# Observational constraints and exact plane wave and spherical solutions in Einstein-Aether Theory.

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There are theoretical reasons to suspect that Lorentz-invariance, a cornerstone of modern physics, may be violated at very high energy levels. To study the effects of Lorentz-invariance in the classical regime, we consider Einstein-aether theory, a modified theory of gravity in which the metric is coupled to a unit timelike vector field called the "aether." This vector field picks out a preferred frame of reference, and generates a "matter-like" stress-energy tensor T æµν . The theory is associated with solutions for black holes and gravitational waves that differ from those of Einsteinian General Relativity. We investigate both the observational constraints on the parameters of the theory as well as the consequences of the theory for plane wave radiation, and the gravitational collapse of the aether itself. We find that the four coupling constants of the theory (ci, i=1,2,3,4) are tightly constrained by astronomical observations, and while multiple plane wave solutions exist most of them are ruled out by observation, leaving several viable candidates, a few of which are the same as General Relativity. For vacuum spherically-symmetric solutions, for the first time we find a simple, closed-form solution for static aether which does not violate the constraints.