### 1.1 Gravitational-wave generation formalisms

The basic problem we face is to relate the asymptotic gravitational-wave form generated by some isolated source, at the location of some detector in the wave zone of the source, to the stress-energy tensor of the matter fields. For general sources it is hopeless to solve the problem via a rigorous deduction within the exact theory of general relativity, and we have to resort to approximation methods, keeping in mind that, sadly, such methods are often not related in a very precise mathematical way to the first principles of the theory. Therefore, a general wave-generation formalism must somehow manage the non-linearity of the field equations by imposing some suitable approximation series in one or several small physical parameters. Of ourse the ultimate aim of approximation methods is to extract from the theory some firm predictions for the outcome of experiments such as VIRGO and LIGO. Some important approximations that we shall use in this article are the post-Newtonian method (or non-linear -expansion), the post-Minkowskian method or non-linear iteration (-expansion), the multipole decomposition in irreducible representations of the rotation group (or equivalently -expansion in the source radius), and the far-zone expansion (-expansion in the distance). In particular, the post-Newtonian expansion has provided us in the past with our best insights into the problems of motion and radiation in general relativity. The most successful wave-generation formalisms make a gourmet cocktail of all these approximation methods. For reviews on analytic approximations and applications to the motion and the gravitational wave-generation see Refs. [1435354144150813]. The post-Newtonian approximation is valid under the assumptions of a weak gravitational field inside the source (we shall see later how to model neutron stars and black holes), and of slow internal motions. The main problem with this approximation is its domain of validity, which is limited to the near zone of the source - the region surrounding the source that is of small extent with respect to the wavelength of waves. A serious consequence is the a priori inability of the post-Newtonian expansion to incorporate the boundary conditions at infinity, which determine the radiation reaction force in the source’s local equations of motion. The post-Minkowskian expansion, by contrast, is uniformly valid, as soon as the source is weakly self-gravitating, over all space-time. In a sense, the post-Minkowskian method is more fundamental than the post-Newtonian one; it can be regarded as an “upstream” approximation with respect to the post-Newtonian expansion, because each coefficient of the post-Minkowskian series can in turn be re-expanded in a post-Newtonian fashion. Therefore, a way to take into account the boundary conditions at infinity in the post-Newtonian series is first to perform the post-Minkowskian expansion. Notice that the post-Minkowskian method is also upstream (in the previous sense) with respect to the multipole expansion, when considered outside the source, and with respect to the far-zone expansion, when considered far from the source.

The most “downstream” approximation that we shall use in this article is the post-Newtonian one; therefore this is the approximation that dictates the allowed physical properties of our matter source. We assume mainly that the source is at once slowly moving and weakly stressed, and we abbreviate this by saying that the source is post-Newtonian. For post-Newtonian sources, the parameter defined from the components of the matter stress-energy tensor and the source’s Newtonian potential by

is much less than one. This parameter represents essentially a slow motion estimate , where denotes a typical internal velocity. By a slight abuse of notation, following Chandrasekhar et al. [404241], we shall henceforth write , even though is dimensionless whereas has the dimension of a velocity. The small post-Newtonian remainders will be denoted . Thus, in the case of post-Newtonian sources. We have for sources with negligible self-gravity, and whose dynamics are therefore driven by non-gravitational forces. However, we shall generally assume that the source is self-gravitating; in that case we see that it is necessarily weakly (but not negligibly) self-gravitating, i.e. . Note that the adjective “slow-motion” is a bit clumsy because we shall in fact consider very relativistic sources such as inspiralling compact binaries, for which can be as large as in the last rotations, and whose description necessitates the control of high post-Newtonian approximations.

The lowest-order wave generation formalism, in the Newtonian limit , is the famous quadrupole formalism of Einstein [68] and Landau and Lifchitz [97]. This formalism can also be referred to as Newtonian because the evolution of the quadrupole moment of the source is computed using Newton’s laws of gravity. It expresses the gravitational field in a transverse and traceless (TT) coordinate system, covering the far zone of the source, as

where is the distance to the source, is the unit direction from the source to the observer, and is the TT projection operator, with being the projector onto the plane orthogonal to . The source’s quadrupole moment takes the familiar Newtonian form
where is the Newtonian mass density. The total gravitational power emitted by the source in all directions is given by the Einstein quadrupole formula
Our notation stands for the total gravitational “luminosity” of the source. The cardinal virtues of the Einstein-Landau-Lifchitz quadrupole formalism are its generality - the only restrictions are that the source be Newtonian and bounded - its simplicity, as it necessitates only the computation of the time derivatives of the Newtonian quadrupole moment (using the Newtonian laws of motion), and, most importantly, its agreement with the observation of the dynamics of the Hulse-Taylor binary pulsar PSR 1913+16 [140141139]. Indeed the prediction of the quadrupole formalism for the waves emitted by the binary pulsar system comes from applying Eq. (4) to a system of two point masses moving on an eccentric orbit (the classic reference is Peters and Mathews [117]; see also Refs. [71148]). Then, relying on the energy equation
where is the Newtonian binary’s center-of-mass energy, we deduce from Kepler’s third law the expression of the “observable”, that is, the change in the orbital period of the pulsar, or , as a function of itself. From the binary pulsar test, we can say that the post-Newtonian corrections to the quadrupole formalism, which we shall compute in this article, have already received, in the case of compact binaries, strong observational support (in addition to having, as we shall demonstrate, a sound theoretical basis). The multipole expansion is one of the most useful tools of physics, but its use in general relativity is difficult because of the non-linearity of the theory and the tensorial character of the gravitational interaction. In the stationary case, the multipole moments are determined by the expansion of the metric at spatial infinity, while, in the case of non-stationary fields, the moments, starting with the quadrupole, are defined at future null infinity. The multipole moments have been extensively studied in the linearized theory, which ignores the gravitational forces inside the source. Early studies have extended the formula (4) to include the current-quadrupole and mass-octupole moments [111110], and obtained the corresponding formulas for linear momentum [1111101124] and angular momentum [11646]. The general structure of the infinite multipole series in the linearized theory was investigated by several works [126127119142], from which it emerged that the expansion is characterized by two and only two sets of moments: mass-type and current-type moments. Below we shall use a particular multipole decomposition of the linearized (vacuum) metric, parametrized by symmetric and trace-free (STF) mass and current moments, as given by Thorne [142]. The explicit expressions of the multipole moments (for instance in STF guise) as integrals over the source, valid in the linearized theory but irrespective of a slow motion hypothesis, are completely known [101393857].

In the full non-linear theory, the (radiative) multipole moments can be read off the coefficient of in the expansion of the metric when , with a null coordinate . The solutions of the field equations in the form of a far-field expansion (power series in ) have been constructed, and their properties elucidated, by Bondi et al. [32] and Sachs [128]. The precise way under which such radiative space-times fall off asymptotically has been formulated geometrically by Penrose [114115] in the concept of an asymptotically simple space-time (see also Ref. [76]). The resulting Bondi-Sachs-Penrose approach is very powerful, but it can answer a priori only a part of the problem, because it gives information on the field only in the limit where , which cannot be connected in a direct way to the actual behaviour of the source. In particular the multipole moments that one considers in this approach are those measured at infinity - we call them the radiative multipole moments. These moments are distinct, because of non-linearities, from some more natural source multipole moments, which are defined operationally by means of explicit integrals extending over the matter and gravitational fields.

An alternative way of defining the multipole expansion within the complete non-linear theory is that of Blanchet and Damour [143], following pioneering work by Bonnor and collaborators [33343581] and Thorne [142]. In this approach the basic multipole moments are the source moments, rather than the radiative ones. In a first stage, the moments are left unspecified, as being some arbitrary functions of time, supposed to describe an actual physical source. They are iterated by means of a post-Minkowskian expansion of the vacuum field equations (valid in the source’s exterior). Technically, the post-Minkowskian approximation scheme is greatly simplified by the assumption of a multipolar expansion, as one can consider separately the iteration of the different multipole pieces composing the exterior field (whereas, the direct attack of the post-Minkowskian expansion, valid at once inside and outside the source, faces some calculational difficulties [14747]). In this “multipolar-post-Minkowskian” formalism, which is physically valid over the entire weak-field region outside the source, and in particular in the wave zone (up to future null infinity), the radiative multipole moments are obtained in the form of some non-linear functionals of the more basic source moments. A priori, the method is not limited to post-Newtonian sources, however we shall see that, in the current situation, the closed-form expressions of the source multipole moments can be established only in the case where the source is post-Newtonian [611]. The reason is that in this case the domain of validity of the post-Newtonian iteration (viz. the near zone) overlaps the exterior weak-field region, so that there exists an intermediate zone in which the post-Newtonian and multipolar expansions can be matched together. This is a standard application of the method of matched asymptotic expansions in general relativity [3736].