Initial Conditions for Bubble Universes

TL;DR

This paper proposes initial conditions for bubble universes in string cosmology that prevent singularities and ensure stability, using Casimir energy effects to set low-entropy starting points without violating thermodynamics or causing instabilities.

Contribution

It introduces a simple, non-anthropic initial condition proposal for bubble universes that guarantees non-perturbative stability and addresses the initial entropy problem.

Findings

Proposes initial conditions ensuring stability of bubble universes.

Shows Casimir energy can set low-entropy initial states.

Ensures compatibility with string theory stability constraints.

Abstract

The "bubble universes" of Coleman and De Luccia play a crucial role in string cosmology. Since our own Universe is supposed to be of this kind, bubble cosmology should supply definite answers to the long-standing questions regarding cosmological initial conditions. In particular, it must explain how an initial singularity is avoided, and also how the initial conditions for Inflation were established. We argue that the simplest non-anthropic approach to these problems involves a requirement that the spatial sections defined by distinguished bubble observers should not be allowed to have arbitrarily small volumes. Casimir energy is a popular candidate for a quantum effect which can ensure this, but [because it violates energy conditions] there is a danger that it could lead to non-perturbative instabilities in string theory. We make a simple proposal for the initial conditions of a bubble universe, and show that our proposal ensures that the system is non-perturbatively stable. Thus, low-entropy conditions can be established at the beginning of a bubble universe without violating the Second Law of thermodynamics and without leading to instability in string theory. These conditions are inherited from the ambient spacetime.