Numerical simulations of binary mergers are a core activity, as they are the keystone to understanding LIGO data on black hole collisions, and multi-messenger data on mergers involving neutron stars. Refining existing numerical codes to include more realistic microphysics is part of a broader effort with researchers in Astrophysics to combine strong-field gravity with all the intricacies of the application of physics in astronomical environments. Confronting gravitational wave data with theory, in particular, to discover if there is novel physics beyond classical Einstein gravity, will require overcoming many mathematical challenges presently posed by candidate modified or alternative theories to general relativity. At the same time, new data analysis strategies are needed to search for rare or unusual sources of gravitational waves, so that the flood of data we hope to come from future LIGO observing runs can fully be mined.

Even as we use Einsteinâ€™s theory of general relativity to understand strong field gravitational phenomena, we are convinced that it cannot be a final and complete description of gravity, first, because it conflicts with quantum mechanics, and second because general relativity predicts the existence of singularities, such as at the center of black holes or in the distant past of cosmological histories, where geometry itself may break down. Formulating and understanding quantum gravity is a long-term goal that the PGI will advance. Efforts in this direction are linked to other goals: for example, understanding possible discrete geometries at the Planck scale should ultimately synergize with the spacetime discretization strategies that are at the core of numerical simulations of black hole mergers.

The accelerating expansion of the universe poses another puzzle because by itself gravity causes expansion to decelerate. We are thus led to ask: What modifies Einsteinâ€™s theory of gravity at the largest scales we see today? The problem is again one of strong-field gravity in the sense that the spacetime geometry of the universe is far from flat space at the Hubble scale, even exhibiting some properties akin to black hole horizons. We must ask whether this geometry is stable and whether the physics driving the current epoch of accelerated expansion might ultimately be related to the way initial conditions are fixed in the very early universe.

A unifying theme for our work is the physics of geometries far from the flat space of our everyday experience. Highly curved spacetime is deeply entwined in the physics of black hole singularities, quantum gravity, the early universe, and the production of gravitational waves from black hole mergers. From abstract theory to empirical modeling, the dynamics of curved spacetimes are a touchstone for the physics we aim to explore in the PGI.

Led by participating faculty Simone Giombi, Igor Klebanov, Frans Pretorius, Paul Steinhardt, and Herman Verlinde, our work on gravity recognizes and will build upon the rich context of other advances in physics, astronomy, and mathematics, in the expectation of forging new links among fields. For example, collisions and phase structure of black holes in negatively curved spacetimes are useful and widely studied as probes of non-equilibrium phenomena in strongly coupled field theories, with relevance to systems as diverse as nuclear collisions and superconductors. Nuclear collisions in turn help elucidate the nuclear equation of state, which controls the structure of neutron stars as well as the phase transition of quarks to protons and neutrons in the early universe.