BSSN formalism

Summary

The BSSN formalism, named after Baumgarte, Shapiro, Shibata and Nakamura, is a reformulation of the evolution equations of the ADM formalism. It splits the spatial metric into a conformal factor and a conformal metric of unit determinant. It splits the extrinsic curvature into its trace and trace-free part, and it adds three conformal connection functions as new variables. Unlike the ADM evolution equations, the resulting system is strongly hyperbolic for common gauge choices, and it gives stable long-time simulations. Together with the moving puncture gauge, it is one of the standard methods of numerical relativity for black hole binaries.