In this thesis we present two different research projects in theoretical high energy physics. The first part deals with inflationary dynamics of the early universe and its observable consequences in measurements of the cosmic microwave background radiation. Here we investigate how inhomogeneous initial conditions for single field inflation with an initial fast-roll stage can alter the standard inflationary predictions for the power spectrum of the cosmic microwave background at largest scales.;In the second part of the thesis we calculate long distance spin effects in gravitational scattering processes. Both classical and quantum corrections arise at the one loop level and are shown to have universal structures with forms and coefficients independent of the spins of the scattered particles. We also include calculations of electromagnetic and of mixed electromagnetic-gravitational scattering.
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