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Abstract

Einstein’s General Relativity is the most successful theory of gravity ever formulated and it has been tested to outstanding precision on a wide range of scales. However, the standard cosmological model based on it requires an unknown dark energy, modelled as a fine-tuned cosmological constant, to explain the universe’s current accelerated expansion. Since gravity is not as well-tested on large cosmological scales as within our Solar System, modified gravity theories are a valid alternative. Even beyond cosmology, the true nature of gravity remains elusive. For example, the field equations of gravity theories can be derived as equations of state from purely thermodynamical considerations. This leads to identifying General Relativity with an equilibrium state of gravity and modified gravity with a non-equilibrium one. In this thesis, we present a novel approach to the thermodynamics of modified gravity which provides a concrete realisation of this idea. Applying a non-equilibrium thermodynamical description to the effective fluid describing scalar-tensor gravity, a “thermodynamics of gravitational theories” naturally emerges. Applications of this framework to cosmology, extensions to different classes of modified theories, and the formulation of two complementary descriptions based on temperature and chemical potential sketch a unifying picture of the landscape of gravity theories.