Formation of voids in the Universe within the Lemaitre-Tolman model
  We develop models of void formation starting from a small initial fluctuation at recombination and growing to a realistic present day density profile in agreement with observations of voids. The model construction is an extension of previously developed algorithms for finding a Lemaitre-Tolman metric that evolves between two profiles of either density or velocity specified at two times. Of the 4 profiles of concern -- those of density and velocity at recombination and at the present day -- two can be specified and the other two follow from the derived model. We find that, in order to reproduce the present-day void density profiles, the initial velocity profile is more important than the initial density profile. Extrapolation of current CMB observations to the scales relevant to proto-voids is very uncertain. Even so, we find that it is very difficult to make both the initial density and velocity fluctuation amplitudes small enough, and still obtain a realistic void by today. 
Krzysztof Bolejko, Andrzej Krasinski, Charles Hellaby