I will report on the recent proposal by Kolb, Matarrese and Riotto (astro-ph/0506534) that the observed acceleration of the Universe can be the result of the backreaction of cosmological perturbations on sub-Hubble (i.e. observable) scales, rather than the effect of a negative-pressure dark-energy fluid or a modification of general relativity. Through the effective Friedmann equations describing an inhomogeneous Universe after smoothing, acceleration in our local Hubble patch is shown to be possible even if fluid elements do not individually undergo accelerated expansion.
This invalidates the no-go theorem that there can be no acceleration in our local Hubble patch if the Universe only contains irrotational dust. A study of the time behavior of cosmological perturbations in two different gauges (comoving gauge and Poisson gauge in the weak-field limit) allows to estimate their backreaction on the average Friedmann dynamics. Possible indications that acceleration in our Hubble patch may indeed originate from the backreaction of cosmological perturbations on observable scales are discussed.