Black Hole Spin Evolution

Einstein's general theory of relativity says that gravity is a curvature of spacetime caused by the presence of matter. If the curvature is fairly weak, classical Newtonian physics can explain most of what we observe in nature, such as the regular motions of the planets. Very massive or dense objects would generate much stronger gravity, and the most compact objects imaginable are predicted by general relativity to have such strong gravity that nothing, not even light, could escape their grip. These objects are called "black holes."

Black holes are believed to lie, seemingly paradoxically, at the heart of some of the most luminous objects in the universe: quasars, active galactic nuclei, and possibly gamma-ray bursts. While black holes themselves are perfectly absorbing, they are surrounded by a hot plasma that radiates throughout the electromagnetic spectrum; the accreted gas is heated during its descent toward the black hole's event horizon. The massive, dark objects observed in the centers of galaxies and some of the stellar-mass compact objects observed in binary systems are also believed to be such black holes.

Black hole solutions of Einstein's equations have three parameters: mass M, spin J, and charge Q. Of these, Q is likely to be negligible in astrophysical contexts because electric charge is shorted out by the surrounding plasma. Thus while much of the variation in the observational appearance of black holes is likely caused by variation in the external parameters such as the angle between black hole spin vector and line of sight, the gas accretion flow geometry and accretion rate M, and other environmental factors, some might also arise from variation in black hole spin j = J/M² = a/M.

Using the resources of the National Center for Supercomputing Applications, Professors Charles Gammie and Stuart Shapiro and their students have been making tremendous progress in developing computational techniques for the study of relativistic plasmas near black holes. The figure above is a "snapshot" of one such simulation, that of a weakly magnetized Fishbone–Moncrief torus around a j = 0.75 black hole. Color corresponds to the logarithm of the density; red is high density, and black is low. (Click here for a higher-resolution image.)

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