Ultralight dark photon as a model for early universe dark matter

TL;DR

This paper proposes that ultralight dark photons could have influenced early universe expansion, potentially resolving the Hubble tension by fitting the observed Hubble constant.

Contribution

It introduces a model where dark photons behave as radiation early on and as cold dark matter later, affecting cosmological parameters and addressing the Hubble tension.

Findings

Dark photon mass of order 10^{-27}-10^{-25} eV fits H_0 = 73 km/s/Mpc

Model modifies early universe expansion rate and sound horizon

Potential resolution of the Hubble tension with dark photon dynamics

Abstract

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.