An introduction to the multi-grid method for numerical relativists

The multi-grid method, which has made a considerable impact on both theoretical and applied numerical analysis in the past decade, is reviewed within the context of the solution of boundary value problems in 3 + 1 numerical relativity. The basic principles of operation of a multi-grid algorithm are discussed and, with the aid of numerical experiments on exactly soluble model problems, the method is compared to more traditional techniques such as SOR. The results of application of the method to a set of axisymmetric problems for black hole initial data, previously determined by relaxation techniques, are presented.     

Critical collapse of a complex scalar field with angular momentum

We report a new critical solution found at the threshold of axisymmetric gravitational collapse of a complex scalar field with angular momentum. To carry angular momentum the scalar field cannot be axisymmetric; however, its azimuthal dependence is defined so that the resulting stress-energy tensor and spacetime metric are axisymmetric. The critical solution found is nonspherical, discretely self-similar with an echoing exponent Delta=0.42(+/-4%), and exhibits a scaling exponent gamma=0.11(+/-10%) in near-critical collapse. Our simulations suggest that the solution is universal (within the imposed symmetry class), modulo a family-dependent constant, complex phase.