Research at the IGPG:

In the fall of 1993, Penn State established a Center for Gravitational Physics and Geometry (CGPG) to enhance inter-disciplinary research in Astronomy and Astrophysics, Mathematics, and Physics. In the fall of 2001, based on the proposal submitted by the core CGPG faculty, the National Science Foundation established a Physics Frontier Center for Gravitational Physics (CGWP) at Penn State. In 2003, the CGPG celebrated its tenth anniversary through an international conference entitled Gravitation: A Decennial Perspective. It also underwent an external review. The conference and the report of the External Committee brought out the growth in size and stature that the Center had undergone during the decade. In recognition of this growth the Center was restructured and made Institute for Gravitational Physics and Geometry in the spring of 2004.

The Whitmore faculty (supported by the gravitational physics program of NSF) consists of Abhay Ashtekar, Martin Bojowald, Sam Finn, Pablo Laguna, Ben Owen, and Deirdre Shoemaker. Their main research interests are:

Quantum Gravity:
  The Institute has played a seminal role in the development of a fully non-perturbative approach to quantum gravity. In this approach, there is no background space-time; matter fields as well as geometry are `born quantum mechanically'. In the mid-nineties, a specific, detailed theory of quantum geometry was developed with careful attention to mathematical issues such as functional integrals, measures and operators. The theory is manifestly finite. Just as differential geometry provides the mathematical language for formulating classical gravitational theories, so does quantum geometry for quantum gravitational theories. The emphasis then shifted to applications. The theory was successfully used to calculate black hole entropy from first principles and has led to a resolution of the big-bang singularity. In both cases, the fundamental discreteness predicted by quantum geometry plays the key role. The current emphasis is on extracting further physical predictions. For example: What is the description of the black hole evaporation process in full quantum gravity? Does the `fundamental' theory reproduce perturbative quantum field theory in the low energy regime? Can one pin-point why and where the perturbation theory fails in quantum gravity? What are the predictions of loop quantum cosmology to the physics of the early universe? Spin-foam models provide a path integral approach to quantum gravity. What is their precise relation to canonical quantum gravity? What, in detail, is the physics of these models?

Classical General Relativity:
  From its inception, the Institute has had a strong effort in this area, especially the study of the asymptotic structure of space-time, gravitational radiation theory, applications of symplectic geometry to gravitational physics, and the development of the `close-limit' approximation to study black hole collisions. Over the past five years, the emphasis has been on topics that have ramifications to numerical relativity and gravitational wave phenomenology. These include the development of the isolated horizon framework to extract physics from numerical simulations of strong field geometry near black holes, the hyperbolic formulations of Einstein's equations, and the study of gauge conditions suitable for numerical relativity. Focus of the current effort is on deepening the analytic understanding of dynamical black holes. Specifically, properties of dynamical horizons are being studied using geometric analysis and causal structure methods and results are being applied to numerical relativity and quantum gravity. On another front, the radiation reaction problem for black hole binaries is being analyzed in conjunction with numerical methods.

Numerical Relativity: 
  Since its creation, the Institute has housed a strong effort in numerical relativity. The main driving force of the numerical relativity research at the Institute has been the simulation of the in-spiral and coalescence of binary black hole systems. The solution to the black hole collision problem requires input not only from numerical analysis and computer science, but also from astrophysics, geometry, mathematics of partial differential equations, to name a few. Because of its multi-disciplinary nature, the Institute is able to provide in-house expertise in most of these areas. In addition, the Institute participates extensively in numerous collaborative efforts. The Institute effort has not been limited to the numerical implementation of a given formulation of the Einstein equations. Researchers at the Institute are also engaged in looking at mathematical issues such as the consistency of the conditions applied in the outer boundary of the computational domain as well as the hyperbolic and well-posed nature of the Einstein evolution equations. Codes developed at the Institute have pioneered the "excision technique" to deal with the black hole singularity and have evolve a single black hole singularity moving throughout the computational domain, major milestones in reaching the goal of binary evolutions. Complementary to the evolution effort, we at the Institute are involved in the construction of binary black hole initial data sets, including data sets in the close-limit approximation. A clear example of the synergistic atmosphere at the Institute is the collaboration between analytical and numerical relativists in applying the isolated horizon machinery to numerically generated space-times. This work has the potential to become an extremely valuable tool in numerical relativity.

Gravitational Wave Physics:
  Research in gravitational wave physics and astrophysics focuses on the challenges and rewards of gravitational wave observations. The challenges are experimental, phenomenological and theoretical; the rewards are the prospect of new tests of fundamental physics and a fundamentally new way of looking at the Universe. As the new detectors come on-line the central question of how we use the observations they make to learn about astrophysics or to test the fundamental physics of gravity come to the fore: Can we definitively show the existence of black holes? Can we bound the crustal strength of rapidly rotating neutron stars? Can we distinguish between gamma-ray bursts triggered by hypernovae collapse and those triggered by binary coalescence? How do we understand unanticipated gravitational wave bursts, signaling perhaps new sources and new astronomy? As we develop new detectors, the questions of how we design those new detectors to optimize the concrete science they can do come to the fore: what are the properties of a detector well-suited for searching for a stochastic gravitational wave background? Inspiraling black hole binary systems? Finally, the questions of interpretation and source prediction, which arise in the context of gravitational wave detection, gives a special focus to, e.g., analytical work on the radiation reaction problem in relativity or numerical relativity studies of single and binary black hole systems.