Ultralight dark photon as a model for early universe dark matter
  Dark photon is a massive vector field which interacts only with the physical
photon through the kinetic mixing. This coupling is assumed to be weak so that
the dark photon becomes almost unobservable in processes with elementary
particles, but can serve as a dark matter particle. We argue that in very early
Universe ($z>3000$) this vector field may have the equation of state of
radiation ($w=1/3$) but later behaves as cold dark matter ($w=0$). This may
slightly change the expansion rate of the Universe at early time and reduce the
value of the sound horizon of baryon acoustic oscillations (standard ruler). As
a result, in this model the value of the Hubble constant appears to be larger
than that in the standard $\Lambda$CDM model. In particular, it is sufficient
to have the dark photon mass of order $m\sim 10^{-27}-10^{-25}$ eV to fit the
value of the Hubble constant to $H_0 = 73$ km$\cdot$s$^{-1}$Mpc$^{-1}$ thus
resolving the Hubble tension.
