Szekeres models: a covariant approach
  We exploit the 1+1+2 formalism to covariantly describe the inhomogeneous and
anisotropic Szekeres models. It is shown that an \emph{average scale length}
can be defined \emph{covariantly} which satisfies a 2d equation of motion
driven from the \emph{effective gravitational mass} (EGM) contained in the dust
cloud. The contributions to the EGM are encoded to the energy density of the
dust fluid and the free gravitational field $E_{ab}$. We show that the
quasi-symmetric property of the Szekeres models is justified through the
existence of 3 independent \emph{Intrinsic Killing Vector Fields (IKVFs)}. In
addition the notions of the Apparent and Absolute Apparent Horizons are briefly
discussed and we give an alternative gauge-invariant form to define them in
terms of the kinematical variables of the spacelike congruences. We argue that
the proposed program can be used in order to express Sachs' optical equations
in a covariant form and analyze the confrontation of a spatially inhomogeneous
irrotational overdense fluid model with the observational data.
