Boundary Conditions and Predictions of Quantum Cosmology

TL;DR

This paper discusses the boundary conditions and predictive frameworks in quantum cosmology, focusing on the Hartle-Hawking no-boundary proposal and its challenges, including Susskind's critique.

Contribution

It analyzes the boundary condition proposals in quantum cosmology, especially the Hartle-Hawking no-boundary model, and examines their successes and criticisms.

Findings

The no-boundary proposal has partial successes in quantum cosmology.

Zero-loop approximation simplifies the path integral calculations.

Susskind's critique challenges the validity of the no-boundary proposal.

Abstract

A complete model of the universe needs at least three parts: (1) a complete set of physical variables and dynamical laws for them, (2) the correct solution of the dynamical laws, and (3) the connection with conscious experience. In quantum cosmology, item (1) is often called a `theory of everything,' and item (2) is the quantum state of the cosmos. Hartle and Hawking have made the `no-boundary' proposal, that the wavefunction of the universe is given by a path integral over all compact Euclidean 4-dimensional geometries and matter fields that have the 3-dimensional argument of the wavefunction on their one and only boundary. This proposal has had several partial successes, mainly when one takes the zero-loop approximation of summing over a small number of complex extrema of the action. However, it has also been severely challenged by an argument by Susskind.