  The Einstein-Aether theory provides a simple, dynamical mechanism for
breaking Lorentz invariance. It does so within a generally covariant context
and may emerge from quantum effects in more fundamental theories. The theory
leads to a preferred frame and can have distinct experimental signatures. In
this letter, we perform a comprehensive study of the cosmological effects of
the Einstein-Aether theory and use observational data to constrain it. Allied
to previously determined consistency and experimental constraints, we find that
an Einstein-Aether universe can fit experimental data over a wide range of its
parameter space, but requires a specific rescaling of the other cosmological
densities.
