Dr. Ted Jacobson

University of Maryland at College Park
Growth of the Vacuum in Quantum Cosmology

Space and time are assumed to be continuous in standard physics, but there are several reasons to suspect that this is only an approximation to a more basic level of description in which the world is discrete. This would be analogous to the fact that matter appears continuous, but is actually composed of discrete atoms. The expansion of the universe would presumably tend to spread out the "atoms of space-time" requiring that, unlike in ordinary matter, new atoms are created as time goes on. The research in this proposal is aimed at understanding possible observable consequences of such creation, and at the challenge to modify the laws of physics to allow for it.

The possibility that the space-time continuum is a low resolution approximation to a more structured, discrete plenum will be probed, focusing on the setting of an expanding universe. Both a phenomenological continuum model and discrete models will be studied. The former assumes new field modes are born free at the cutoff scale, and cosmological expansion then brings them into interaction with the rest of the vacuum via scale dependent interactions. The resulting vacuum energy and pressure will be studied. Could this process could drive inflation and/or explain the current dark energy? Could it produce a small number of highly energetic or massive quanta?

Quantum cosmology via path integration over discrete geometries will also be studied, the main question being whether this could deliver a semiclassical regime with a growing vacuum that remains near the adiabatic ground state. The logical status of the probability interpretation of this framework will also be studied.

Discrete quantum field theory on a causal set will also be investigated, to determine whether a growing universe can be accommodated by weakening the previously introduced local unitarity and commutativity axioms so that they apply only in one time direction.

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