Fundamental Physics and Cosmology

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This award funds the research activities of Professor Alexander Vilenkin at Tufts University.Professor Vilenkin will study the evolution and possible observational signatures of cosmic strings. Cosmic strings are very thin and highly energetic linear objects. Many theories of particle physics predict that such strings must have formed in the early universe, and if so they can produce a variety of effects which are observable at the present time. In particular, such strings could be responsible for a background of gravitational waves and for cosmic rays at the highest energies, which are hard to explain through conventional astrophysical processes. A detection of cosmic strings would be of great importance, as it would open a unique window into the physics of very high energies, inaccessible in particle accelerators. Another direction of Professor Vilenkin's research is the study of bubble formation in a high energy phase of the universe. Bubbles such as these are regions of space in which the laws of physics are in a different "phase", just as water and ice are the same material in different phases. A small bubble of space with a lower energy can be created spontaneously and then rapidly expand. Bubble formation plays a central role in a number of cosmological models, including the theory of inflation (under which the universe is believed to have expanded very rapidly at initial times), but some aspects of this process are still poorly understood. Professor Vilenkin will develop new methods for calculating the probability of bubble creation. This research is in the national interest in that it furthers the development of basic science in the United States. Professor Vilenkin will also continue his efforts to communicate these ideas to the general public.At a more technical level, Professor Vilenkin will study the interaction of cosmic strings with black holes. An oscillating loop of string can be captured by a black hole. The loop can then grow in size by extracting the black hole's rotational energy. It may also self-intersect and split, ejecting a smaller loop. A large number of string loops may be produced as this cycle continues. The probability of loop capture significantly depends on how loops are distributed in space, in particular on the extent to which they are clustered in galaxies. Professor Vilenkin will use numerical simulations to study the gravitational clustering of strings and the interaction of string loops with rotating black holes. The results will be used to derive predictions for observational signatures of strings, especially for the gravitational wave background. The bubble nucleation probability is usually calculated using the standard instanton approach, but this method fails in many relevant situations. Professor Vilenkin will use the Wigner distribution and numerical simulations to study the alternative stochastic method and to determine its limits of applicability.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.