New data from cosmology have determined many parameters of our uni- verse to unprecedented accuracy. Among the many significant results is the determination that much of the energy in the universe is dark and of a form consistent with a positive cosmological constant. Understanding such a result appears to require a theory of quantum gravity. The best candidate for such a theory, string theory, contains a mechanism for producing dark energy and understanding its approximate value, but it is crucial that this model be falsifiable and predictive. The PI proposes to undertake a broad program of research in high energy theory, focusing on the aspects of string theory most relevant to cosmology, with the ultimate goal of uncovering stringy effects which can be observed. He will continue his work studying the effects of high-scale physics and string theory on large-scale cosmological phenomena such as the Cosmic Microwave Background and the curvature of the universe. He will research the physics of black hole and cosmological horizons and singularities using string theory dualities and other methods. As a final component of the program he will continue and extend his recent work in particle physics in anticipation of the arrival of new data. The Broader Impact of the proposal will be that the PI will develop a pilot program at NYU to help promote the success of undergraduates belonging to groups traditionally underrepresented in physics and the physical sciences. The program will attempt to overcome the isolation such students often experience by facilitating the formation of a peer group and by providing additional support where necessary. In addition, the PI will lecture at the Physics Club of New York, a public forum consisting of area high school teachers and members of the public, and will participate in school tours and interactions with two particular local high schools.
This project supported the PI, Matthew Kleban, together with several postdoctoral researchers and graduate students, in pursuing a program of research at the interface between high-energy particle physics, string theory, cosmology, and quantum gravity. One of the biggest challenges facing physicists studying string theory and quantum gravity is the difficulty in finding ways to test competing theories. Observational cosmology provides a unique opportunity, because it is a window into the early universe when both quantum effects and gravity were very strong. The primary goal of this research program was to produce testable predictions for cosmology from string theory. String theory predicts that exponentially rapid expansion -- cosmic inflation -- is taking place all around us at a very rapid rate, and that our observable universe fits in a bubble embedded in this eternally expanding whole. Further, our bubble nucleated during a first-order phase transition (the sudden, spontaneous appearance of a region of a new phase) from this parent state. If this prediction is correct, collisions with other bubble universes will occur. During the course of this project remarkable progress was made in understanding and quantifying the effects of these collisions. The impact produces "cosmic wakes" that propagate across our universe, create specific patterns in the cosmic microwave background, and disturb the large scale structure of matter. With a combination of analytic and numerical work we made quantitative predictions for the effects on both the temperature and polarization of the cosmic microwave background radiation. A number of groups of astrphysicists are actively engaged in searching data for these traces. In addition, our research uncovered a surprising and unexpected result -- that an observation of non-zero spatial curvature in our universe can conclusively falsify the existence of such an eternally inflating multiverse. Due to the difficulty in finding tests of string theory and the multiverse, some have criticized it as impossible to falsify and hence unscientific. Instead, our results show that a future measurement of positive spatial curvature -- even at a level well below the current observational limit -- would with very high confidence rule out the possibility that our bubble nucleated from an eternally inflating region, and hence falsify the multiverse. Finally, in the course of this research we discovered an entirely new class of models for inflation. Observational evidence strongly supports a finite (not necessarily eternal) period of inflation in the early universe, but the mechanism that produced this exponential expansion is unknown. Furthermore, most of the models proposed to explain it suffer from the difficulty that they are fine-tuned or in some other way difficult to trigger. By contrast, the new type of inflation -- termed unwinding inflation -- has the advantage that it begins spontaneously as a a result of precisely the same phase transition that produced our bubble. It also makes several characteristic predictions for the detailed spectrum of fluctuations in the temperature of the Cosmic Microwave Background radiation. These predictions too are under active investigation by teams of astrophysicists. If they are confirmed, this would provide evidence not just for string theory, but for the existence of extra, compact dimensions of space. The unambiguous detection of a cosmic bubble collision or the signatures of unwinding inflation would be a transformative discovery, revolutionizing our understanding of the universe and humanity's place within it. It would demonstrate that our observable universe is a tiny part of a vastly larger multiverse populated by bubbles containing regions of highly exotic physics. It would confirm a prediction of string theory, provide crucial evidence for the nature of dark energy, and permanently alter our view of the big bang. While it may be that no such signals are present, or are too faint to be observed, our work has made such a discovery possible.
