"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
Inflationary cosmology, combined with recent developments in string theory, is pointing to a major paradigm shift: from a nearly homogeneous and isotropic universe to an extremely inho-mogeneous "multiverse", where much of the volume is still in the state of explosive inflationary expansion. We live in a finite "bubble" where inflation has ended, and other bubbles with diverse properties are constantly being formed. A major goal of the proposed research is to learn how this multiverse scenario can be tested observationally. All possible events will happen an infinite number of times in an eternally infllating 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". To determine the probability measure of the multiverse, different measure prescriptions will be studied to check for internal inconsistencies and for conflict with the data. Some measure candidates have already been ruled out in this way, and the hope is that there aren't that many viable prescriptions satisfying some minimal set of requirements. Another idea to be pursued has recently been suggested by Garriga and Vilenkin. They conjecture that the dynamics of the inflationary multiverse is holographically encoded at the future infinity, where it is described by a lower-dimensional Euclidean field theory. The measure is then defined by imposing an ultraviolet cutoff in that theory. This is an attractive approach, as it opens the possibility of deriving the measure from first principles. The most spectacular observational test of the multiverse scenario would be a direct observation of a collision of our bubble with another one. Observational effects of such a collision and the probability for us to be close enough to detect one will be investigated.
Another major topic of the proposed research is the study of the evolution and observational signatures of topological defects, which can be copiously produced at phase transitions in the early universe. The main focus here will be on cosmic string networks and on "necklaces", consisting of monopoles connected by strings, which are the most promising defects from the observational point of view. Numerical simulations and analytic modeling will be used to study the evolution of string networks, and the results will be used to make accurate predictions for gravitational waves, high-energy particle fluxes, and other observable effects. Detection of topological defects would open a new window into the early history of the universe and into particle physics at extremely high energies. The broader impacts are that the proposed research will be done in collaboration with graduate students and postdocs. The P.I. will continue his efforts to communicate the results to the general public. This especially applies to the multiverse worldview, which has far-reaching implications beyond physics. The PI has already written a book for lay-persons called "Many Worlds in One" and His work has been featured in numerous newspaper and magazine articles in US, Europe, Russia, and Japan, and in many popular books.
