Frequency variation for in vacuo photon propagation in the
  Standard-Model Extension
  In the presence of Lorentz Symmetry Violation (LSV) associated with the
Standard-Model Extension (SME), we have recently shown the non-conservation of
the energy-momentum tensor of a light-wave crossing an Electro-Magnetic (EM)
background field even when the latter and the LSV are constant. Incidentally,
for a space-time dependent LSV, the presence of an EM field is not necessary.
Herein, we infer that in a particle description, the energy non-conservation
for a photon implies violation of frequency invariance in vacuo giving rise to
a red or blue shift. We discuss the potential consequences on cosmology.
