This award funds the research activities of Professor Sergey Dubovskiy at New York University.
The accelerated expansion of the Universe, first observed in 1998, is one of the most surprising discoveries in fundamental physics over the past several decades. Currently, the two most successful theoretical explanations of this observation are either that the laws of gravity change at cosmological distances, or that our Universe is randomly selected from a huge number of universes with different vacuum energies and laws of physics. Definite evidence favoring either of these two proposals will not only have crucial impact on our current picture of the Universe, but may even affect our understanding of the kinds of physics questions which may be amenable to traditional investigations. In his research, Professor Dubovskiy aims to develop methods to test these two alternative scenarios observationally and to develop new ideas capable of explaining the accelerated expansion of the Universe. One critical component of his work is to study the String Axiverse scenario --- a recent proposal by Professor Dubovskiy and his collaborators which describes how the existence of alternative universes may acquire observational support if a plethora of ultra-light particles ("axions") are discovered in our own Universe. An especially intriguing possibility being investigated by Professor Dubovskiy is to use the ongoing observations of astrophysical black holes as a tool to discover the axions predicted in the axiverse scenario.
This project is also envisioned to have significant broader impacts. Professor Dubovskiy will involve graduate students and postdocs in his research, and thereby provide critical training to junior physicists beginning research in the fundamental physics. He also intends to give public lectures on his research results, including those aimed at high-school students, and develop new course curricula based on the most recent theoretical and observational results in particle physics and cosmology.
All of non-gravitational physics around us can be described within a framework of relativistic quantum field theory with an unprecedented accuracy. However, combining this description with gravity remains a major challenge. One of the characteristic properties of relativistic quantum field theories is that they become scale-free at short distances. On the other hand, gravitational theories are expected to keep memory of the fundamental length scale (the Planck scale) at arbitrarily high energies. This length scale is expected to represent the shortest distance scale below which the notion of smooth space-time looses its meaning and no local description of physics is possible. However, naive attempts to achieve this, for example, by introducing a fundamental discrete structure (space-time lattice) face tremendous theoretical and observational problems because it is extremely hard to combine such a discrete structure with Lorentz symmetry, which is tested observationally with a great precision. As a major outcome of this project a class of exactly solvable Lorentz invariant models was constructed, which exhibit a new type of asymptotic behavior at high energies and never loose memory of the fundamental distance scale.This new behavior was called asymptotic fragility. In spite of their simplicity, existing asymptotically fragile models exhibit surprisingly many properties expected from a more mature gravitational theories. In particular, they do not allow for sharply defined local observables. This is achieved without introducing any discrete structures. Instead, these theories simply do not allow to predict sharply defined local observables, in spite of possessing scattering amplitudes defined at all energies. The simplest example of such a model is realized by excitations living on the worldsheet of a fundamental string. As one of the applications of these ideas it was demonstrated that asymptotically fragile models challenge the conventional notion of naturalness. Asymptotically fragile theories suggest the existence of a third possibility, beyond conventional natural theories and anthropic landscape, for addressing the electroweak hierarchy and cosmological constant problems. A major underlying theme for much of the recent progress in theoretical physics is a surprising intimate relation between gravitational physics and gauge theories, similar to quantum chromodynamics, which is responsible for the confinement of quarks inside nucleons. This proved to be true in the course of this project as well. It turns out that ideas and tools developed in the study of asymptotically fragile theories can be applied to describe the properties of confining strings (fluxtubes) holding quarks together. This lead to a major progress in theoretical understanding of the existing high quality lattice data, studying the properties of confining flux tubes. In particular, this lead to a discovery of a new unusal particle---a first massive excitation on the worldsheet of confining strings. Unlike conventional particles, this one is not free to propagate in all three spatial dimensions, but instead is confined on a flux tube. Rather unexpectedly, it corresponds to the twist-like excitation of a flux tube.
