An effective description of an initial state is a method for representing the signatures of new physics in the short-distance structure of a quantum state. The expectation value of the energy-momentum tensor for a field in such a state contains new divergences that arise when summing over this new structure. These divergences occur only at the initial time at which the state is defined and therefore can be cancelled by including a set of purely geometric counterterms that also are confined to this initial surface. We describe this gravitational renormalization of the divergences in the energy-momentum tensor for a free scalar field in an isotropically expanding inflationary background. We also show that the backreaction from these new short-distance features of the state is small when compared with the leading vacuum energy contained in the field.
Collins, Hael and Holman, R., "The Renormalization of the Energy-momentum Tensor for an Effective Initial State" (2006). Physics Department Faculty Publication Series. 5.
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