On Anthropic Solutions of the Cosmological Constant Problem

TL;DR

The paper critically examines anthropic explanations for the cosmological constant within M-theory, assessing three explicit model classes inspired by Bousso-Polchinski: a two large dimensions (d=2) BP-like scenario, wrapped branes on Calabi–Yau cycles, and a rationalized irrational axion. It argues that coupling these ideas to the gauge hierarchy imposes strong constraints, often forcing a low vacuum-energy scale and predicting overly small CMB perturbations, thereby limiting the viability of such models. The authors emphasize that, unless the physics is embedded in a large, robust landscape with generic features, anthropic reasoning risks predicting ensemble- rather than unique low-energy physics, which they find philosophically and scientifically troubling. They further explore anthropic vacuum selection in M-theory as a potential way to filter out unacceptable vacua, while remaining skeptical about broad anthropic explanations beyond the cosmological constant.

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.