Negative refraction in outer space?
  Mediums which do not support the propagation of plane waves with negative phase velocity (NPV) when viewed at rest can support NPV propagation when they are viewed in a reference frame which is uniformly translated at sufficiently high velocity. Thus, relativistic negative refraction may be exploited in astronomical scenarios. 
Tom G. Mackay (1), Akhlesh Lakhtakia (2) ((1) University of Edinburgh,   (2) Pennsylvania State University)