Ted Jacobson

University of Maryland
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. 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 effective Lagrangian below the cutoff will be computed using perturbative methods. This will be used to study field dynamics and vacuum stress-energy. 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? Could it mediate leptogenesis? Quantum causal histories, an approach to discrete quantum field theory on a causal set, will also be investigated in two regards: i) to determine whether a growing universe with growing Hilbert space can be accommodated by an approach analogous to the no boundary wavefunction, and ii) to study the status of causal horizons and the evolution of the entropy outside them, to explore the relation to the generalized second law, and to try to infer constraints on the dynamics of causal structure.

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