Gravitational plane waves in Einstein-aether theory
  In this paper, we systematically study spacetimes of gravitational plane
waves in Einstein-aether theory. Due to the presence of the timelike aether
vector field, now the problem in general becomes overdetermined. In particular,
for the linearly polarized plane waves, there are five independent vacuum
Einstein-aether field equations for three unknown functions. Therefore,
solutions exist only for particular choices of the four free parameters
$c_{i}$'s of the theory. We find that there exist eight cases, in two of which
any form of gravitational plane waves can exist, similar to that in general
relativity, while in the other six cases, gravitational plane waves exist only
in particular forms. Beyond these eight cases, solutions either do not exist or
are trivial (simply representing a Minkowski spacetime with a constant or
dynamical aether field.).
