Gravitational Radiation from Black Holes

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

This award supports studies of the revolutionary prediction of Einstein's general theory of relativity that accelerated bodies produce gravitational waves, analogous to the way electric charges produce electromagnetic waves. Waves, such as water, sound, elastic and electromagnetic waves, are a prominent feature of physical systems. Previous work on this project involved international experts spanning applied mathematics, computation, gravitational theory and astrophysics and led to new methods which have found application to a broad class of problems, e.g. seismology. In a historic scientific event leading to a Nobel prize award, gravitational waves were recently detected by a Laser Interferometric Gravity Wave Observatory (LIGO). This initial detection was a resounding confirmation of Einstein's theory. In collaboration with the European Virgo gravitational wave observatory, it opens a new form of astronomy which will expand our knowledge of the universe. The powerful gravitational waves in the initial detection resulted from the inspiral and merger of two co-orbiting black holes. The excitement generated by this dramatic event has led to overflow audiences at public lectures and has inspired young talent to enter the field. Although Einstein's equations describe the production of gravitational waves, their complexity and nonlinearity make a purely analytic approach inadequate. This necessitated the introduction of computational methods into general relativity. A main goal of the project is to develop new methods for computing the gravitational waveform and other physical properties of binary black hole inspirals, such as the energy-momentum and angular momentum loss. The approach utilizes characteristic evolution based upon the light cones on which the radiation propagates. Most computational work is based upon time evolution using the Cauchy initial value problem. The reformulation of the initial value problem for general relativity in terms of light cones by Bondi and by Penrose resulted in the first clear understanding of gravitational radiation. Characteristic evolution has led to the first code which could successfully locate, track, evolve and compute the waveform for a single distorted, spinning and moving black hole. However, the focusing of the light cones in a binary problem leads to interior caustics which prevent a purely characteristic approach. On the other hand, present Cauchy codes require a finite artificial outer boundary which introduces ambiguity and error in extracting the waveform. The global strategy pursued here combines Cauchy evolution in the interior region containing the black holes with characteristic evolution in the exterior region extending along the light cones to infinity. This integrates the complementary strengths of Cauchy and characteristic evolution and has led to a characteristic wave extraction tool which is available to the numerical relativity community as part of the Einstein Toolkit. 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.