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Abstract

A recent dynamical formulation at a derivative level partial derivative (3)g for fluid spacetime geometries (M,g,u), that employs the concept of evolution systems in a first-order symmetric hyperbolic format, implies the existence in the Weyl curvature branch of a set of timelike characteristic three-surfaces associated with the propagation speed upsilon = 1/2 relative to fluid-comoving observers. We show it is a physical role of the constraint equations to prevent realization of jump discontinuities in the derivatives of the related initial data so that Weyl curvature modes propagating along these three-surfaces cannot be activated. In addition we introduce a new, illustrative first-order symmetric hyperbolic evolution system at a derivative level partial derivative (2)g for baryotropic perfect fluid cosmological models that are invariant under the transformation of an Abelian G(2) isometry group.