Ponderable aether
  We consider a Lorentz-violating theory of gravity where the aether vector is
taken to be nondynamical. This "ponderable aether theory" is almost the same as
Einstein-aether theory (where the aether vector is dynamical), but involves
additional integration constants arising due to the loss of initial value
constraints. One of these produces an effective energy density for the aether
fluid, similar to the appearance of dark matter in projectable Ho\v{r}ava
gravity and the mimetic dark matter theory. Here we investigate the extent to
which this energy density can reproduce the phenomenology of dark matter.
Although it is indistinguishable from cold dark matter in homogeneous,
isotropic cosmology, it encounters phenomenological problems in both
spherically symmetric configurations and cosmological perturbations.
Furthermore, inflationary considerations lead us to expect a tiny value for the
ponderable aether energy density today unless a sourcing effect is added to the
theory. The theory then effectively reduces to dynamical Einstein-aether
theory, rendering moot the question of whether an aether must be dynamical in
order to be consistent.
