Dr. Alexander Vilenkin
Tufts University

Project Title:
The Measure Problem in the Inflationary Multiverse

Co-Investigators:
Jaume Garriga, Universitat de Barcelona

Summary:
Recent developments in cosmology suggest that much of the universe is in a state of explosive, accelerated expansion, called inflation. We live in a finite "bubble" where inflation has ended, and other bubbles with diverse properties are constantly being formed. All possible events will happen an infinite number of times in such an eternally inflating "multiverse." Unless we learn how to compare these infinities, we will not be able to make any predictions at all. This is known as "the measure problem''. Some of the proposed probability measures predict that "Boltzmann brains'' - hallucinating disembodied brains appearing as quantum fluctuations - greatly outnumber regular observers. We shall investigate how this and other paradoxes can be used to restrict the set of possible measures. We shall also explore how the probability measure can be derived from the fundamental theory, such as quantum gravity. Another direction of the proposed research is the study of bubble collisions. The most spectacular observational test of the multiverse scenario would be a direct observation of a collision of our bubble with another one. We shall investigate observational signatures of such a collision and the probability for us to be close enough to detect one.



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