Black-Hole Spin Precession and its Astrophysical Implications

University Of Texas At Dallas, Richardson TX

Investigators

Abstract

Gravity is one of the four fundamental forces of nature and is described by Einstein's theory of general relativity. One of the most extreme predictions of general relativity is the existence of black holes, compact objects whose gravity is so intense that not even light can escape from them. Investigating gravity in such an extreme regime provides one of the most promising opportunities to learn more about this fundamental force. Another prediction of general relativity, that binary black holes emit gravitational waves that stretch and squeeze spacetime, was recently confirmed by the NSF-funded Laser Interferometer Gravitational-wave Observatory (LIGO). Black holes are fully described by their masses and spins, but the spins of binary black holes are generally misaligned with their orbital plane, much like the 23 degree misalignment between the Earth's rotation axis and its orbit about the Sun responsible for the seasons. General relativity causes black-hole spins to change direction or precess; this precession leaves detectable signatures in the gravitational waves observed by LIGO. This award supports studies on black-hole spin precession and the extent to which it can be constrained by gravitational-wave detectors like LIGO. This work helps scientists to interpret LIGO observations and thus better understand gravity and the astrophysical origin of black holes responsible for their spin misalignments. The PI and collaborators will also develop an interactive virtual-reality simulation of precessing black holes and the gravitational waves they emit which will be available free online and presented at local public libraries and museums through existing outreach networks of the University of Texas at Dallas. These simulations help the public virtually experience the effects of gravitational waves and enhance public interest and support for science research and education. This goal of this proposal is to provide a comprehensive picture of the origin and implications of misalignments between binary black-hole spins and their orbital angular momentum. Population-synthesis codes will be used to explore how initial spin misalignments depend on various aspects of astrophysical black-hole formation like mass transfer, tidal alignment, and supernova recoils. The spin distributions generated by these codes will be evolved from black-hole formation until they begin to emit detectable gravitational waves using the PI's newly developed technique for efficiently calculating spin precession on the inspiral timescale. Once the binaries enter the sensitivity band of ground-based detectors like LIGO, publicly available codes in the LIGO Algorithm Library Suite will be used to generate waveforms and analyze how well the binary black-hole parameters that determine spin precession can be estimated for realistic signal-to-noise ratios. The PI and collaborators will also perform and analyze numerical-relativity simulations to assess the reliability of post-Newtonian predictions for spin precession in the final orbits before black-hole merger. This research provides the basis for astrophysical model selection, combining constraints on spin misalignments from many events to distinguish astrophysical models of black-hole formation, such as dynamical formation in a globular cluster (for which spins should be isotropic) versus formation from stellar binaries (which should exhibit a tendency towards aligned spins). The final project will consider how spin precession between black hole formation and merger affects predictions for final black-hole masses, spins, and gravitational recoils. This project has implications both for the stellar-mass black holes observed by LIGO and recoiling quasars spatially or kinematically offset from their host galaxies sought in galaxy surveys.