  We consider the gravitational effects of a single, fixed-norm,
Lorentz-violating timelike vector field. In a cosmological background, such a
vector field acts to rescale the effective value of Newton's constant. The
energy density of this vector field precisely tracks the energy density of the
rest of the universe, but with the opposite sign, so that the universe
experiences a slower rate of expansion for a given matter content. This vector
field similarly rescales Newton's constant in the Newtonian limit, although by
a different factor. We put constraints on the parameters of the theory using
the predictions of primordial nucleosynthesis, demonstrating that the norm of
the vector field should be less than the Planck scale by an order of magnitude
or more.
