In theoretical physics, back-reaction (or backreaction) is often necessary to calculate the self-consistent behaviour of a particle or an object in an external field.
When a particle is considered to have no mass or to have an infinitesimal charge, this can be described as saying that we deal with a probe and that back-reaction is neglected.
In inhomogeneous cosmology, in which structure formation is taken into account in a general-relativistic model of the Universe, the term "backreaction" is used for a measure of the non-commutativity of the averaging procedure (which comes from the non-linearity of Einstein field equations) and the dynamical evolution of spatial slices of space-time.
[1] As of 2017[update], the role of backreaction in possibly leading to an alternative to dark energy is an open question of debate among cosmologists.
[2][3] The existence of a homogeneity length scale can be considered to be that at which the calculations with and without backreaction give the same results.