Cosmological constant problems and their solutions

TL;DR

This paper reviews the cosmological constant problems and discusses how anthropic selection, involving scalar or four-form fields, offers potential solutions, highlighting recent particle physics model proposals.

Contribution

It provides a comprehensive review of anthropic solutions to the cosmological constant problems and explores recent models that realize the necessary scalar or four-form field features.

Findings

Anthropic selection can address both cosmological constant problems.

Recent models propose mechanisms for extremely flat potentials or small charges.

Scalar and four-form field models are viable in particle physics contexts.

Abstract

There are now two cosmological constant problems: (i) why the vacuum energy is so small and (ii) why it comes to dominate at about the epoch of galaxy formation. Anthropic selection appears to be the only approach that can naturally resolve both problems. The challenge presented by this approach is that it requires scalar fields with extremely flat potentials or four-form fields coupled to branes with an extremely small charge. Some recent suggestions are reviewed on how such features can arise in particle physics models.