Lectures on the Cosmological Constant Problem

TL;DR

This paper reviews the cosmological constant problem, emphasizing its global nature, and discusses various proposals and modifications to gravity aimed at resolving the issue, including sequestering scenarios.

Contribution

It provides a comprehensive review of the cosmological constant problem, detailing its challenges and evaluating multiple theoretical solutions and modifications to gravity.

Findings

The cosmological constant problem is fundamentally global and persistent.

Sequestering effectively cancels vacuum energy contributions.

Global modifications to gravity are promising solutions.

Abstract

These lectures on the cosmological constant problem were prepared for the X Mexican School on Gravitation and Mathematical Physics. The problem itself is explained in detail, emphasising the importance of radiative instability and the need to repeatedly fine tune as we change our effective description. Weinberg's no go theorem is worked through in detail. I review a number of proposals including Linde's universe multiplication, Coleman's wormholes, the fat graviton, and SLED, to name a few. Large distance modifications of gravity are also discussed, with causality considerations pointing towards a global modification as being the most sensible option. The global nature of the cosmological constant problem is also emphasized, and as a result, the sequestering scenario is reviewed in some detail, demonstrating the cancellation of the Standard Model vacuum energy through a global modification of General Relativity.