In general relativity, the Gibbons–Hawking–York boundary term is a term that needs to be added to the Einstein–Hilbert action when the underlying spacetime manifold has a boundary.
Contents
- Defining hyper surfaces
- Hyper surface orthogonal vector fields
- Induced and transverse metric
- On Proving the main result
- Variation of the Einstein Hilbert term
- Variation of the boundary term
- The non dynamical term
- Variation of modified gravity terms
- A path integral approach to quantum gravity
- Transition amplitudes and the Hamiltons principal function
- Background independent scattering amplitudes
- References
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
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
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
That a boundary term is needed in the gravitational case is due to the fact that
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
Defining hyper-surfaces
In a four-dimensional spacetime manifold, a hypersurface is a three-dimensional submanifold that can be either timelike, spacelike, or null.
A particular hyper-surface
or by giving parametric equations,
where
For example, a two-sphere in three-dimensional Euclidean space can be described either by
where
where
Hyper-surface orthogonal vector fields
We start with the family of hyper-surfaces given by
where different members of the family correspond to different values of the constant
Subtracting off
at
and we require that
if the hyper-surface either spacelike or timelke.
Induced and transverse metric
The three vectors
are tangential to the hyper-surface.
The induced metric is the three-tensor
This acts as a metric tensor on the hyper-surface in the
Because the three vectors
where
We introduce what is called the transverse metric
It isolates the part of the metric that is transverse to the normal
It is easily seen that this four-tensor
projects out the part of a four-vector transverse to the normal
We have
If we define
where
Note that variation subject to the condition
implies that
On Proving the main result
In the following subsections we will first compute the variation of the Einstein-Hilbert term and then the variation of the boundary term, and show that their sum results in
where
where
In the third subsection we elaborate on the meaning of the non-dynamical term.
Variation of the Einstein-Hilbert term
We will use the identity
and the Palatini identity:
which are both obtained in the article Einstein-Hilbert action.
We consider the variation of the Einstein-Hilbert term:
The first term gives us what we need for the left-hand side of the Einstein field equations. We must account for the second term.
By the Palatini identity
We will need Stokes theorem in the form:
where
We now evaluate
It is useful to note that
where in the second line we have swapped around
So now
where in the second line we used the identity
Gathering the results we obtain
We next show that the above boundary term will be cancelled by the variation of
Variation of the boundary term
We now turn to the variation of the
We have
where we have used that
where we have use the fact that the tangential derivatives of
which cancels the second integral on the right-hand side of
This produces the correct left-hand side of the Einstein equations. This proves the main result.
The non-dynamical term
We elaborate on the role of
in the gravitational action. As already mentioned above, because this term only depends on
Let us assume that
where we are ignoring the non-dynamical term for the moment. Let us evaluate this for flat spacetime. Choose the boundary
meaning the induced metric is
so that
and diverges as
Variation of modified gravity terms
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:
where
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."
A path integral approach to quantum gravity
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
where
It is argued that one need only specify the three-dimensional induced metric
Now consider the situation where one makes the transition from metric
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
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.
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
where
The functional
Background-independent scattering amplitudes
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
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
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.