IRFP: Semiclassical Quantum Gravity

Haggard Harold M, Berkeley CA

Investigators

Abstract

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award is co-funded by the Office of International Science and Engineering and by the Physics of the Universe Program in the Division of Physics. This award will support a twenty-four-month research fellowship by Dr. Hal M. Haggard to work with Dr. Carlo Rovelli at the Centre de Physique Theorique de Luminy, Marseille, France. There is now substantial evidence that the classical limit of loop gravity is general relativity. This research uses semiclassical techniques to study the first order quantum effects of loop gravity and to clarify the structure of the theory so that these effects can become solid predictions characterizing the quantum nature of gravitation. The work proceeds along three main lines: At the Planck scale, a quantum behavior of the geometry of space is expected. Loop gravity provides a specific realization of this expectation and predicts a granularity of space with each grain having a quantum behavior. The first line of this work investigates the geometrical character of the spin network basis, the basis that captures this discreteness. In particular, a manner for investigating the volume of more complex grains of space, beyond the tetrahedral case usually studied, is being pursued. There is a well known one parameter ambiguity in the loop quantization of gravity, which is coded in the Immirzi parameter. In the second line a new method for investigating the origin of the Immirzi parameter is being investigated. Clarification of the role of this parameter is essential for the theory to make definite predictions. Finally, in the third line, a novel method for the calculation of the perturbative graviton propagator is being investigated. This method uses the tools of semiclassical mechanics and could substantially streamline the calculation of propagators and n-point functions in the theory. Successful development of this method will also allow the calculation of the first order quantum corrections in the theory. Completing this research program will establish loop gravity as a strong candidate for the description of gravity at the Planck scale; a description that not only provides a quantum picture of spacetime at microscopic scales and is compatible with our current understanding of quantum field theory but one that can predict signatures that will be experimentally tested as they enter the domain of technological feasibility. Experimental astrophysics and cosmology are achieving unprecedented measurements of the large-scale universe. This project aims to predict new quantum gravitational phenomena so that we can witness science at its best: prediction and experimental measurement arriving together.