In differential geometry, the
Einstein tensor (also
tracereversed Ricci tensor), named after
Albert EinsteinAlbert Einstein was a Germanborn theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
, is used to express the
curvatureIn mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...
of a
Riemannian manifoldIn Riemannian geometry and the differential geometry of surfaces, a Riemannian manifold or Riemannian space is a real differentiable manifold M in which each tangent space is equipped with an inner product g, a Riemannian metric, which varies smoothly from point to point...
. In
general relativityGeneral relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, the Einstein tensor occurs in the
Einstein field equationsThe Einstein field equations or Einstein's equations are a set of ten equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy...
for
gravitationGravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
describing
spacetimeIn physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being threedimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
curvature in a manner consistent with energy considerations.
Definition
The Einstein tensor
is a rank 2
tensorTensors are geometric objects that describe linear relations between vectors, scalars, and other tensors. Elementary examples include the dot product, the cross product, and linear maps. Vectors and scalars themselves are also tensors. A tensor can be represented as a multidimensional array of...
defined over
Riemannian manifoldIn Riemannian geometry and the differential geometry of surfaces, a Riemannian manifold or Riemannian space is a real differentiable manifold M in which each tangent space is equipped with an inner product g, a Riemannian metric, which varies smoothly from point to point...
s. In indexfree notation it is defined as

where
is the Ricci tensor,
is the
metric tensorIn the mathematical field of differential geometry, a metric tensor is a type of function defined on a manifold which takes as input a pair of tangent vectors v and w and produces a real number g in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean...
and
is the
scalar curvatureIn Riemannian geometry, the scalar curvature is the simplest curvature invariant of a Riemannian manifold. To each point on a Riemannian manifold, it assigns a single real number determined by the intrinsic geometry of the manifold near that point...
. In component form, the previous equation reads as

The Einstein tensor is symmetric

and, like the
stressenergy tensorThe stress–energy tensor is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and nongravitational force fields...
, divergenceless

Explicit form
The Ricci tensor depends only on the
metric tensorIn the mathematical field of differential geometry, a metric tensor is a type of function defined on a manifold which takes as input a pair of tangent vectors v and w and produces a real number g in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean...
, so the Einstein tensor can be defined directly with just the metric tensor. However, this expression is complex and rarely quoted in textbooks. The complexity of this expression can be shown using the formula for the Ricci tensor in terms of
Christoffel symbolsIn mathematics and physics, the Christoffel symbols, named for Elwin Bruno Christoffel , are numerical arrays of real numbers that describe, in coordinates, the effects of parallel transport in curved surfaces and, more generally, manifolds. As such, they are coordinatespace expressions for the...
:

where is the Kronecker tensor and the Christoffel symbol is defined as

Before cancellations, this formula results in individual terms. Cancellations bring this number down somewhat.
In the special case of a locally inertial reference frame near a point, the first derivatives of the metric tensor vanish and the component form of the Einstein tensor is considerably simplified:


where square brackets conventionally denote antisymmetrization over bracketed indices, i.e.


Trace
The traceIn linear algebra, the trace of an nbyn square matrix A is defined to be the sum of the elements on the main diagonal of A, i.e.,...
of the Einstein tensor can be computed by contractIn multilinear algebra, a tensor contraction is an operation on one or more tensors that arises from the natural pairing of a finitedimensional vector space and its dual. In components, it is expressed as a sum of products of scalar components of the tensor caused by applying the summation...
ing the equation in the definition with the metric tensorIn the mathematical field of differential geometry, a metric tensor is a type of function defined on a manifold which takes as input a pair of tangent vectors v and w and produces a real number g in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean...
. In dimensions (of arbitrary signature):

The special case of 4 dimensions in physics (3 space, 1 time) gives , the trace of the Einstein tensor, as the negative of , the Ricci tensor's trace. Thus another name for the Einstein tensor is the tracereversed Ricci tensor.
Use in general relativity
The Einstein tensor allows the Einstein field equationsThe Einstein field equations or Einstein's equations are a set of ten equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy...
(without a cosmological constantIn physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...
) to be written in the concise form:

which becomes in geometrized units,

From the explicit form of the Einstein tensor, the Einstein tensor is a nonlinear function of the metric tensor, but is linear in the second partial derivativeIn mathematics, a partial derivative of a function of several variables is its derivative with respect to one of those variables, with the others held constant...
s of the metric. As a symmetric 2nd rank tensor, the Einstein tensor has 10 independent components in a 4dimensional space. It follows that the Einstein field equations are a set of 10 quasilinear secondorder partial differential equations for the metric tensor.
The Bianchi identities can also be easily expressed with the aid of the Einstein tensor:

The Bianchi identities automatically ensure the conservation of the stressenergy tensorThe stress–energy tensor is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and nongravitational force fields...
in curved spacetimes:

The geometric significance of the Einstein tensor is highlighted by this identity. In coordinate frames respecting the gauge condition

an exact conservation law for the stress tensor density can be stated:

 .
The Einstein tensor plays the role of distinguishing these frames.
See also
 Mathematics of general relativity
The mathematics of general relativity refers to various mathematical structures and techniques that are used in studying and formulating Albert Einstein's theory of general relativity. The main tools used in this geometrical theory of gravitation are tensor fields defined on a Lorentzian manifold...
 General relativity resources