Big-Bang is a Boundary Condition

TL;DR

This paper proposes a new formalism in quantum gravity where the big-bang singularity is interpreted as a boundary condition in a finite field space, resolving issues with wave-function interpretation and evolution.

Contribution

It introduces embedding coordinates as dynamical variables, extending general relativity to include a larger gauge symmetry and reinterpreting the big-bang as a boundary condition.

Findings

Wave-functions can be consistently defined with an invariant inner product.

Big-bang appears as a boundary in the finite field space.

The formalism applies to both full theory and minisuperspace approximation.

Abstract

There is a common expectation that the big-bang singularity must be resolved in quantum gravity but it is not clear how this can be achieved. A major obstacle here is the difficulty of interpreting wave-functions in quantum gravity. The standard quantum mechanical framework requires a notion of time evolution and a proper definition of an invariant inner product having a probability interpretation, both of which are seemingly problematic in quantum gravity. We show that these two issues can actually be solved by introducing the embedding coordinates as dynamical variables \`a la Isham and Kuchar. The extended theory is identical to general relativity but has a larger group of gauge symmetries. The Wheeler-DeWitt equations describe the change of the wave-function from one arbitrary spacelike slice to another, however the constraint algebra makes this evolution purely kinematical and furthermore enforces the wave-function to be constrained in the subspace of zero-energy states. An inner product can also be introduced having all the necessary requirements. In this formalism big-bang appears as a finite field space boundary on which certain boundary conditions must be imposed for mathematical consistency. We explicitly illustrate this point both in the full theory and in the minisuperspace approximation.