and galactic winds, this heating process is more diffuse and efficient also in low-density regions.

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Matteo Viel explained, “Normally, cosmic filaments have been used to probe small-scale properties of dark matter, while in this case we use for the first time the data from the intergalactic low red-shift medium as a calorimeter, to check if all the heating processes we know are sufficient to reproduce the data. We find that this is not the case: something is missing, which we model as a contribution produced by the dark photon.”

The work identified the mass and mixing of the dark photon with the Standard Model photon needed to reconcile the discrepancy between observations and simulation. This effort could lead to further theoretical and observational investigations to explore the exciting possibility that the dark photon could constitute dark matter.

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