  Theories of low-energy Lorentz violation by a fixed-norm "aether" vector
field with two-derivative kinetic terms have a globally bounded Hamiltonian and
are perturbatively stable only if the vector is timelike and the kinetic term
in the action takes the form of a sigma model. Here we investigate the
phenomenological properties of this theory. We first consider the propagation
of modes in the presence of gravity, and show that there is a unique choice of
curvature coupling that leads to a theory without superluminal modes.
Experimental constraints on this theory come from a number of sources, and we
examine bounds in a two-dimensional parameter space. We then consider the
cosmological evolution of the aether, arguing that the vector will naturally
evolve to be orthogonal to constant-density hypersurfaces in a
Friedmann-Robertson-Walker cosmology. Finally, we examine cosmological
evolution in the presence of an extra compact dimension of space, concluding
that a vector can maintain a constant projection along the extra dimension in
an expanding universe only when the expansion is exponential.
