theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain, and predict List of natural phenomena, natural phenomena. This is in contrast to experimental p ...

, back-reaction (or backreaction) is often necessary to calculate the self-consistent behaviour of a

particle In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

or an object in an external field.

Intuitive definition

When a particle is considered to have no mass or to have an

infinitesimal In mathematics, an infinitesimal number is a non-zero quantity that is closer to 0 than any non-zero real number is. The word ''infinitesimal'' comes from a 17th-century Modern Latin coinage ''infinitesimus'', which originally referred to the " ...

charge, this can be described as saying that we deal with a probe and that back-reaction is neglected. However, a real object also carries (in general) a mass and a charge itself. These properties imply that the model of the original environment needs to be modified to reach self-consistency. For example, a particle can be described as helping to curve the space in

general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

. Taking into account the constraints implied on the model by the particle's properties – the back-reaction – is one way of reaching a more accurate model than if those constraints are ignored.

Cosmology

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 :

G_(\overline) \neq \overline = \frac\overline

(which comes from the non-linearity of Einstein field equations) and the dynamical evolution of spatial slices of space-time. , the role of backreaction in possibly leading to an alternative to

dark energy In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure format ...

is an open question of debate among cosmologists. 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. , the existence of such a scale needs experimental confirmation.

References

External links

Theoretical physics Physical cosmology {{theoretical-physics-stub