ABSTRACT
A rigorous, general relativistic treatment of the structure formation in the universe requires people to deal with a bit of formalism. The great advantage of the Lemaitre-Tolman-Bondi (LTB) model is that is an exact solution of the field equation of the general relativity, providing a natural framework for discussing local inhomogeneities. Because the model is isotropic by construction, it may be too simplified w.r.t. the physical situation of interest. However, it constitutes a toy model that can add physical insight into complicated problems and/or give alternative interpretation of the observations. In this respect, the observations of the high redshift supernovae Ia and the consequent evidence for an accelerating universe renewed interest for isotropic but inhomogeneous cos-mological models described by the LTB solution. The LTB is a toy model which nonetheless points out interesting physical effects. Slow spatial variation of the Hubble rate in an inhomogeneous model can produce a magnitude-redshift relation consistent with the data.
