Abstract
Ground-based observatories have been collecting 0.2-20 TeV gamma rays from
blazars for about twenty years. These gamma rays can experience absorption
along the line of sight due to interactions with the extragalactic background
light (EBL). In this paper, we investigate the most extensive set of TeV
spectra from blazars collected so far, twice as large as any other studied. We
first show that the gamma-ray optical depth can be reduced to the convolution
product of an EBL kernel with the EBL intensity. We extract the EBL intensity
from the gamma-ray spectra, show that it is preferred at 11 sigma to a null
intensity, and unveil the broad-band spectrum of the EBL from mid-UV to far IR.
Our measurement shows that the total radiative content of the universe between
0.1 and 1000 microns represents 6.5+/-1.2% of the brightness of the CMB. This
is slightly above the accumulated emission of stars and galaxies and constrains
the unresolved sources that could have reionized the universe. We also propose
a data-driven method to estimate the Hubble constant based on the comparison of
local and gamma-ray measurements of the EBL, yielding H0 = 88 +/- 8(stat)
+/-13(sys) km/s/Mpc. After setting the most stringent upper-limits on the
redshift of four TeV blazars, we investigate the 106 intrinsic gamma-ray
spectra in our sample and find no significant evidence for anomalies. We do not
find evidence for the so-called "pair-production anomaly" at large optical
depths, which has been used previously to place lower limits on the coupling of
TeV gamma rays with axion-like particles. Finally, we investigate the impact of
a modification of the pair-creation threshold due to a Lorentz invariance
violation. A mild excess prevents us from ruling out an effect at the Planck
energy and we constrain for the first time the energy scale of the modification
to values larger than sixty percent of the Planck energy.
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