TASI Lectures on the Cosmological Constant

TL;DR

The paper surveys the cosmological constant problem, arguing that quantum vacuum energy and gravity collectively resist resolution by straightforward modifications. It evaluates a wide range of ideas, identifying robust obstructions, and highlights the shift from seeking a vanishing $\Lambda$ to embracing a variable vacuum energy within a vast string-theory landscape. A central focus is the Brown-Teitelboim mechanism extended to a discretuum of four-form fluxes, yielding a dense spectrum of metastable vacua and a natural route to small $\Lambda$ compatible with structure formation. The discussion then outlines how string theory's membrane/flux structure and moduli stabilization underpin a statistical, anthropic picture of the observed vacuum energy, with measurable predictions framed by landscape statistics and measure choices such as the causal diamond.

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.