## 16 Gravitational Green’s functions

### 16.1 Equations of linearized gravity

We are given a background spacetime for which the metric satisfies the Einstein field equations in vacuum. We then perturb the metric from to

The metric perturbation is assumed to be small, and when working out the Einstein field equations to be satisfied by the new metric , we work consistently to first order in . To simplify the expressions we use the trace-reversed potentials defined by
and we impose the Lorenz gauge condition,
In this equation, and in all others below, indices are raised and lowered with the background metric . Similarly, the connection involved in Eq. (16.3), and in all other equations below, is the one that is compatible with the background metric. If is the perturbing energy-momentum tensor, then by virtue of the linearized Einstein field equations the perturbation field obeys the wave equation
in which is the wave operator and the Riemann tensor. In first-order perturbation theory, the energy-momentum tensor must be conserved in the background spacetime: .

The solution to the wave equation is written as

in terms of a Green’s function that satisfies [161]
where is a parallel propagator and an invariant Dirac functional. The parallel propagators are inserted on the right-hand side of Eq. (16.6) to keep the index structure of the equation consistent from side to side; in particular, both sides of the equation are symmetric in and , and in and . It is easy to check that by virtue of Eq. (16.6), the perturbation field of Eq. (16.5) satisfies the wave equation of Eq. (16.4). Once is known, the metric perturbation can be reconstructed from the relation .

We will assume that the retarded Green’s function , which is nonzero if is in the causal future of , and the advanced Green’s function , which is nonzero if is in the causal past of , exist as distributions and can be defined globally in the entire background spacetime.

### 16.2 Hadamard construction of the Green’s functions

Assuming throughout this subsection that is in the normal convex neighbourhood of , we make the ansatz

where , are the light-cone distributions introduced in Section 13.2, and where , are smooth bitensors.

To conveniently manipulate the Green’s functions we shift by a small positive quantity . The Green’s functions are then recovered by the taking the limit of

as . When we substitute this into the left-hand side of Eq. (16.6) and then take the limit, we obtain

after a routine computation similar to the one presented at the beginning of Section 14.2. Comparison with Eq. (16.6) returns: (i) the equations
and
that determine ; (ii) the equation
that determine , the restriction of on the light cone ; and (iii) the wave equation
that determines inside the light cone.

Eq. (16.9) can be integrated along the unique geodesic that links to . The initial conditions are provided by Eq. (16.8), and if we set , we find that these equations reduce to Eqs. (14.7) and (14.6), respectively. According to Eq. (14.8), then, we have

which reduces to
near coincidence, with denoting the affine-parameter distance between and ; there is no term of order because by assumption, the background Ricci tensor vanishes at (as it does in the entire spacetime). Differentiation of this relation gives
and eventually,
this last result follows from the fact that is antisymmetric in the last pair of indices.

Similarly, Eq. (16.10) can be integrated along each null geodesic that generates the null cone . The initial values are obtained by taking the coincidence limit of this equation, using Eqs. (16.8), (16.16), and the additional relation . We arrive at

With the characteristic data obtained by integrating Eq. (16.10), the wave equation of Eq. (16.11) admits a unique solution.

To summarize, the retarded and advanced gravitational Green’s functions are given by Eq. (16.7) with given by Eq. (16.12) and determined by Eq. (16.11) and the characteristic data constructed with Eqs. (16.10) and (16.17). It should be emphasized that the construction provided in this subsection is restricted to , the normal convex neighbourhood of the reference point .

### 16.3 Reciprocity and Kirchhoff representation

The (globally defined) gravitational Green’s functions satisfy the reciprocity relation

The derivation of this result is virtually identical to what was presented in Sections 14.3 and 15.3. A direct consequence of the reciprocity relation is the statement

The Kirchhoff representation for the trace-reversed gravitational perturbation is formulated as follows. Suppose that satisfies the homogeneous version of Eq. (16.4) and that initial values , are specified on a spacelike hypersurface . Then the value of the perturbation field at a point in the future of is given by

where is a surface element on ; is the future-directed unit normal and is the invariant volume element on the hypersurface. The derivation of Eq. (16.20) is virtually identical to what was presented in Sections 14.4 and 15.3.

### 16.4 Relation with electromagnetic and scalar Green’s functions

In a spacetime that satisfies the Einstein field equations in vacuum, so that everywhere in the spacetime, the (retarded and advanced) gravitational Green’s functions satisfy the identities [144]

and
where are the corresponding electromagnetic Green’s functions, and the corresponding scalar Green’s functions.

To prove Eq. (16.21) we differentiate Eq. (16.6) covariantly with respect to , use Eq. (13.3) to work on the right-hand side, and invoke Ricci’s identity to permute the ordering of the covariant derivatives on the left-hand side. After simplification and involvement of the Ricci-flat condition (which, together with the Bianchi identities, implies that ), we arrive at the equation

Because this is also the differential equation satisfied by , and because the solutions are chosen to satisfy the same boundary conditions, we have established the validity of Eq. (16.21).

The identity of Eq. (16.22) follows simply from the fact that and satisfy the same tensorial wave equation in a Ricci-flat spacetime.

### 16.5 Singular and regular Green’s functions

We shall now construct singular and regular Green’s functions for the linearized gravitational field. The treatment here parallels closely what was presented in Sections 14.5 and 15.5.

We begin by introducing the bitensor with properties

H1:
satisfies the homogeneous wave equation,
H2:
is symmetric in its indices and arguments,
H3:
agrees with the retarded Green’s function if is in the chronological future of ,
H4:
agrees with the advanced Green’s function if is in the chronological past of ,

It is easy to prove that property H4 follows from H2, H3, and the reciprocity relation (16.18) satisfied by the retarded and advanced Green’s functions. That such a bitensor exists can be argued along the same lines as those presented in Section 14.5.

Equipped with we define the singular Green’s function to be

This comes with the properties
S1:
satisfies the inhomogeneous wave equation,
S2:
is symmetric in its indices and arguments,
S3:
vanishes if is in the chronological past or future of ,

These can be established as consequences of H1 H4 and the properties of the retarded and advanced Green’s functions.

The regular two-point function is then defined by

and it comes with the properties
R1:
satisfies the homogeneous wave equation,
R2:
agrees with the retarded Green’s function if is in the chronological future of ,
R3:
vanishes if is in the chronological past of ,

Those follow immediately from S1 S3 and the properties of the retarded Green’s function.

When is restricted to the normal convex neighbourhood of , we have the explicit relations

From these we see clearly that the singular Green’s function does not distinguish between past and future (property S2), and that its support excludes (property S3). We see also that the regular two-point function coincides with in (property R2), and that its support does not include (property R3).