Abstract
High energy photons from astrophysical sources are unique probes for some predictions of candidate theories of Quantum Gravity (QG). In particular, Imaging atmospheric Cherenkov telescope (IACTs) are instruments optimised for astronomical observations in the energy range spanning from a few tens of GeV to ∼100 TeV, which makes them excellent instruments to search for effects of QG. In this article, we will review QG effects which can be tested with IACTs, most notably the Lorentz invariance violation (LIV) and its consequences. It is often represented and modelled with photon dispersion relation modified by introducing energy-dependent terms. We will describe the analysis methods employed in the different studies, allowing for careful discussion and comparison of the results obtained with IACTs for more than two decades. Loosely following historical development of the field, we will observe how the analysis methods were refined and improved over time, and analyse why some studies were more sensitive than others. Finally, we will discuss the future of the field, presenting ideas for improving the analysis sensitivity and directions in which the research could develop.
Highlights
Introduction and MotivationPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
Another possible source of systematic effects are secondary gamma rays, which can be produced through one of the following processes: (i) hadrons accelerated within a source interact with the surrounding electromagnetic fields to produce neutral pions, which decay into gamma rays, (ii) gamma rays emitted from the source interact with magnetic fields to produce electron-positron pairs, which can create secondary gamma rays either through annihilation, or by inverse-Compton scattering of lower-energy photons
Though not formulated yet, the theory of Quantum Gravity (QG) is expected to resolve what happens in extreme gravitational potentials, such as singularities within black holes predicted by the general theory of relativity, or early universe, and to push us in the direction of formulating the unification theory describing all interactions
Summary
The general theory of relativity is a beautiful and elegant theory, which connects the local matter and energy content to the curvature of spacetime, giving a classical description of gravity. It has been heavily tested and scrutinised ever since Albert Einstein proposed it in 1915 [1]. It is expected that there exists a more fundamental quantum theory of gravity, which can handle these extreme situations. What allows us to hope that we will measure an effect of QG?
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