Using the Extragalactic Gamma-Ray Background to Constrain the Hubble Constant and Matter Density of the Universe

The attenuation produced by extragalactic background light (EBL) in gamma-ray spectra of blazars has been used to constrain the Hubble constant (H-0) and matter density (Omega(m)) of the universe. We propose to estimate H-0 and Omega(m) using the well-measured > 10 GeV extragalactic gamma-ray background (EGB). This suggestion is based on the fact that the > 10 GeV EGB is totally explained by the emissions from blazars, and an EBL-absorption cutoff occurs at similar to 50 GeV in the EGB spectrum. We fit the > 10 GeV EGB data with modeled EGB spectrum. This results in H-0 64.9(-4.3)(+4.6) km s(-1) Mpc(-1) and Omega(m) = 0.31(-0.14)(+0.13). Note that the uncertainties may be underestimated due to the limit of our realization for EBL model. H-0 and Omega(m) are degenerate in our method. Independent determination of Omega(m) by other methods would improve the constraint on H-0.