On Anthropic Solutions of the Cosmological Constant Problem

TL;DR

This paper explores anthropic solutions to the cosmological constant problem, analyzing models with constraints from the gauge hierarchy problem, and discusses their implications and limitations.

Contribution

It introduces three classes of models satisfying anthropic constraints on the cosmological constant and examines their challenges and implications.

Findings

Models predict constants are chosen from an ensemble, not uniquely determined.

Most models face issues explaining microwave background fluctuations.

Anthropic reasoning may offer a solution to string theory's vacuum selection problem.

Abstract

Motivated by recent work of Bousso and Polchinski (BP), we study theories which explain the small value of the cosmological constant using the anthropic principle. We argue that simultaneous solution of the gauge hierarchy problem is a strong constraint on any such theory. We exhibit three classes of models which satisfy these constraints. The first is a version of the BP model with precisely two large dimensions. The second involves 6-branes and antibranes wrapped on supersymmetric 3-cycles of Calabi-Yau manifolds, and the third is a version of the irrational axion model. All of them have possible problems in explaining the size of microwave background fluctuations. We also find that most models of this type predict that all constants in the low energy Lagrangian, as well as the gauge groups and representation content, are chosen from an ensemble and cannot be uniquely determined from the fundamental theory. In our opinion, this significantly reduces the appeal of this kind of solution of the cosmological constant problem. On the other hand, we argue that the vacuum selection problem of string theory might plausibly have an anthropic, cosmological solution.