# Ricci decomposition

In semi-Riemannian geometry, the **Ricci decomposition** is a way of breaking up the Riemann curvature tensor of a pseudo-Riemannian manifold into pieces with useful individual algebraic properties. This decomposition is of fundamental importance in Riemannian- and pseudo-Riemannian geometry.

## The pieces appearing in the decomposition

The decomposition is

The three pieces are:

- the
*scalar part*, the tensor - the
*semi-traceless part*, the tensor - the
*fully traceless part*, the Weyl tensor

Each piece possesses all the algebraic symmetries of the Riemann tensor itself, but has additional properties.

The decomposition can have different signs, depending on the Ricci curvature convention, and only makes sense if the dimension satisfies .

The scalar part

is built using the scalar curvature , where is the Ricci curvature, and a tensor constructed algebraically from the metric tensor ,

The semi-traceless part

is constructed algebraically using the metric tensor and the *traceless part* of the Ricci tensor

where is the metric tensor.

The Weyl tensor or *conformal curvature tensor* is completely traceless, in the sense that taking the trace, or contraction, over any pair of indices gives zero. Hermann Weyl showed that this tensor measures the deviation of a semi-Riemannian manifold from *conformal flatness*; if it vanishes, the manifold is (locally) conformally equivalent to a flat manifold.

No additional differentiation is needed anywhere in this construction.

In the case of a Lorentzian manifold, , the Einstein tensor has, by design, a trace which is just the negative of the Ricci scalar, and one may check that the traceless part of the Einstein tensor agrees with the traceless part of the Ricci tensor.

*Terminological note:* the notation is standard in the modern literature, the notations are commonly used but not standardized, and there is no standard notation for the scalar part.

## Mathematical definition

Mathematically, the Ricci decomposition is the decomposition of the space of all tensors having the symmetries of the Riemann tensor into its irreducible representations for the action of the orthogonal group Template:Harv. Let *V* be an *n*-dimensional vector space, equipped with a metric tensor (of possibly mixed signature). Here *V* is modeled on the cotangent space at a point, so that a curvature tensor *R* (with all indices lowered) is an element of the tensor product *V*⊗*V*⊗*V*⊗*V*. The curvature tensor is skew symmetric in its first and last two entries:

and obeys the interchange symmetry

for all *x*,*y*,*z*,*w* ∈ *V*^{∗}. As a result *R* is an element of the subspace *S*^{2}Λ^{2}*V*, the second symmetric power of the second exterior power of *V*. A curvature tensor must also satisfy the Bianchi identity, meaning that it is in the kernel of the linear map

The space **R***V* = ker *b* in *S*^{2}Λ^{2}*V* is the space of algebraic curvature tensors. The Ricci decomposition is the decomposition of this space into irreducible factors. The Ricci contraction mapping

is given by

This associates a symmetric 2-form to an algebraic curvature tensor. Conversely, given a pair of symmetric 2-forms *h* and *k*, the Kulkarni–Nomizu product of *h* and *k*

produces an algebraic curvature tensor.

If *n* > 4, then there is an orthogonal decomposition into (unique) irreducible subspaces

**R***V*=**S***V*⊕**E***V*⊕**C***V*

where

- , where is the space of real scalars
- , where
*S*Template:Su*V*is the space of trace-free symmetric 2-forms

The parts *S*, *E*, and *C* of the Ricci decomposition of a given Riemann tensor *R* are the orthogonal projections of *R* onto these invariant factors. In particular,

is an orthogonal decomposition in the sense that

This decomposition expresses the space of tensors with Riemann symmetries as a direct sum of the scalar submodule, the Ricci submodule, and Weyl submodule, respectively. Each of these modules is an irreducible representation for the orthogonal group Template:Harv, and thus the Ricci decomposition is a special case of the splitting of a module for a semisimple Lie group into its irreducible factors. In dimension 4, the Weyl module decomposes further into a pair of irreducible factors for the special orthogonal group: the self-dual and antiself-dual parts *W*^{+} and *W*^{−}.

## Physical interpretation

The Ricci decomposition can be interpreted physically in Einstein's theory of general relativity, where it is sometimes called the *Géhéniau-Debever decomposition*. In this theory, the Einstein field equation

where is the stress–energy tensor describing the amount and motion of all matter and all nongravitational field energy and momentum, states that the Ricci tensor—or equivalently, the Einstein tensor—represents that part of the gravitational field which is due to the *immediate presence* of nongravitational energy and momentum. The Weyl tensor represents the part of the gravitational field which can propagate as a gravitational wave through a region containing no matter or nongravitational fields. Regions of spacetime in which the Weyl tensor vanishes contain no gravitational radiation and are also conformally flat.

## See also

- Bel decomposition of the Riemann tensor
- Conformal geometry
- Petrov classification
- Plebanski tensor
- Ricci calculus
- Schouten tensor
- Trace-free Ricci tensor

## References

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