2013ApJ...771L..34D

A measurement of the expansion rate of the universe (that is, the Hubble constant, H₀) is derived here using the γ-ray attenuation observed in the spectra of γ-ray sources produced by the interaction of extragalactic γ-ray photons with the photons of the extragalactic background light (EBL). The Hubble constant determined with our technique, for a ΛCDM cosmology, is H_O = 71.8_–13.8^+7.2(syst) km/s.M/pc. This value is compatible with present-day measurements using well-established methods such as local distance ladders and cosmological probes. The recent detection of the cosmic γ-ray horizon (CGRH) from multiwavelength observations of blazars, together with the advances in the knowledge of the EBL, allow us to measure the expansion rate of the universe. This estimate of the Hubble constant shows that γ-ray astronomy has reached a mature enough state to provide cosmological measurements, which may become more competitive in the future with the construction of the Cherenkov Telescope Array. We find that the maximum dependence of the CGRH on the Hubble constant is approximately between redshifts 0.04 and 0.1, thus this is a smoking gun for planning future observational efforts. Other cosmological parameters, such as the total dark matter density Ω_m and the dark energy equation of state w, are explored as well.