Measurement of the expansion rate of the Universe from \gamma-ray
  attenuation
  A measurement of the expansion rate of the Universe (that is the Hubble
constant, H0) is derived here using the gamma-ray attenuation observed in the
spectra of gamma-ray sources produced by the interaction of extragalactic
gamma-ray photons with the photons of the extragalactic background light (EBL).
The Hubble constant that is determined with our technique, for a Lambda CDM
cosmology, is H0=71.8_{-5.6}^{+4.6}(stat)_{-13.8}^{+7.2}(syst) km s^{-1}
Mpc^{-1}. 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 gamma-ray horizon (CGRH) from multiwavelength
observation 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 gamma-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 Omega_m and the dark energy equation of state w, are explored as
well.
