A science highlight in 2022 was the development of the first hydrodynamical simulations of the intergalactic medium (IGM) to incorporate heating by dark photons. Photons are the mediator of the electromagnetic force between charged particles, such as electrons and protons. We expect particles in the dark sector to interact with each other through such forces too, exchanging new particles called “dark photons” with each other. The dark photon can be massive, and if they are very light, can be produced in the early Universe to make up the dark matter. Dark photons can also mix with the regular photon, leading to possible cosmological signatures of dark photons. In regions of space where the mass of the dark photon matches the effective plasma mass of the photon, conversions from dark photons to photons can occur. The converted photons are then rapidly absorbed by the IGM in those regions, heating the gas up. If these dark photons are indeed the missing dark matter, then a measurable effect on the temperature of intergalactic gas is predicted.
We have therefore compared our simulations to observations of the Lyman-alpha forest from the Comic Origins Spectrograph (COS) on the Hubble Space Telescope, finding that some heating from dark photons is indeed consistent with the measured Lyman-alpha line widths. The image shows the main result, which was published in Physical Review Letters (Bolton et al. 2022, PRL, 129, 211102). The top panel shows the results of fitting the Doppler parameter distribution and column density distribution function of the Lyman-alpha forest at z=0.1 assuming a maximal contribution of dark photon heating to the line widths. The contours show the projection of the 68% and 95% intervals for the mass and mixing parameter of the dark photon. The colours correspond to different assumptions about the uncertainty of the intergalactic medium temperature at z = 2. The bottom panel shows the corresponding best-fit models compared to the COS observational data. The solid grey curve shows a result with no dark photon heating.