The Cosmological Constant and the String Landscape
Joseph Polchinski
TL;DR
Polchinski surveys the cosmological constant problem and articulates a framework in which a vast landscape of string vacua could environmental-select a small, nonzero $\rho_V$. He contrasts fixed-$\Lambda$ and adjustable-$\Lambda$ approaches, arguing that string theory—via constructions like KKLT—provides a dense discretuum of metastable vacua near $\rho_V=0$, potentially addressing the observed vacuum energy through cosmological dynamics and anthropic selection. The discussion extends to phenomenology, outlining how the landscape impacts $\theta_{\rm QCD}$, baryon lifetime, the dark-energy equation of state $w$, generational structure, and the amplitude $Q$ of primordial fluctuations, while remaining cognizant of unresolved issues around nonperturbative de Sitter realizations and measure problems in eternal inflation. While the landscape offers a compelling, testable direction for environmental explanations, substantial theoretical hurdles—most notably a nonperturbative definition of string theory in de Sitter space and a robust measure—must be overcome to solidify predictive power. Overall, the work argues for a paradigm shift: the smallness of the cosmological constant may reflect ambient selection across a multiverse of vacua, rather than a unique, dynamical calculation within a single theory.
Abstract
Theories of the cosmological constant fall into two classes, those in which the vacuum energy is fixed by the fundamental theory and those in which it is adjustable in some way. For each class we discuss key challenges. The string theory landscape is an example of an adjustment mechanism. We discuss the status of this idea, and future directions.