Exploiting gauge and constraint freedom in hyperbolic formulations of Einstein’s equations

We present many-parameter families of strongly and symmetric hyperbolic formulations of Einstein’s equations that include quite general algebraic and live gauge conditions for the lapse. The first system that we present has $30$ variables and incorporates an algebraic relationship between the lapse and the determinant of the three metric that generalizes the densitized lapse prescription. The second system has $34$ variables and uses a family of live gauges that generalizes the Bona-Masso slicing conditions. These systems have free parameters even after imposing hyperbolicity and are expected to be useful in 3D numerical evolutions. We discuss under what conditions there are no superluminal characteristic speeds.