Quasi-local variables and scalar averaging in LTB dust models
  We introduce quasi--local (QL) scalar variables in spherically symmetric LTB
models. If the QL scalars are defined as functionals, they become weighed
averages that generalize the standard proper volume averages on space slices
orthogonal to the 4-velocity. We examine the connection between QL functions
and functionals and the "back-reaction" term $Q$ in the context of Buchert's
scalar averaging formalism. With the help of the QL scalars we provide rigorous
proof that back--reaction is positive for (i) all LTB models with negative and
asymptotically negative spatial curvature, and (ii) models with positive
curvature decaying to zero asymptotically in the radial direction. We show by
means of qualitative, but robust, arguments that generic LTB models exist,
either with clump or void profiles, for which an "effective" acceleration
associated with Buchert's formalism can mimic the effects of dark energy.
