  We elaborate on the proposal that the observed acceleration of the Universe
is the result of the backreaction of cosmological perturbations, 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, we demonstrate that acceleration in our
local Hubble patch is 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. We then study perturbatively the time behavior of
general-relativistic cosmological perturbations, applying, where possible, the
renormalization group to regularize the dynamics. We show that an instability
occurs in the perturbative expansion involving sub-Hubble modes. Whether this
is an indication that acceleration in our Hubble patch originates from the
backreaction of cosmological perturbations on observable scales requires a
fully non-perturbative approach.
