`Positivity Bounds in Effective Field Theories of Gravity'

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The project will explore the very recent theoretical developments in effective quantum field theories, in which the tools of the S-matrix program of the 1960's have been shown to provide powerful constraints on the consistency of low energy effective field theories, in particular cosmological effective field theories that can describe the inflationary or dark energy phases of expansion of the universe. These constraints arise from the overarching requirements that a consistent Lorentz invariant quantum theory should satisfy unitarity, crossing symmetry, locality and causality. In recent work within the theoretical physics group at Imperial College, these requirements have been shown to impose an infinite number of bounds on the parameters in an effective field theory expansion that can be used to actively constrain candidate theories of gravity and cosmological models. In particular these constraints are already being used to significantly constrain the parameter space of massive gravity theories, which have been considered as candidate theories of gravity at cosmological scales with the potential to tackle the cosmological constant. The project will continue this exploration and put constraints on theories of multiple spin 2 and higher fields.