De-Sitter-spacetime instability from a nonstandard vector field
  It is found that de-Sitter spacetime, the constant-curvature matter-free solution of the Einstein equations with a positive cosmological constant, becomes classically unstable due to the dynamic effects of a certain type of vector field (fundamentally different from a gauge field). The perturbed de-Sitter universe evolves towards a final exotic singularity. The relevant vector-field configurations violate the strong and dominant energy conditions. 
V. Emelyanov, F. R. Klinkhamer
gr-qc hep-th