We discuss three applications of a gauge theory of gravity to rotating astrophysical systems. The theory employs gauge fields in a flat Minkowski background spacetime to describe gravitational interactions. The iron fluorescence line observed in AGN is discussed, assuming that the line originates from matter in an accretion disk around a Kerr (rotating) black hole. Gauge-theory gravity, expressed in the language of Geometric Algebra, allows very efficient numerical calculation of photon paths. From these paths we are able to infer the line shape of the iron line. Comparison with observational data allows us to constrain the black hole parameters, and, for the first time, infer an emissivity profile for the accretion disk. The topological constraints imposed by gauge-theory gravity are exploited to investigate the nature of the Kerr singularity. This reveals a simple physical picture of a ring of matter moving at the speed of light which surrounds a sheet of pure isotropic tension. Implications for the end-points of collapse processes are discussed. Finally we consider rigidly-rotating cosmic strings. It is shown that a solution in the literature has an unphysical stress-energy tensor on the axis. Well defined solutions are presented for an ideal two-dimensional fluid. The exterior vacuum solution admits closed timelike curves and exerts a confining force.
A. N. Lasenby, C. J. L. Doran, Y. Dabrowski and A. D. Challinor, Rotating astrophysical systems and a gauge theory approach to gravity, In N. Sanchez and A. Zichini, editors, Current Topics in Astrofundamental Physics, Erice 1996. (World Scientific Publishing Co., 1997)p. 380-403