Planck-scale models of the Universe

TL;DR

This paper explores Planck-scale models of the universe, such as quantum causal histories and spin foams, proposing a statistical physics approach to quantum gravity without a background spacetime, aiming to derive classical spacetime as a low-energy limit.

Contribution

It introduces a framework for quantum cosmology at Planck energies using spin foam and quantum causal history models, emphasizing a background-independent, statistical physics perspective.

Findings

Models are formulated via partition functions similar to lattice gauge theories.

Suggests gravity and spacetime emerge as low-energy approximations.

Proposes treating quantum gravity as a statistical physics problem.

Abstract

Suppose the usual description of spacetime as a 4-dimensional manifold with a Lorentzian metric breaks down at Planck energies. Can we still construct sensible theoretical models of the universe? Are they testable? Do they lead to a consistent quantum cosmology? Is this cosmology different than the standard one? The answer is yes, to all these questions, assuming that quantum theory is still valid at this scale. I describe the basic features of such models, mainly quantum causal histories and spin foams. They are given by a partition function, similar to spin systems and lattice gauge theories. This suggests that we should treat this approach to quantum gravity as a problem in statistical physics, but with significant complications: there is no background and, in particular, no external time. Gravity and the familiar 3+1 manifold spacetime are to be derived as the low-energy continuum approximation to these models.