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 ...
, Gauss–Bonnet gravity, also referred to as Einstein–Gauss–Bonnet gravity, is a modification of the
Einstein–Hilbert action
The Einstein–Hilbert action in general relativity is the action that yields the Einstein field equations through the stationary-action principle. With the metric signature, the gravitational part of the action is given as
:S = \int R \sqrt ...
to include the
Gauss–Bonnet term (named after
Carl Friedrich Gauss
Johann Carl Friedrich Gauss (; ; ; 30 April 177723 February 1855) was a German mathematician, astronomer, geodesist, and physicist, who contributed to many fields in mathematics and science. He was director of the Göttingen Observatory and ...
and
Pierre Ossian Bonnet
Pierre Ossian Bonnet (; 22 December 1819, Montpellier – 22 June 1892, Paris) was a French mathematician. He made some important contributions to the differential geometry of surfaces, including the Gauss–Bonnet theorem.
Biography Early yea ...
)
:
,
where
:
.
This term is only nontrivial in 4+1D or greater, and as such, only applies to extra dimensional models. In 3+1D, it reduces to a topological
surface term. This follows from the
generalized Gauss–Bonnet theorem
A generalization is a form of abstraction whereby common properties of specific instances are formulated as general concepts or claims. Generalizations posit the existence of a domain or set of elements, as well as one or more common characteri ...
on a 4D manifold
:
.
In lower dimensions, it identically vanishes.
Despite being quadratic in the
Riemann tensor (and
Ricci tensor
In differential geometry, the Ricci curvature tensor, named after Gregorio Ricci-Curbastro, is a geometric object which is determined by a choice of Riemannian or pseudo-Riemannian metric on a manifold. It can be considered, broadly, as a measure ...
), terms containing more than 2 partial derivatives of the
metric
Metric or metrical may refer to:
Measuring
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
...
cancel out, making the
Euler–Lagrange equations second order quasilinear partial differential equations
In mathematics, a partial differential equation (PDE) is an equation which involves a multivariable function and one or more of its partial derivatives.
The function is often thought of as an "unknown" that solves the equation, similar to how ...
in the metric. Consequently, there are no additional dynamical degrees of freedom, as in say
f(R) gravity
In physics, ''f''(''R'') is a type of modified gravity theory which generalizes Einstein's general relativity. ''f''(''R'') gravity is actually a family of theories, each one defined by a different function, , of the Ricci scalar, . The simpl ...
.
Gauss–Bonnet gravity has also been shown to be connected to
classical electrodynamics
Classical electromagnetism or classical electrodynamics is a branch of physics focused on the study of interactions between electric charges and currents using an extension of the classical Newtonian model. It is, therefore, a classical field th ...
by means of complete gauge invariance with respect to
Noether's theorem
Noether's theorem states that every continuous symmetry of the action of a physical system with conservative forces has a corresponding conservation law. This is the first of two theorems (see Noether's second theorem) published by the mat ...
.
More generally, we may consider a
:
term for some function ''f''. Nonlinearities in ''f'' render this coupling nontrivial even in 3+1D. Therefore, fourth order terms reappear with the nonlinearities.
See also
*
Einstein–Hilbert action
The Einstein–Hilbert action in general relativity is the action that yields the Einstein field equations through the stationary-action principle. With the metric signature, the gravitational part of the action is given as
:S = \int R \sqrt ...
* f(R, G, T) or f(R, T, G) gravity
*
f(R) gravity
In physics, ''f''(''R'') is a type of modified gravity theory which generalizes Einstein's general relativity. ''f''(''R'') gravity is actually a family of theories, each one defined by a different function, , of the Ricci scalar, . The simpl ...
*
Lovelock gravity
References
Theories of gravity
{{relativity-stub