Infrared modifications to General Relativity

      Recent cosmological observations indicate a late-time acceleration of our observable universe. This has stimulated various infrared modifications of General Relativity which are relevant at cosmological scales and hope to provide a dynamical explanation for dark energy. In this talk, I will discuss two infrared modified gravity theories - specifically non-locally modified gravity theories and shift symmetric Galileon models. I will first consider the non-local model of Maggiore and Mancarella, comprising of the Einstein-Hilbert action in addition to a non-local R −2R term associated with a mass scale m. This theory successfully accounts for the cosmo-logical evolution of our universe and provides corrections about the Schwarzschild background. However, we will demonstrate that the theory fails to provide real perturbative corrections about the Reissner-Nordstr ̈om background when the electromagnetic field is minimally coupled. This motivates us to consider the modified coupling of electromagnetism to this theory via the Kaluza ansatz. The Kaluza reduction introduces non-local terms involving the electromagnetic field to the pure gravitational non-local theory. An iterative approach is provided to perturbatively solve the equations of motion to arbitrary order in m2 about any known solution of General Relativity. The resulting non-local electromagnetic fields could be relevant in particle trajectories near the horizons and future graviatational wave observations. I will conclude the seminar with a brief discussion of covariant Galileon actions which respect an additional internal shift symmetry. We will demonstrate that these models do not predict the late-time acceleration of the universe.