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.