  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.
