Is backreaction in cosmology a relativistic effect? On the need for an extension of Newton's theory to non-Euclidean topologies

Understand

Cosmological backreaction corresponds to the effect of inhomogeneities of structure on the global expansion of the Universe.

• The main question surrounding this phenomenon is whether or not it is important enough to lead to measurable effects on the scale factor evolution eventually explaining its acceleration or the Hubble tension.
• One of the most important result on this subject is the Buchert-Ehlers theorem (Buchert \& Ehlers, 1997) stating that backreaction is exactly zero when calculated using Newton's theory of gravitation, which may not be the case in general relativity.
• It is generally said that this result implies that backreaction is a purely relativistic effect.