Geodesic-light-cone coordinates and the Bianchi I spacetime
  The geodesic-light-cone (GLC) coordinates are a useful tool to analyse light
propagation and observations in cosmological models. In this article, we
propose a detailed, pedagogical, and rigorous introduction to this coordinate
system, explore its gauge degrees of freedom, and emphasize its interest when
geometric optics is at stake. We then apply the GLC formalism to the
homogeneous and anisotropic Bianchi I cosmology. More than a simple
illustration, this application (i) allows us to show that the Weinberg
conjecture according to which gravitational lensing does not affect the proper
area of constant-redshift surfaces is significantly violated in a globally
anisotropic universe; and (ii) offers a glimpse into new ways to constrain
cosmic isotropy from the Hubble diagram.
