The Einstein–Hilbert action is the basis for the most elementary variational principle from which the field equations of general relativity can be defined. However, the use of the Einstein–Hilbert action is appropriate only when the underlying spacetime manifold is closed, i.e., a manifold which is both compact and without boundary. In the event that the manifold has a boundary , the action should be supplemented by a boundary term so that the variational principle is well-defined.
The necessity of such a boundary term was first realised by York and later refined in a minor way by Gibbons and Hawking.
For a manifold that is not closed, the appropriate action is
where is the Einstein–Hilbert action, is the Gibbons–Hawking–York boundary term, is the induced metric (see section below on definitions) on the boundary, its determinant, is the trace of the second fundamental form, is equal to where is timelike and where is spacelike, and are the coordinates on the boundary. Varying the action with respect to the metric , subject to the condition
gives the Einstein equations; the addition of the boundary term means that in performing the variation, the geometry of the boundary encoded in the transverse metric is fixed (see section below). There remains ambiguity in the action up to an arbitrary functional of the induced metric .
That a boundary term is needed in the gravitational case is due to the fact that , the gravitational Lagrangian density, contains second derivatives of the metric tensor. This is a non-typical feature of field theories, which are usually formulated in terms of Lagrangians that involve first derivatives of fields to be varied over only.
The GHY term is desirable, as it possesses a number of other key features. When passing to the hamiltonian formalism, it is necessary to include the GHY term in order to reproduce the correct Arnowitt-Deser-Misner energy (ADM energy). The term is required to ensure the path integral (a la Hawking) for quantum gravity has the correct composition properties. When calculating black hole entropy using the euclidean semiclassical approach, the entire contribution comes from the GHY term. This term has had more recent applications in loop quantum gravity in calculating transition amplitudes and background-independent scattering amplitudes.
In order to a finite value for the action, we may have to subtract off a surface term for flat spacetime:
where is the extrinsic curvature of the boundary imbedded flat spacetime. As is invariant under variations of this addition term does not effect the field equations, as such this is referred to as the non-dynamical term.
We start with the family of hyper-surfaces given by
where different members of the family correspond to different values of the constant . Consider two neighbouring points and with coordinates and , respectively, lying in the same hyper-surface. We then have to first order
Subtracting off from this equation gives
at . This implies that is normal to the hyper-surface. A unit normal can be introduced in the case where the hyper-surface is not null. This is defined by
and we require that point in the direction of increasing . It can then easily be checked that is given by
where is the unit normal to and , and are coordinates on the boundary. And where where , is an invariant three-dimensional volume element on the hyper-surface. In our particular case we take .
We now evaluate on the boundary , keeping in mind that on , . Taking this into account we have
It is useful to note that
where in the second line we have swapped around and and used that the metric is symmetric. It is then not difficult to work out .
where in the second line we used the identity , and in the third line we have used the anti-symmetry in and . As vanishes everywhere on the boundary, , its tangential derivatives must also vanish: . It follows that . So finally we have
Gathering the results we obtain
We next show that the above boundary term will be cancelled by the variation of .
in the gravitational action. As already mentioned above, because this term only depends on , its variation with respect to gives zero and so does not effect the field equations, its purpose is to change the numerical value of the action. As such we will refer to it as the non-dynamical term.
Let us assume that is a solution of the vacuum field equations, in which case the Ricci scalar vanishes. The numerical value of the gravitational action is then
where we are ignoring the non-dynamical term for the moment. Let us evaluate this for flat spacetime. Choose the boundary to consist of two hyper-surfaces of constant time value and a large three-cylinder at (that is, the product of a finite interval and a three-sphere of radius ). We have on the hyper-surfaces of constant time. On the three cylinder, in coordinates intrinsic to the hyper-surface, the line element is
meaning the induced metric is
so that . The unit normal is , so . Then
and diverges as , that is, when the spatial boundary is pushed to infinity, even when the is bounded by two hyper-surfaces of constant time. One would expect the same problem for curved spacetimes that are asymptotically flat (there is no problem if the spacetime is compact). This problem is remedied by the non-dynamical term. The difference will be well defined in the limit .
There are many theories which attempt to modify General Relativity in different ways, for example f(R) gravity replaces R, the Ricci scalar in the Einstein-Hilbert action with a function f(R). Guarnizo et al. found the boundary term for a general f(R) theory. They found that "the modified action in the metric formalism of f(R) gravity plus a Gibbons- York-Hawking like boundary term must be written as:
By using the ADM decomposition and introducing extra auxiliary fields, in 2009 Deruelle et al. found a method to find the boundary term for "gravity theories whose Lagrangian is an arbitrary function of the Riemann tensor."
As mentioned at the beginning, the GHY term is required to ensure the path integral (a la Hawking et al.) for quantum gravity has the correct composition properties.
This older approach to path-integral quantum gravity had a number of difficulties and unsolved problems. The starting point in this approach is Feynman's idea that one can represent the amplitude
to go from the state with metric and matter fields on a surface to a state with metric and matter fields on a surface , as a sum over all field configurations and which take the boundary values of the fields on the surfaces and . We write
where is a measure on the space of all field configurations and , is the action of the fields, and the integral is taken over all fields which have the given values on and .
It is argued that one need only specify the three-dimensional induced metric on the boundary.
Now consider the situation where one makes the transition from metric , on a surface , to a metric , on a surface and then on to a metric on a later surface
One would like to have the usual composition rule
expressing that the amplitude to go from the initial to final state to be obtained by summing over all states on the intermediate surface .
Let be the metric between and and be the metric between and . Although the induced metric of and will agree on , the normal derivative of at will not in general be equal to that of at . Taking the implications of this into account, it can then be shown that the composition rule will hold if and only if we include the GHY boundary term.
In the next section it is demonstrated how this path integral approach to quantum gravity leads to the concept of black hole temperature and intrinsic quantum mechanical entropy.
Calculating black hole entropy using the euclidean semiclassical approach
Transition amplitudes and the Hamilton's principal function
In the quantum theory, the object that corresponds to the Hamilton's principal function is the transition amplitude. Consider gravity defined on a compact region of spacetime, with the topology of a four dimensional ball. The boundary of this region is a three-dimensional space with the topology of a three-sphere, which we call . In pure gravity without cosmological constant, since the Ricci scalar vanishes on solutions of Einstein's equations, the bulk action vanishes and the Hamilton's principal function is given entirely in terms of the boundary term,
where is the extrinsic curvature of the boundary, is the three-metric induced on the boundary, and are coordinates on the boundary.
The functional is a highly non-trivial functional to compute; this is because the extrinsic curvature is determined by the bulk solution singled out by the boundary intrinsic geometry. As such is non-local. Knowing the general dependence of from is equivalent to knowing the general solution of the Einstein equations.
Loop quantum gravity is formulated in a background-independent language. No spacetime is assumed a priori, but rather it is built up by the states of theory themselves - however scattering amplitudes are derived from -point functions (Correlation function (quantum field theory)) and these, formulated in conventional quantum field theory, are functions of points of a background space-time. The relation between the background-independent formalism and the conventional formalism of quantum field theory on a given spacetime is far from obvious, and it is far from obvious how to recover low-energy quantities from the full background-independent theory. One would like to derive the -point functions of the theory from the background-independent formalism, in order to compare them with the standard perturbative expansion of quantum general relativity and therefore check that loop quantum gravity yields the correct low-energy limit.
A strategy for addressing this problem has been suggested; the idea is to study the boundary amplitude, or transition amplitude of a compact region of spacetime, namely a path integral over a finite space-time region, seen as a function of the boundary value of the field. In conventional quantum field theory, this boundary amplitude is well–defined and codes the physical information of the theory; it does so in quantum gravity as well, but in a fully background–independent manner. A generally covariant definition of -point functions can then be based on the idea that the distance between physical points –arguments of the -point function is determined by the state of the gravitational field on the boundary of the spacetime region considered.
The key observation is that in gravity the boundary data include the gravitational field, hence the geometry of the boundary, hence all relevant relative distances and time separations. In other words, the boundary formulation realizes very elegantly in the quantum context the complete identification between spacetime geometry and dynamical fields.
^Guarnizo, Alejandro; Castaneda, Leonardo; Tejeiro, Juan M. (2010). "Boundary Term in Metric f(R) Gravity: Field Equations in the Metric Formalism". General Relativity and Gravitation. 42 (11): 2713–2728. arXiv:1002.0617. doi:10.1007/s10714-010-1012-6.
^Deruelle, Nathalie; Sasaki, Misao; Sendouda, Yuuiti; Yamauchi, Daisuke (2009). "Hamiltonian formulation of f(Riemann) theories of gravity". Progress of Theoretical Physics. 123: 169–185. arXiv:0908.0679. doi:10.1143/PTP.123.169.
^For example see the book "Hawking on the big bang and black holes" by Stephen Hawking, chapter 15.
^L. Modesto, C. Rovelli:Particle scattering in loop quantum gravity, Phys Rev Lett 95 (2005) 191301
^R Oeckl, A ‘general boundary’ formulation for quantum mechanics and quantum gravity, Phys Lett B575 (2003) 318-324 ; Schrodinger's cat and the clock: lessons for quantum gravity, Class Quant Grav 20 (2003) 5371-5380l
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^F. Conrady, L. Doplicher, R. Oeckl, C. Rovelli, M. Testa, Minkowski vacuum in background independent quantum gravity, Phys Rev D69 (2004) 064019.