New Directions in Cosmology and Gravitational Theory: Cosmological Bounces and Bouncing Cosmologies

Our goal is to develop, explore and test theories of the origin, evolution and future of the universe that challenge the standard view that the universe began with a big bang about 14 billion years ago. According to this new paradigm, what has been long believed to be a beginning was actually a transition — known as a ‘bounce’ — from a preceding period of contraction to the current period of expansion. The observed properties of the universe today trace back to events that occurred during the period of contraction and the bounce. In some of the most interesting versions, space-time exists eternally through regular cycles of expansion, contraction and bounce, driven by the same fields that are responsible for dark energy. Significant progress has been made in the last few years in identifying the basic elements of this picture. In this program, Paul Steinhardt and Anna Ijjas will continue their efforts to create new tools for solving fully nonlinear equations of Einstein’s general relativity and beyond, for the purpose of testing the viability of the models and identifying their predictions for forthcoming observations ranging from the Large Hadron Collider to the Simons Observatory. They also plan to investigate the connections to fundamental physics and the implications for gravitational theory in other situations involving space-time singularities.

Paul Steinhardt’s research spans problems in particle and string physics, astrophysics, cosmology, condensed matter physics and geophysics. He is one of the original architects of the inflationary model, an important modification of the standard big-bang picture that explains the large-scale homogeneity, geometry and structure of the universe. He was among the first to show that quantum fluctuations combined with inflation can generate a nearly scale-invariant spectrum of density variations. Then, expanding these ideas, he was the first to demonstrate that inflation can be eternal, the result of which is the multiverse, an effect that obliterates the predictability of inflationary theory. The multiverse and other problems ultimately caused him to turn his attention to bouncing cosmology, which avoids the multiverse and other problems of the big-bang model. In related research, he introduced the concept of ‘quintessence,’ a dynamical form of dark energy that may account for the recently discovered cosmic acceleration and various novel models for dark matter, including self-interacting dark matter (SIDM).

Anna Ijjas’ research lies at the intersection of gravitational theory and cosmology. She is a pioneer in applying mathematical and numerical relativity to cosmology with the goal of developing novel theories that explain the origin, structure and evolution of our universe. Her work has already led to several advances in this new field, including the first example of a smooth nonsingular cosmological bounce as well as the development of novel mathematical and numerical nonperturbative methods for studying the effects of modifications of Einstein theory of gravity on cosmology and black holes. Her interest in bouncing cosmologies was triggered by the Planck2013 release when she was the first to point out that the simplest inflationary models had been eliminated by the then-new data. Before turning to cosmology, Ijjas completed an award-winning Ph.D. in the philosophy of physics, which has influenced her current research.