Title: The Measure Problem in Eternally Inflating Universes
Speaker: Alan Guth, Victor F. Weisskopf Professor of Physics, Massachusetts Institute of Technology
Almost all models of inflation lead to eternal inflation. In such models, inflation never stops completely. Instead, the inflating region grows indefinitely, while pieces break off to form what I like to call "pocket universes," within which inflation stops and big bang evolution ensues. We would be living in one of this infinite number of pocket universes. These models have the property that anything that can happen will happen an infinite number of times. Thus, any distinction between the probable and the improbable involves comparing infinities, which has no unique definition. The measure problem involves finding a way to assign definite probabilities under these circumstances.
After describing the problem, I will discuss measures based on global time cutoffs. I will define these measures, and then discuss some thought-experiment tests that can be applied. For example, proper-time cutoff measure seems to be ruled out by the "youngness paradox." I will also discuss Boltzmann brains, which provide another method of possibly ruling out some measure proposals. Finally, I will discuss my understanding of the status of the measure problem.