Silent Universes

The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero pressure, which implies that the fluid world-lines are geodesic (u˙a = 0); (2) the assumption that the dust is irrotational (ωa = 0); and (3) the assumption that the magnetic Weyl curvature is zero (Hab = 0). In general, however, the solutions admit no symmetries. The dynamical systems approach used here was first systematically applied to these models by Bruni et al. (1995). In analyzing these solutions the authors make use of the evolution equations for the kinematic quantities and the Weyl curvature. The solutions describe the evolution of an expanding or collapsing distribution of dust in which there is no exchange of information between different fluid elements, either by sound waves or by gravitational waves. For this reason the models in this class have been called "silent universes".