Fundamental Physics and Cosmology

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This award funds the research activities of Professor Vilenkin at Tufts University.The theory of cosmic inflation, which was proposed as a speculative hypothesis 35 years ago, has by now accumulated substantial observational support and has become the leading cosmological paradigm. This theory has also led to a major change in our global view of the universe. According to the new worldview, much of the volume in the universe is in the state of explosive inflationary expansion. We live in a "bubble" where inflation has ended, but it will never end in the entire universe. The total volume of inflating regions continues to grow, and other bubbles with diverse properties are constantly being formed. The major goal of the proposed research is to learn how this "multiverse" scenario can be tested observationally and to investigate its implications for the beginning of the universe. The multiverse worldview has far-reaching implications beyond physics. It provides a dramatically new perspective on the global structure of the universe and on our place in it. Research in this area thus advances the national interest by promoting the progress of science in one of its most fundamental directions: the understanding of the properties of the universe. Professor Vilenkin will also involve graduate students and postdocs in his research, and thereby provide critical training for junior physicists beginning research in this field. He will continue his efforts to communicate the ideas of modern cosmology to the general public, in the form of public lectures, popular articles, and interviews for radio and TV. He will also write a textbook, emphasizing the new worldview, based on his undergraduate cosmology course, which is aimed at students with no preparation in science.More technically, Professor Vilenkin will investigate the possibility of a direct test of the multiverse scenario by observing collisions of our bubble with other bubbles, or with topological defects that could nucleate in the inflating background. Possible observational effects of such collisions will be studied using both analytic and numerical methods. A complementary approach is to analyze the statistical properties of the multiverse and use them to derive predictions for the observed values of the constants of Nature in our bubble. This approach will be pursued, with the main focus on the statistics of different states in the multiverse and of the transition rates between them. The general problem of making predictions in the multiverse, known as the measure problem, will also be addressed.