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In differential geometry, the Einstein tensor (also trace-reversed Ricci tensor), named after Albert Einstein, is used to express the curvature of a Riemannian manifold. In general relativity, the Einstein tensor occurs in the Einstein field equations for gravitation describing spacetime curvature in a manner consistent with energy considerations.
The Einstein tensor is symmetric
and, like the stress–energy tensor, divergenceless
The Ricci tensor depends only on the metric tensor, 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 symbols:
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.
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 trace-reversed Ricci tensor.
Use in general relativity
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 derivatives of the metric. As a symmetric 2nd rank tensor, the Einstein tensor has 10 independent components in a 4-dimensional space. It follows that the Einstein field equations are a set of 10 quasilinear second-order 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 covariant conservation of the stress–energy tensor in curved spacetimes:
The geometric significance of the Einstein tensor is highlighted by this identity. In coordinate frames respecting the gauge condition
an ordinary conservation law for the stress tensor density can be stated:
The Einstein tensor plays the role of distinguishing these frames.
David Lovelock has shown that, in a four dimensional differentiable manifold, the Einstein tensor is the only tensorial and divergence-free function of the and at most their first and second partial derivatives.    We also note that the Einstein field equation is not the only equation which satisfies the three conditions:
- Resemble but generalize Newton-Poisson gravitational equation
- Applied to all coordinate systems, and
- Guarantee local covariant conservation of energy–momentum for any metric tensor.
Many alternatives theory have been proposed, like Einstein–Cartan theory, etc... that also satisfies the above conditions.
- Lovelock, D. (1971). "The Einstein Tensor and Its Generalizations". Journal of Mathematical Physics 12 (3): 498–502. Bibcode:1971JMP....12..498L. doi:10.1063/1.1665613.
- Lovelock, D. (1969). "The uniqueness of the Einstein field equations in a four-dimensional space". Archive for Rational Mechanics and Analysis 33 (1): 54–70. Bibcode:1969ArRMA..33...54L. doi:10.1007/BF00248156.
- Farhoudi, M. (2009). "Lovelock Tensor as Generalized Einstein Tensor". General Relativity and Gravitation 41 (1): 17–29.
- Rindler, Wolfgang (2001). Relativity: Special, General, and Cosmological. Oxford University Press. p. 299. ISBN 0-19-850836-0.
- Schutz, Bernard (May 31, 2009). A First Course in General Relativity (2 ed.). Cambridge University Press. p. 185. ISBN 0-521-88705-4.
- Ohanian, Hans C.; Remo Ruffini (1994). Gravitation and Spacetime (Second edition ed.). W. W. Norton & Company. ISBN 0-393-96501-5.
- Martin, John Legat (1995). General Relativity: A First Course for Physicists. Prentice Hall International Series in Physics and Applied Physics (Revised edition ed.). Prentice Hall. ISBN 0-13-291196-5.