Aether Unleashed
  We follow a low-energy effective theory approach to identify the general
class of theories that describes a vector field (of unconstrained norm) coupled
to gravity. The resulting set may be regarded as a generalization of the
conventional vector-tensor theories, and as a high-momentum completion of
aether models. We study the conditions that a viable cosmology, Newtonian limit
and absence of classical and quantum instabilities impose on the parameters of
our class of models, and compare these constraints with those derived in
previously studied and related cases. The most stringent conditions arise from
the quantum stability of the theory, which allows dynamical cosmological
solutions only for a non-Maxwellian kinetic term. The gravitational constant in
the Newtonian limit turns to be scale dependent, suggesting connections to dark
matter and degravitation. This class of theories has a very rich gravitational
phenomenology, and offers an ample but simple testing ground to study
modifications of gravity and their cosmological implications.
