The no boundary density matrix

TL;DR

This paper explores a no-boundary approach to the universe's density matrix, involving classical solutions with no boundary conditions, and discusses implications for cosmological probabilities and bubble solutions.

Contribution

It introduces a novel no-boundary prescription for subregion density matrices and interprets Coleman de Luccia bubbles within this framework.

Findings

Certain geometries yield phenomenologically unacceptable probabilities.

Some bubble solutions provide acceptable local maxima of probability.

The approach extends the Hartle-Hawking proposal to subregions.

Abstract

We discuss a no-boundary proposal for a subregion of the universe. In the classical approximation, this density matrix involves finding a specific classical solution of the equations of motion with no boundary. Beyond the usual no boundary condition at early times, we also have another no boundary condition in the region we trace out. We can find the prescription by starting from the usual Hartle-Hawking proposal for the wavefunction on a full slice and tracing out the unobserved region in the classical approximation. We discuss some specific subregions and compute the corresponding solutions. These geometries lead to phenomenologically unacceptable probabilities, as expected. We also discuss how the usual Coleman de Luccia bubble solutions can be interpreted as a possible no boundary contribution to the density matrix of the universe. These geometries lead to local (but not global) maxima of the probability that are phenomenologically acceptable.