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

Abstract: In this colloquium-level account, I describe the cosmological constant prob-lem: 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 ex-plain 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 spec-ulative 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 tha