2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/183816We study backreaction analytically using the parabolic Lemaitre-Tolman-Bondi universe as a toy model. We calculate the average expansion rate and energy density on two different hypersurfaces and compare the results. We also consider the Hubble law and find that backreaction slows down the expansion if measured with proper time, but speeds it up if measured with energy density.11 pages, JCAP-style. v2: Published version. One added reference, no major changesGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - Theorytext