The Cosmological Constant Problem, Dark Energy, and the Landscape of String Theory

TL;DR

This paper discusses the cosmological constant problem, the nature of dark energy as vacuum energy, and explores how string theory's landscape and multiverse concept can explain the smallness of vacuum energy and its coincidence with matter density.

Contribution

It provides a comprehensive overview of the cosmological constant problem and introduces the string theory landscape as a potential explanation for dark energy's small value.

Findings

Vacuum energy accounts for dark energy causing universe's accelerated expansion.

String theory landscape suggests a multiverse with varied vacuum energies.

Multiverse framework explains the smallness and coincidence of observed vacuum energy.

Abstract

In this colloquium-level account, I describe the cosmological constant problem: why is the energy of empty space at least 60 orders of magnitude smaller than several known contributions to it from the Standard Model of particle physics? I explain why the "dark energy" responsible for the accelerated expansion of the universe is almost certainly vacuum energy. The second half of the paper explores a more speculative subject. The vacuum landscape of string theory leads to a multiverse in which many different three-dimensional vacua coexist, albeit in widely separated regions. This can explain both the smallness of the observed vacuum energy and the coincidence that its magnitude is comparable to the present matter density.