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

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Title:		The Cosmological Constant Problem, Dark Energy, and the Landscape of String Theory
Authors:		Bousso, Raphael
Publication:		eprint arXiv:1203.0307
Publication Date:		03/2012
Origin:		ARXIV
Keywords:		Astrophysics - Cosmology and Extragalactic Astrophysics, General Relativity and Quantum Cosmology, High Energy Physics - Phenomenology, High Energy Physics - Theory
Comment:		22 pages, 4 figures. To appear in the proceedings of "Subnuclear Physics: Past, Present and Future", Pontificial Academy of Sciences, Vatican (October 2011). v2: minor editing, typo in eq. (3.3) fixed
Bibliographic Code:		2012arXiv1203.0307B

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.

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