Zenith Grant Awardee
Raphael Bousso
University of California, Berkeley
Co-Investigators
Project Title
The Future of the Multiverse
Project Summary
The arrow of time is the question of why the universe started out so well ordered that even today, it remains far from the state of maximum disorder that it strives towards. A popular but somewhat ad-hoc strategy is to look for a law that requires the universe to begin orderly. In eternal inflation, this cannot work. Instead, the emergence of an arrow of time depends in on the lifetime of the vacua populating the landscape of string theory–a fascinating connection that I will explore. Eternal inflation leads to infinities that must be tamed to make predictions. Leading approaches to this problem tell us to count everything that happens until a particular time. This "cutoff" was regarded as a mere computational trick, but recent work indicates that it comes with heavy baggage. Some ob jects may last to the end of time, when they are disrupted for no reason other than the cutoff. We must understand this bizarre conclusion in a more physical way, by identifying a real mechanism that brings about the end of time and justifies the cutoff. The only alternative is to find an entirely new way of thinking about eternal inflation.
Technical Abstract
This project will focus on two issues. Eternal inflation transforms an old problem–the origin of the arrow of time–and introduces a new problem, the end of time.
In eternal inflation, special initial conditions are erased by cosmological attractor behavior. Therefore, the arrow of time can only emerge dynamically. Recent work indicates that it will indeed emerge, if all metastable de Sitter vacua decay sufficiently fast. I intend to demonstrate this connection rigorously. This would reduce the arrow of time problem to a nontrivial constraint on the landscape of string theory, which can in principle be checked.
Time cutoffs on eternal inflation are widely used to regulate divergences and compute probabilities. Recent work has shown that such cutoffs are necessarily physical, since they are encountered by a finite fraction of matter systems. My research will investigate the implications of this result: Either geometric cutoffs are not the right way to compute probabilities, in which case we will need to develop entirely new tools for the study of eternal inflation. Or the end of time is a real possibility, which challenges us to identify a physical mechanism underlying the cutoff.
QSpace Latest
PressRelease: Precision experiment puts pressure on quantum collapse theories
Quantum mechanics, the theory governing the microscopic world, is famously counterintuitive. A particle can exist in a superposition of multiple states, such as different positions, until a measurement is performed. At that point, the wavefunction describing that particle appears to ‘collapse’ to a single outcome. This puzzle lies at the heart of the measurement problem, famously illustrated by Schrödinger’s cat, suspended between life and death until observed. The XENONnT detector, which was designed to be sensitive to rare physics events, has tightened constraints on one family of possible solutions to the measurement problem, known as ‘collapse theories.’ The work, which was partially funded by FQxI, was reported in Physical Review Letters in March 2026. Image credit: XENON Collaboration.