Measurement of the expansion rate of the Universe from γ-ray attenuation
Denis Wouters
irfu / SPP
Mardi 22/10/2013, 13:30-14:30

Measurement of the expansion rate of the Universe from γ-ray attenuation

 A measurement of the expansion rate of the Universe (that is the Hubble cons
tant, H0) is derived here using the gamma-ray attenuation observed in the spectr
a of gamma-ray sources produced by the interaction of extragalactic gamma-ray ph
otons with the photons of the extragalactic background light (EBL). The Hubble c
onstant that is determined with our technique, for a Lambda CDM cosmology, is H_0
=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 cos
mic gamma-ray horizon (CGRH) from multiwavelength observation of blazars, togeth
er with the advances in the knowledge of the EBL, allow us to measure the expans
ion rate of the Universe. This estimate of the Hubble constant shows that gamma-
ray astronomy has reached a mature enough state to provide cosmological measurem
ents, which may become more competitive in the future with the construction of t
he 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 p
arameters, such as the total dark matter density Omega_m and the dark energy equ
ation of state w, are explored as well.

Contact : Jean-marc LE-GOFF


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