Extragalactic Background Light inhomogeneities and Lorentz-Invariance- Violation in gamma-gamma absorption and Compton scattering
Abstract
Very-high-energy gamma ray photons (VHE; E > 100 GeV) from distant gamma ray objects
(e.g. blazars) are expected to be absorbed by the diffuse extragalactic background light (EBL),
which leads to a high-energy cut-off in a blazar's spectral energy distribution (SED). But
recent observations of cosmological gamma ray sources, after correction for the standard EBL
absorption, have been interpreted by some authors that the Universe is more transparent to
VHE gamma rays than expected from our current knowledge of the EBL energy density and
cosmological evolution. These unexpected VHE gamma ray signatures are currently one of the
subjects of intensive research.
One of the suggested solutions to this problem is the hypothesis that a reduced EBL opacity
results from the EBL energy density inhomogeneities in particular if the line of sight to a blazar
is passing through a cosmic void (under-dense region) in intergalactic space.
In this thesis, we start by studying the effects of such inhomogeneities on the energy density of
the EBL and the resulting gamma-gamma opacity, speciffically, by investigating the effects of
cosmic void along the line of sight to a distant blazar. First, we studied the possibility of one
single void and then the possibility of multiple voids, by assuming an accumulation of voids (10
voids) of typical radii R = 100 h -1 Mpc centred at a redshift of zv = 0:3 along the line of sight
to an object (for example, a blazar) located at redshift zv = 0:6. We conclude that spectral
hardening of the VHE gamma ray spectrum for blazars (e.g. PKS 1424+240), after correction
for the EBL gamma ray attenuation, is most likely not an artifact of an over-estimation of the
EBL absorption due to cosmic inhomogeneities.
In the second part of this thesis, we considered the impact of the Lorentz Invariance Violation
(LIV) effect on the gamma-gamma opacity of the Universe to VHE gamma rays propagating
from a distant object, compared with the possibility of multiple voids along the line of sight
(LOS) to the same object, and we investigated the impact of the LIV effect on the Compton
effect. Both subluminal and superluminal modifications of the dispersion relation of photons
are considered. In the subluminal scenario, the LIV effects may result in a significant reduction
due to the gamma-gamma absorption for photons with energies & 10 TeV. However, the
effect is not expected to be sufficient to explain the apparent spectral hardening of several
observed VHE gamma ray blazars in the energy range from 100 GeV up to few TeVs, even
when including effects of the EBL inhomogeneities in the distributions of matter and light in
the intergalactic space. superluminal modifications of the dispersion relation of photons lead to
a further enhancement of the EBL gamma-gamma absorption. We consider, for the first time,
the influence of LIV on the Compton effect. We find that the modified Compton scattering
process due to the LIV effect becomes relevant only for photons with energies, E & 1 PeV. In
the case of a superluminal modification of the photon dispersion relation, both the kinematic
recoil effect and the Klein-Nishina suppression of the cross-section are reduced. However, we
argue that the impact of LIV e ect on the Compton scattering process is unlikely to be of
astrophysical significance.