  In the context of second order perturbation theory, cosmological backreaction
is seen to rescale both time and the scale factor. The issue of the homogeneous
limit of long-wavelength perturbations is addressed and backreaction is
quantified in terms of a gauge-invariant metric function that is the true
physical degree of freedom in the homogeneous limit. The time integral of this
metric function controls whether backreaction hastens or delays the expansion
of the universe. As an example, late-time acceleration of the universe is shown
to be inconsistent with a perturbative approach. Any tendency to accelerate the
expansion requires negative non-adiabatic pressure fluctuations.

Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Theory