Abstract
In the conventional framework for cosmological dynamics the scale factor
\$a(t)\$ is assumed to obey the `background' Friedmann equation for a perfectly
homogeneous universe while particles move according to equations of motions
driven by the gravity sourced by the density fluctuations. It has been
suggested that the emergence of structure modifies the evolution of \$a(t)\$ via
`kinematic' backreaction and that this may avoid the need for dark energy. Here
we show that the conventional equations are exact in Newtonian gravity -- which
should accurately describe the low-\$z\$ universe -- and there is no
approximation in the use of the homogeneous universe equation for \$a(t)\$. We
conclude that there is no backreaction of structure on \$a(t)\$ and that the need
for dark energy cannot be avoided in this way.
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