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

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.

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

References in corpus (12)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Three Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis
• Report of the Dark Energy Task Force
• Holographic probabilities in eternal inflation
• The Cosmological Constant and the String Landscape
• Predicting the Cosmological Constant from the Causal Entropic Principle
• de Sitter Space in Non-Critical String Theory
• Eternal Inflation, Bubble Collisions, and the Disintegration of the Persistence of Memory
• Eternal inflation predicts that time will end
• A geometric solution to the coincidence problem, and the size of the landscape as the origin of hierarchy
• The Landscape "avant la lettre"
• Saturating the holographic entropy bound

Cited by in corpus (5)

• Gauge theory extension to include number scaling by boson field: Effects on some aspects of physics and geometry
• Quantum gravity and the cosmological constant: lessons from two-dimensional dilaton gravity
• Classical, quantum, and phenomenological aspects of dark energy models
• The exoplanets analogy to the Multiverse
• Quantum state of fields in $SU(\infty )$ Quantum Gravity ($SU(\infty )$-QGR)