Abstract. The arrival of TeV-energy photons from distant galaxies is expected to be affected by their QED interaction with intergalactic radiation fields through electron-positron pair production. In theories where high-energy photons violate Lorentz symmetry, the kinematics of the process γ + γ → e + + e − is altered and the cross-section suppressed. Consequently, one would expect more of the highest-energy photons to arrive if QED is modified by Lorentz violation than if it is not. We estimate the sensitivity of Cherenkov Telescope Array (CTA) to changes in the γ-ray horizon of the Universe due to Lorentz violation, and find that it should be competitive with other leading constraints.

We present new observational constraints on Lorentz violating Hořava-Lifshitz cosmological scenarios using an updated cosmological data set from Cosmic Microwave Background (Planck CMB), expansion rates of elliptical and lenticular galaxies, JLA compilation (Joint Light-Curve Analysis) data for Type Ia supernovae (SneIa), Baryon Acoustic Oscillations (BAO) and priors on the Hubble parameter with an alternative parametrisation of the equations. Unlike in other approaches we consider the curvature parameter Ω k as a free parameter in the analysis we considered the parameters Ω k and ∆N ν as completely free, which helped to place new, updated bounds on several of the theory parameters. Remarkably, the detailed balance scenario exhibits positive spatial curvature to more than 3σ, whereas for further theory generalizations we found evidence for positive spatial curvature at 1σ. This could create circumstantial evidence from observations and could be used to single out distinct formulations and scenarios.

We modify the standard relativistic dispersion relation in a way which breaks Lorentz symmetry -the effect is predicted in a high-energy regime of some modern theories of quantum gravity. We show that it is possible to realise this scenario within the framework of Rainbow Gravity which introduces two new energy-dependent functions f 1 (E) and f 2 (E) into the dispersion relation. Additionally, we assume that the gravitational constant G and the cosmological constant Λ also depend on energy E and introduce the scaling function h(E) in order to express this dependence. For cosmological applications we specify the functions f 1 and f 2 in order to fit massless particles which allows us to derive modified cosmological equations. Finally, by using Hubble+SNIa+BAO(BOSS+Lyman α)+CMB data, we constrain the energy scale E LV to be at least of the order of 10 16 GeV at 1σ which is the GUT scale or even higher 10 17 GeV at 3σ. Our claim is that this energy can be interpreted as the decoupling scale of massless particles from spacetime Lorentz violating effects.

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