Observation angles, Fermi coordinates, and the Geodesic-Light-Cone gauge
  We show that the angular directions locally measured by a static geodesic
observer in a generic cosmological background and expressed in the system of
Fermi Normal Coordinates always coincide with those expressed in the
Geodesic-Light-Cone (GLC) gauge, up to a local transformation which exploits
the residual gauge freedom of the GLC coordinates. This is not the case for
other gauges - like, for instance, the synchronous and longitudinal gauge -
commonly used in the context of observational cosmology. We also make an
explicit proposal for the GLC gauge-fixing condition that ensures a full
identification of its angles with the observational ones.
