TASI Lectures on the Cosmological Constant

TL;DR

This paper reviews the cosmological constant problem, discussing observational constraints, the interpretation of dark energy as vacuum energy, and the potential for a variable Lambda within string theory's landscape of vacua.

Contribution

It provides a comprehensive overview of the cosmological constant problem, including theoretical approaches and the role of string theory in potential solutions.

Findings

Lambda is constrained by observational data

Dark energy is consistent with vacuum energy interpretation

String theory's landscape offers a framework for variable Lambda

Abstract

The energy density of the vacuum, Lambda, is at least 60 orders of magnitude smaller than several known contributions to it. Approaches to this problem are tightly constrained by data ranging from elementary observations to precision experiments. Absent overwhelming evidence to the contrary, dark energy can only be interpreted as vacuum energy, so the venerable assumption that Lambda=0 conflicts with observation. The possibility remains that Lambda is fundamentally variable, though constant over large spacetime regions. This can explain the observed value, but only in a theory satisfying a number of restrictive kinematic and dynamical conditions. String theory offers a concrete realization through its landscape of metastable vacua.