Populating the Landscape: A Top Down Approach
S. W. Hawking, Thomas Hertog
TL;DR
Hawking and Hertog introduce a top-down cosmology that combines the string landscape with the no-boundary proposal, computing universe histories from late-time boundary conditions. Using a no-boundary path integral, they identify dominant saddle points (e.g., Hawking–Moss instantons) that yield inflationary, nearly flat universes and enable prediction of primordial spectra from the instanton background. In a simple double-well landscape, only a subset of vacua acquire significant amplitudes, illustrating how a few histories dominate and how observational signatures (e.g., CMB fluctuations) test the framework. The approach provides a principled alternative to eternal inflation, with predictions grounded in the quantum state of the universe and final-boundary constraints, offering concrete avenues for empirical discrimination among cosmological scenarios.
Abstract
We put forward a framework for cosmology that combines the string landscape with no boundary initial conditions. In this framework, amplitudes for alternative histories for the universe are calculated with final boundary conditions only. This leads to a top down approach to cosmology, in which the histories of the universe depend on the precise question asked. We study the observational consequences of no boundary initial conditions on the landscape, and outline a scheme to test the theory. This is illustrated in a simple model landscape that admits several alternative inflationary histories for the universe. Only a few of the possible vacua in the landscape will be populated. We also discuss in what respect the top down approach differs from other approaches to cosmology in the string landscape, like eternal inflation.