Precision Cosmology and the Landscape

TL;DR

The paper argues that precision cosmology reveals a nonzero cosmological constant consistent with an environmental, landscape view of vacuum energy in string theory. It presents a mechanism based on multiple four-form fluxes that yields a densely spaced discretuum of vacua, compatible with Weinberg's bound and structure formation. It analyzes probabilistic predictions in an eternally inflating multiverse, contrasting global measures with a local causal-diamond approach, and proposes an entropic weighting to derive prior-free predictions for $\Lambda$ and the observable size of the universe. The work highlights the need to rethink spacetime on the largest scales and to develop statistical, thermodynamic principles to extract robust predictions from the string landscape.

Abstract

After reviewing the cosmological constant problem - why is Lambda not huge? - I outline the two basic approaches that had emerged by the late 1980s, and note that each made a clear prediction. Precision cosmological experiments now indicate that the cosmological constant is nonzero. This result strongly favors the environmental approach, in which vacuum energy can vary discretely among widely separated regions in the universe. The need to explain this variation from first principles constitutes an observational constraint on fundamental theory. I review arguments that string theory satisfies this constraint, as it contains a dense discretuum of metastable vacua. The enormous landscape of vacua calls for novel, statistical methods of deriving predictions, and it prompts us to reexamine our description of spacetime on the largest scales. I discuss the effects of cosmological dynamics, and I speculate that weighting vacua by their entropy production may allow for prior-free predictions that do not resort to explicitly anthropic arguments.