Research in Classical and Quantum Gravity

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This awards supports research in gravitational physics at the University of California, Santa Barbara. Many of the deepest problems in theoretical physics revolve around combining Einstein's theory of general relativity with quantum theory. The resulting theory is called "quantum gravity" and is needed to better understand the origin of the universe, the nature of space and time on small scales, and what happens inside black holes. The research supported by this grant will use the latest techniques and tools to try to answer some of these fundamental problems. An essential part of the work is the training of graduate students and postdoctoral researchers in the knowledge and techniques that are central to understanding and discovery in gravitational physics. Through a range of forums from public lectures, to informing media reporters, the Principal Investigators will disseminate the directions and results of their research to a broad audience. Society at large will benefit by increasing their understanding of science and the world they live in.The research involves three inter-related programs of research. The first works to understand what sort of spacetimes are needed to describe gravity at the quantum level, taking into account the quantum fluctuations that may range far beyond familiar classical solutions. In particular, the light-cone structure of such spacetimes may be far from classical, and may even be singular. We will investigate the implications of such effects for distant observers, focusing on properties called unitarity and boundary causality. And while it is now understood to be consistent to include fluctuations that cause the spacetime to change topology, the investigators will study whether fluctuations at this level are in fact required by fundamental principles. A second part of the project further investigates the physical consequences of fluctuating topology and the associated processes by which a large universe may interact with a physically-separate closed universe (often called a baby universe). It was shown recently that this at least partially resolves the nearly 50-year-old black hole information problem, but the actual physics experienced by an observer inside an old evaporating black hole remains to be fully understood. The project will also study associated implications for the above-mentioned closed universes, and thus for cosmology and for our universe as a whole. The final part probes the 50-year-old cosmic censorship hypothesis and the structure of singularities in classical gravity. Known violations of certain versions of cosmic censorship with AdS boundary conditions will be analyzed in more detail with an eye toward 1) connections to the so-called weak gravity conjecture of high energy physics and 2) developing possible further counter-examples in asymptotically flat spacetimes. The fact that certain black hole singularities show strong sensitivity to parameters will also be explored. The methods employed in all three programs of research will be primarily analytic, though these will be supplemented with numerical calculations on desktop computers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.