Understanding NonLocality in Quantum Gravity

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The formulation of a consistent theory of quantum gravity is one of the most outstanding unsolved problems in Theoretical Physics, which has attracted interest since the middle of the last century. In the past decades several promising approaches to quantum gravity have been proposed. Despite their intrinsic differences, many of them seem to predict the emergence of nonlocality at the microscopic level, i.e. at short distances and high energies, indicating that the gravitational interaction is nonlocal in nature rather than point-like. This feature could be the key to solving open issues in gravitational physics - such as classical curvature singularities and quantum divergences - as nonlocality naturally introduces a physical cut-off scale. Nonlocal physics is expected to manifest through specific non-polynomial form factors in the quantum gravitational Lagrangian whose derivation, however, is still pending in all known approaches. The aim of the proposed project is to derive and use fundamental consistency requirements of causality, stability, unitarity, and healthy high-energy behaviour to constrain the viable form factors, and thus the space of allowed quantum field theories of gravity. The study will be model-independent but its implications can severely limit the feasibility of various quantum gravity programs. This project will take the field of nonlocal gravity beyond its current state-of-the-art by deriving novel causality constraints, constructing for the first time the Hamiltonian for infinite derivative Lagrangians, and pioneering a complete one-loop computation of the quantum effective action. The unique scenario to test new physics beyond general relativity provided by the new era of precision cosmology and gravitational-wave astronomy makes this project highly timely. Achieving the proposed goals can place nonlocal theories of gravity on firmer ground and lay the foundation for future phenomenological applications in cosmology and astrophysics.