The description of gravitational waves in geometric scalar gravity

It is investigated the gravitational waves phenomena in the geometric scalar theory of gravity (GSG), a class of theories such that gravity is described by a single scalar field. The associated physical metric describing the spacetime is constructed from a disformal transformation of Minkowski geometry. In this theory, a weak field approximation gives rise to a description similar to that one obtained in general relativity, although the gravitational waves in GSG have a characteristic longitudinal polarization mode, besides others modes that are observer dependent. We also analyze the energy carried by the gravitational waves as well as how their emission affects the orbital period of a binary system.


I. INTRODUCTION
Although general relativity (GR) has been a very successful gravitational theory during the last century, many proposals for modification of Einstein original formulation appeared in the literature over the past decades. Most of these ideas come up within the cosmological scenario, where GR only works if unknown components, like dark matter or dark energy, are introduced. Such alternative descriptions are basically variations of Einstein's theory, either assuming most general Lagrangians for the gravitational field or adding new fields together with the metric.
Unlike those variations of GR, it was recently proposed a theory of gravity in the realm of purely scalar theories, introducing some crucial modifications from the previous attempts that took place before the emergence of GR [1]. It represents the gravitational field with a single scalar function Φ, that obeys a non-linear dynamics. 1 Interaction with matter fields is given only trough a minimal coupling to the physical metric q µν , constructed from a disformal transformation of a auxiliary and flat metric γ µν , namely with, and the short notation ∂ µ = ∂/∂x µ . A complete theory can only be set if one defines the functions A and B, and also the Lagrangian of the scalar field. Then, a field equation, characterizing the theory, can be derived. We refer to this class of gravitational theories as geometric scalar gravity (GSG).
In early communications on GSG, it was explored a specific set of those functions defining the theory, which shows that it is possible to go further in representing the gravitational field as a single scalar, giving realistic descriptions of the solar system and cosmology [1,2]. An analysis of GSG within the so called parametrized post-Newtonian formalism was also made and, although the theory is not covered by the formalism, a limited situation indicate a good agreement with the observations [3]. Intending to improve the understanding of how GSG deals with gravitational interaction, the present work develops the theoretical description and characterization of gravitational waves (GW).
The direct detections of GW by LIGO and Virgo collaborations initiated a new era of testing gravitational theories. It enables to construct constraints over a series of theoretical mechanisms associated with GW's physics, but the crucial point relies on the observed waveform and how a theory can reproduce it [4]. Notwithstanding, this is not the scope of this work. We are mainly focused in analyzing the GW fundamentals on the perspective of GSG, studying their propagation, polarization modes and defining an appropriated tensor to describe the energy and momentum carried by the waves. As an example, we analyze the binary system, deriving an expression for the orbital variation that should be caused by the loss of energy due to gravitational radiation. An expected confrontation with observational data from binary pulsars is still not possible since a post-Keplerian analysis is needed in order to determine the masses of the system components according to GSG. This is a distinct treatment of what is present here and, therefore, is left to a future work.
The paper is organized as follows. In section II is presented a brief overview of GSG in order to introduce to the reader the main features of this theory. The following section describes the theory's weak field approximation. In section IV the study of the propagation and vibration modes associated to gravitational waves is made. The definition of a energymomentum tensor for the linear waves is treated in section V. Generation of waves, including the computation of the orbital variation of binary systems due to the emission of GW, is discussed in section VI and the last section presents our concluding remarks. Also, two appendices were introduced in order to complement the middle steps of calculations present in section VI.

II. OVERVIEW OF GEOMETRIC SCALAR GRAVITY
GSG is a class of gravitational theories which identifies the gravitational field to a single real scalar function Φ, satisfying a non-linear dynamic described by the action where γ is the determinant of the Minkowski metric and w is defined in eq. (2). Metric signature convention is (+, −, −, −). The physical metric is constructed from the gravitational field according to the expression (1) and its contravariant form is obtained from the definition of an inverse metric, q µα q αν = δ µ ν , namely, where, In order to describe the interaction of the scalar gravitational field with matter, GSG makes the fundamental hypothesis, according to Einstein's proposal, that gravity is a geometric phenomenon. Thus, it is assumed that the interaction with Φ is given only through a minimal coupling with the gravitational metric q µν . The matter action in GSG is then described as A complete theory should specify the metric's functions A and B together with the Lagrangian of the scalar field L, in order to be possible to derive its field equation. Up to now in the literature, it has been explored the case in which the following choice is made, Using the standard definition of the energy momentum tensor in terms of a metric structure, we set Then, the dynamics of the scalar field is described by the equation where the indicates the d'Alembertian operator constructed with the physical metric q µν , κ is a coupling constant and the source term χ is provided by where ' ; ' indicates a covariant derivative with respect to the physical metric, V = dV /dΦ and The choices made in (7)- (9) are such that the resulting theory satisfies the Newtonian limit, the classical gravitational tests and the spherically symmetric vacuum solution is given by the Schwarzschild geometry. Moreover, in the absence of any matter fields, Φ is a free wave propagating in the metric q µν [5]. More details concerning the fundamentals of GSG and how this specific model can successfully describe the solar system physics and cosmology can be found in [1][2][3]. In the present work we will consider only this model. To work with different functions α, β and L, all the process of constructing the field equation of the theory has to be redone, as well as it should be checked the feasibility of the new theory.

III. WEAK FIELD APPROXIMATION
To discuss linear gravitational waves we should consider an isolated system, distant from any source, embedded in a homogenous and isotropic universe. At a particular moment of time and specific distance from the isolated system, the background metric can be transformed to assume a flat Minkowskian form, resulting in a geometric structure given by, where η µν = diag(1, −1, −1, −1) and h µν represents the first order perturbations.
In this sense, the weak-field approximation of GSG consists in a small deviation of a cosmological solution φ 0 . Thus, we set In order to construct the geometric structure as in (18), for simplicity, we start with a coordinate systemx µ , where the auxiliary metric γ µν assumes the usual diagonal form indicated as η µν , and we expand the kinetic term and the metric coefficients as follows, The subindex " 0 " identifies quantities constructed with φ 0 according to basic expressions given in the previous section. The gravitational metric takes the form where,q and,h With the following coordinate transformation the desired structure is achieved, where In this new coordinate system, x 0 is equivalente to the cosmological time and where H 0 is the Hubble parameter (please see [2] for more details on GSG cosmology). 2 Then, the perturbed metric becomes, The corresponding covariant expression for (27) is obtained from the definition q µα q αν = δ µ ν . It reads where, Equations (27) and (30) shows that in the weak field limit the indices are lowered and raised by the Minkowski background metric.
Note that, the perturbed metric (29) can also be derived from the expansion of the exact form given in (4) starting already with the coordinates x µ , where the auxiliary metric γ µν takes the form The resulting covariant expression can be written as with In reference [3] a distinct weak field approximation was made where the scalar field was expanded around a vanishing background value. Although consistent, that scheme is not suitable for the description of GW, due to a term ∂ µ φ∂ ν φ/w that is present in h µν .
Oscillatory solutions would then lead to a singular behavior of the metric, evidencing that the background cosmological scenario can not be neglected.

A. The cosmological backgroung
Before proceeding in the analysis of GW in GSG, let us clarify important points of the cosmological background described by φ 0 . To a more detailed discussion about the cosmology in GSG we refer to the reader the analysis present in [2]. By considering the scalar field as a function of coordinatex 0 only, the metric arising is of Friedman-Robertson-Walker type with a flat spatial section. The cosmological time is achieved by the time transformation given by the first expression in (26) and the scale factor, said a 0 (x 0 ), is related with the φ 0 as follows, The dynamical equation (12) contains two regimes classified by the term a consequence of the particular choice of the scalar field Lagrangian. The case where α 0 < , with a barotropic fluid as source, describes a eternal universe without singularities. The universe has a bouncing, followed by a early accelerated phase and a final decelerated expansion. The problematic value α 0 = 3 is unattainable, in other words, the minimal value of the scale factor a 0 is always greater than 1/ √ 3. A distinct behavior occurs for the solutions with barotropic fluids in the region where α 0 > 3 (a 0 < 1/ √ 3); the universe starts from a initial singularity, it expands to a certain maximum value of the scale factor, smaller than 1/ √ 3, and then returns to a final singular point. This two regions are then disjoint classes of cosmological solutions. In the present work, we will consider only the case since it represents a class of more realistic descriptions of the universe.

IV. PROPAGATION AND POLARIZATION OF GRAVITATIONAL WAVES
At the level of the dynamical equation we can consider φ 0 as a constant, since its timescale variation is longer compared to the dynamical timescale for the local system. Expanding the left hand side of Eq. (12) and neglecting second order terms, one has, where we refer to Minkowskian d'Alembertian operator as η . Thus, without the presence of sources, one has The perturbed scalar field has oscillatory solutions which propagates at the speed of light.
Once the metric is constructed with the field and its first derivatives, such solutions yields oscillations as GW in the geometric structure of the spacetime. Moreover, it is verified that thus gravitational waves in GSG propagates with the speed of light.

A. Polarization states
The most general (weak) gravitational wave that any metric theory of gravity is able to predict can contain six modes of polarization. Considering plane null waves propagating in a given direction, these modes are related to tetrad components of the irreducible parts of the Riemann tensor, or the Newmann-Penrose quantities (NPQ): Ψ 2 , Ψ 3 , Ψ 4 and Φ 22 (Ψ 3 and Ψ 4 are complex quantities and each one represents two modes of polarization) [6]. The others NPQ are negligible by the weak field approximation, or are described in terms of these four ones.
The linearized dynamical equations of a gravitational theory can automatically vanish some of these NPQ, specifying then the predicted number of polarization states. For instance, in GR only Ψ 4 is not identically zero, which characterizes two transversal polarization modes, called "+" and "×" states. In general, the six polarization modes can not be specified in a observer-independent way because of their behavior under Lorentz transformations. Nevertheless, if we restrict our attention to a set of specific observers, which agree with the GW on the frequency and on the direction of propagation, then is possible to make some observer-invariant statements about the NPQ. Such assertions are on the basis of the so called E(2)-classification of gravitational theories, introduced in ref. [7]: • Class II 6 : If Ψ 2 = 0 , all the standard observers agree with the same nonzero Ψ 2 mode, but the presence or absence of the other modes is observer-dependent.
• Class III 5 : If Ψ 2 = 0 and Ψ 3 = 0 , all the standard observers measure the absence of Ψ 2 and the presence of Ψ 3 , but the presence or absence of all other modes is observer dependent.
The E(2)-classification of GSG is easily obtained by noticing that the Ricci scalar is not identically null. Actually, from the weak field approximation, one has with h = η µν h µν and, using relations (29) together with linearized vacuum field equation This result implies Ψ 2 = 0 (cf. equation (A4) of [7]) and GSG is from the class II 6 . This Thus, the description of GW by GSG carries a substancial distinction from GR, as it predicts the presence of a longitudinal polarization mode. Up to now, the recent detections of GW can not exclude the existence of any one of the six modes of polarization [11]. But in the future, with the appropriated network of detectors, with different orientations, this information can be used to restrict gravitational theories.

V. ENERGY OF THE GRAVITATIONAL WAVE
In order to associate an energy-momentum tensor to the gravitational waves in GSG we follow a standard procedure, identifying the relation between the second and the first order perturbations of the gravitational field [12]. First note that, without approximations, the following relation holds, Thus, taking φ ≈ φ (1) + φ (2) , where the subindexes indicates the order, and computing the second order vacuum field equation, it yields with w (2) = η µν ∂ µ φ (1) ∂ ν φ (1) . The right hand side of this equation contains only the derivatives of the first order field φ (1) , thus it can be interpreted as the source for the second order field generated by the linear waves.
From the general structure of the field equation of GSG, the influence of any energymomentum tensor enters in the equation of motion uniquely through the quantity χ [cf.
equation (13)]. Thus, the energy-momentum tensor of the GW, said Θ µν , must be consistent with, where χ (2) (Θ µν ) means the second order approximation of the source term calculated with the energy-momentum tensor of the gravitational field Θ µν , instead of T µν . Therefore, we which has the same general structure of GSG's field equation.
To describe the energy and momentum carried by the linear waves, the second-order approximation of Θ µν must be quadratic in the first derivatives of φ (1) . This lead us to a specific form for it, with σ and λ being arbitrary constants. The condition (44) returns the relation Any tensor, described like in Eq. (46) and satisfying the above relation, can be used as the energy-momentum tensor of the linear GW in GSG. This ambiguity already appeared in reference [13], where the authors show how to construct the energy-momentum tensor of the gravitational field in GSG, without using approximate methods. In that occasion, they fixed the functions defining the energy tensor by requiring that Θ µν can be derived from the Lagrangian. As expected, their results are consistent with the relation above and are recovered (inside the approximation method) if σ = 2 In what follows we will proceed with the generic expression for Θ µν and look for a specific example, the orbital variations in binary systems, to see how this ambiguity can influence in a observed phenomenon.

VI. ORBITAL VARIATION OF A BINARY SYSTEM
This section focus on deriving an expression for the orbital variation of a binary system, due to the emission of gravitational waves, as it is predicted by GSG. In order to obtain the energy rate emitted by the system one should consider the influence of the source into the dynamics in the linear approximation. Since the left hand side of Eq. (12) reduces to a Minkowskian d'Alembertian when linearized (c.f. eq.(37)), from the method of Green functions, we immediately write down the general solution as, where χ L attends to the first order approximation of the source term [cf. (13)] and t r = t − r/c, with r = | x − x |, is the retarded time.
By considering that the source is far away from the point where we calculate the scalar field (R r , where R = | x|, and r = | x | is the typical distances between the source's components), it is possible to make a multipole expansion 3 . Further assuming that the typical velocities of the source components are non relativistic, it is also possible to expand the time dependent terms of the integrand in a Taylor series. For our purpose here it is sufficient to take only the first term of this expansion. Thus, one has where t R = t − R/c and we have neglected terms of order 1/R 2 .
Most of terms in the above integration contains the scalar field φ , explicitly. To solve them, we have to expand these terms using the correspondent post-Newtonian approximation of the field in the near-zone region [14]. However, to keep the final result up to order G 2 /c 4 , it is only necessary the Newtonian approximation of the near-zone scalar field, namely Φ N .
By the viral theorem, we know that, for slow motions, v 2 ∼ GM/R, where v, M and R are the typical velocity, mass and distances in the source's components, respectively. Thus, The energy-momentum tensor also depends on source velocities with T 0i ∼ v/c and T ij ∼ v 2 /c 2 .
Thus, keeping terms up to order v 2 /c 2 (since κ ∼ v 2 /c 2 ) and using the perturbed metric expressions in (29), one gets The C i ,i term does not contribute by Gauss law. Also, to derive the above expressions we take into account that Specifying the source for the case of a binary system, we have where summation is over the two particles of the system, i.e. n = 1, 2. With these expressions, all the integrals in (49) can be analytically calculated (more details in Appendix where the dot indicates a derivative with respect to retarded time, C attends to constant terms that does not contribute to the radiation, G is the Newtonian gravitational constant as measured today (see Appendix A) and the notation was shortened by the definitions below, Also, we are adopting the usual center of mass notation such that, with r = r 1 − r 2 and M = m 1 + m 2 .
Once we are dealing with a binary system as the source of the gravitational field, we can use the Keplerian orbital parameters to simplify the above expression [15]. The distance between the two masses are, where a is the semimajor axis and e is the eccentricity of the orbit. They are related with the total energy E and the angular momentum L by with E < 0. The fact that L = (m 1 m 2 /M )r 2θ , allow us to derive the following relation, Then, in (58), all time derivatives can be expressed in terms of θ, yielding To calculate the energy-flux that is carried off by GW we use the gravitational energymomentum tensor presented in the previous section. The flux in the radial direction will be c Θ 0r thus, the energy radiated per unit time that is passing through a sphere of radius R, is given by where we have used the fact that At this point, we go further in the approximation scheme in order to get a more treatable expression for the rate of energy loss. Let us consider that the background field is too small, i.e. φ 0 1, and take only the leading order terms. This is realistic since it is always expected that the cosmological influence on local systems are minimal. Expression (66) can be then simplified, reading where f is given by Averaging the energy loss over an orbital period T , where we have, The above integral is directly solved, yielding with To finish, we derive how this loss of energy changes the orbital period of the system. From The result has the same proportionality with the constants G and c, as in GR, but has a rather more involved dependence on the masses and the eccentricity of the orbit.
Note that equation (74) must be negative, otherwise it would imply that the masses are moving away from each other. In other words, the system would be increasing their energy by the emission of GW, an unrealistic situation. The function F is positive, as it can be verified by comparison between the term f 2 and the part involved by the round brackets multiplying e 2 (the only part that could be negative), Since e 2 < 1 for elliptical orbits, it follows that F is always positive. Thus to guaranteė T < 0, we must have λ < 0.
The Keplerian and post-Keplerian orbital parameters of a binary system can be extracted from the timing pulsar observation in a theory-independent way, but the determination of the masses of the pulsar and its companion is only obtained by making use of specific equations relating them to that set of parameters. These relations are particular for each gravitational theory [16]. Thus, a confrontation between the orbital variation of a binary system, as predicted by GSG, and the observational data is only possible after obtaining the so called post-Keplerian parametrization of the theory to extract the mass values according to GSG. 4 We will leave this task to be addressed in a future work.

VII. CONCLUDING REMARKS
We have presented a discussion on gravitational waves (GW) in the context of the geometric scalar gravity (GSG), a class of theories describing the effects of gravity as a consequence of a modification of spacetime metric in terms of a single scalar field. GSG overcomes the serious drawbacks present in all previous attempts to formulate a scalar theory of gravity. Its fundamental idea rests on the proposal that the geometrical structure of the spacetime is described by a disformal transformation of a conformal flat metric. The model analyzed here has already showed several advances within the scalar gravity program, featuring a good representation of the gravitational phenomena both in the solar system domains as well as in cosmology.
Initially it was shown the procedure used to construct the weak field limit in GSG considering an expansion of the scalar field over a background cosmological solution. Within this approximation scheme it was shown that the scalar dynamical equations assumes oscillatory solutions that represent GW in the spacetime structure propagating with light velocity. An important distinction appears in the polarization states of the waves, which is characterized by the presence of a longitudinal mode in GSG. Within the E(2)-classification of gravitational theories, GSG is of the type II 6 , since Ψ 2 = 0. This is the most general class, where the detection of all the other five polarization modes depends on the observer.
It was also discussed how to define an energy-momentum tensor for the linear GW following a field theoretical point of view. An ambiguity emerges since GSG fundamental equation includes a non trivial interaction between matter/energy and the scalar field, leading to non unique expression for the approximated gravitational energy-momentum tensor. As an example fo application of the previous results, an expression for the orbital variation of a binary system, due to the emission of GW, was derived and, in principle, it could be useful to provide the more appropriate energy-momentum tensor for the gravitational waves. However, a confrontation with observational data coming from pulsars can only be performed in posses of the post-Keplerian parameters as predicted by GSG, in order to determine the masses of a binary system components. We hope to come back to this question in the near future.
ACKNOWLEDGMENTS I wish to thank J.C. Fabris and T.R.P. Caramês for dedicated reviews of this work. This research is supported by FAPES and CAPES through the PROFIX program.
influent in determining the metric. On the other hand, in the region where R λ, called near zone, the difference between the t R and t are small and the time derivatives becomes irrelevant in front of the spatial derivatives.
The near zone region is covered by the post-Newtonian approximation of the gravitational field, expanding it in orders of v/c, where v is the typical velocities of the source's components, and considering also slow motion. This is the approximation required for the scalar field when integrating the wave equation. Once the scalar field aways appears multiplied by T µν in the integrand, we only need to know its leading order, i.e. its Newtonian approximation. Thus, equation (48) reduces to where only the zeroth-order terms is considered in the above integral.
From (29) and (30), and using a multipole expansion, one can easily sees that the metric assumes the form where M = 1 c 2 T 00 d 3 x (A3) and G attends for the Newton's gravitational constant as measured today and it is a redefinition of the theory's coupling constant, Note that the dependence of G with the cosmological background field implies its change as a result of the evolution of the universe. This effect has not been evident in the previous analysis of GSG's newtonian limit since the cosmological influence was neglect in those works [1,3]. We will not discuss its implications in the present work, but it certainly shows the importance of take into account the cosmological background when analyzing the Newtonian and post-Newtonian limits of GSG.

Appendix B: More detailed calculations
In this section we aim to be more clear on the calculation of the integrals of the quantities appearing in expressions (51), (52) and (53). We start with the linearized conservation law, ∂ µ T µν = 0 , from where is possible to derive the following expressions,