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[1]

where is the determinant of the metric tensor matrix, is the Ricci scalar, and is the Einstein gravitational constant ( is the gravitational constant and is the speed of light in vacuum). If it converges, the integral is taken over the whole spacetime. If it does not converge, is no longer well-defined, but a modified definition where one integrates over arbitrarily large, relatively compact domains, still yields the Einstein equation as the Euler–Lagrange equation of the Einstein–Hilbert action. The action was proposed[2] by David Hilbert in 1915 as part of his application of the variational principle to a combination of gravity and electromagnetism.[3]: 119 

Discussion

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Deriving equations of motion from an action has several advantages. First, it allows for easy unification of general relativity with other classical field theories (such as Maxwell theory), which are also formulated in terms of an action. In the process, the derivation identifies a natural candidate for the source term coupling the metric to matter fields. Moreover, symmetries of the action allow for easy identification of conserved quantities through Noether's theorem.

In general relativity, the action is usually assumed to be a functional of the metric (and matter fields), and the connection is given by the Levi-Civita connection. The Palatini formulation of general relativity assumes the metric and connection to be independent, and varies with respect to both independently, which makes it possible to include fermionic matter fields with non-integer spin.

The Einstein equations in the presence of matter are given by adding the matter action to the Einstein–Hilbert action.

Derivation of Einstein field equations

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Suppose that the full action of the theory is given by the Einstein–Hilbert term plus a term   describing any matter fields appearing in the theory.

 . (1)

The stationary-action principle then tells us that to recover a physical law, we must demand that the variation of this action with respect to the inverse metric be zero, yielding

 .

Since this equation should hold for any variation  , it implies that

  (2)

is the equation of motion for the metric field. The right hand side of this equation is (by definition) proportional to the stress–energy tensor,[4]

 .

To calculate the left hand side of the equation we need the variations of the Ricci scalar   and the determinant of the metric. These can be obtained by standard textbook calculations such as the one given below, which is strongly based on the one given in Carroll (2004).[5]

Variation of the Ricci scalar

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The variation of the Ricci scalar follows from varying the Riemann curvature tensor, and then the Ricci curvature tensor.

The first step is captured by the Palatini identity

 .

Using the product rule, the variation of the Ricci scalar   then becomes

 

where we also used the metric compatibility  , and renamed the summation indices   in the last term.

When multiplied by  , the term   becomes a total derivative, since for any vector   and any tensor density  , we have

  or  .

By Stokes' theorem, this only yields a boundary term when integrated. The boundary term is in general non-zero, because the integrand depends not only on   but also on its partial derivatives  ; see the article Gibbons–Hawking–York boundary term for details. However, when the variation of the metric   vanishes in a neighbourhood of the boundary or when there is no boundary, this term does not contribute to the variation of the action. Thus, we can forget about this term and simply obtain

 . (3)

at events not in the closure of the boundary.

Variation of the determinant

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Jacobi's formula, the rule for differentiating a determinant, gives:

 ,

or one could transform to a coordinate system where   is diagonal and then apply the product rule to differentiate the product of factors on the main diagonal. Using this we get

 

In the last equality we used the fact that

 

which follows from the rule for differentiating the inverse of a matrix

 .

Thus we conclude that

 . (4)

Equation of motion

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Now that we have all the necessary variations at our disposal, we can insert (3) and (4) into the equation of motion (2) for the metric field to obtain

 , (5)

which is the Einstein field equations, and

 

has been chosen such that the non-relativistic limit yields the usual form of Newton's gravity law, where   is the gravitational constant (see here for details).

Cosmological constant

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When a cosmological constant Λ is included in the Lagrangian, the action:

 

Taking variations with respect to the inverse metric:

 

Using the action principle:

 

Combining this expression with the results obtained before:

 

We can obtain:

 

With  , the expression becomes the field equations with a cosmological constant:

 

See also

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Notes

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  1. ^ Feynman, Richard P. (1995). Feynman Lectures on Gravitation. Addison-Wesley. p. 136, eq. (10.1.2). ISBN 0-201-62734-5.
  2. ^ Hilbert, David (1915), "Die Grundlagen der Physik" [Foundations of Physics], Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen – Mathematisch-Physikalische Klasse (in German), 3: 395–407
  3. ^ Mehra, Jagdish (1987). "Einstein, Hilbert, and the Theory of Gravitation". In Mehra, Jagdish (ed.). The physicist's conception of nature (Reprint ed.). Dordrecht: Reidel. ISBN 978-90-277-2536-3.
  4. ^ Blau, Matthias (August 27, 2024), Lecture Notes on General Relativity (PDF), p. 207
  5. ^ Carroll, Sean M. (2004), Spacetime and Geometry: An Introduction to General Relativity, San Francisco: Addison-Wesley, ISBN 978-0-8053-8732-2

Bibliography

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