Quantum Field Theory and Cosmology

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Quantum gravity is the theory that arises from combining gravity with relativity (how observers compare time and distance) and quantum mechanics (the key principle of which is that every degree of freedom has a tiny amount of motion). It has long been the subject of wild speculation, unchecked by the usual chain of observation and experiment which characterizes hard science. Recent advances in observational and theoretical cosmology have changed this by providing the first quantum gravitational data ever taken, with a vast potential for increase. The very early universe was expanding so rapidly and with such a high degree of acceleration that the usually tiny fluctuations of gravitational fields in empty space were amplified to a still-small but measurable amount. Equally important, these fluctuations were stretched out in space so far that they became effectively fossilized, and were thereby able to survive down to the current epoch. This project will study how these fluctuations affect gravity itself, and also the other forces and particles. Such studies are part of the great transformation of quantum gravity from science fiction to science fact, and the phenomenon they reveal may help us to better understand the current universe.Interactions which involve undifferentiated gravitons and/or massless, minimally coupled scalars can give rise to greatly enhanced quantum effects during primordial inflation. This is the source of cosmological perturbations and recent work has turned from these primary effects to secondary effects arising from loop corrections to the power spectra and other quantities. The PI will address these problems over the next three years by continuing work in six areas: Computing and fully renormalizing graviton loop contributions to 1PI 2-point functions in general gauges; defining nonlinear extensions of cosmological observables based on nonlocal functionals of the metric; computing the one and two loop expectation value of an invariant measure of spacetime expansion; studying nonperturbative secular effects using the scattering of matter quanta by inflationary gravitons; investigating nonlocal modified gravity models of inflation, of late time acceleration and of MOND; and exploring phenomenological implications of inflationary perturbations build up from past evolution.