Extended Theories of Gravity have recently received increasingly attention in issues such as dark matter     and dark energy   . In particular, the theories of gravity have been suggested as a possible alternative to explain the late time cosmic speed-up experienced by our universe  . Such theories avoid the Ostrogradski’s instability that can otherwise prove to be problematic for general higher derivatives theories  . Besides, these theories could be probed by the recent LIGO detections  and by gravitational lensing observations .
On the other hand, it is well known that several types of topological defects may have been created by the vacuum phase transitions in the early universe  . In particular, cosmic strings have been extensively studied in many kinds of alternative gravity theories, notably as scalar-tensor and theories where many aspects and applications were developed    .
In this paper, our main purpose is to study the formation and evolution of wakes in cylindrically symmetric solutions in the framework of theories in vacuum. In particular, we aim to explore the propagation of particles and light in a cosmic string. We compare our results with the wakes formed by cosmic string solutions obtained in General Relativity and the Scalar-Tensor Theories of Gravity.
This paper is structured as follows. In Section 2, we briefly review the theories of gravity (for comprehensive reviews in gravity see  ). In Section 3, for the sake of completeness, we review the obtention of the cylindrically symmetric solutions in vacuum. We give special attention to the cosmic string solution. Is it important to stress that, in this section, we follow the reference  ? In Section 4, the formation and evolution of wakes in the background of a are studied and these are our original work. Finally, in the conclusion we summarize our main results and discuss some perspectives.
2. Gravity in the Metric Formalism—A Brief Review
In this section, we will mainly follow the references .
The action associated with the modified theories of gravity coupled with matter fields is given by:
where is an analytical function of the the Ricci scalar, R, and corresponds to the action associated with the matter fields. By using the metric formalism, the field equations become:
in which is the geometric energy-momentum tensor, namely
The standard minimally coupled energy-momentum tensor, , derived from the matter action, is related to by
Thus the field equations can be written as
Taking the trace of the above equation we get
which express a further scalar degree of freedom that arises in the modified theory. Through this equation it is possible to express in terms of its derivatives and the trace of the matter energy-momentum tensor, as follows
Substituting the above expression into (5) we obtain
From this expression we can see that the combination below
with fixed indices, is independent of the corresponding index. So, the following relation
holds for all and .
3. Vacuum Cylindrically Symmetric Solutions in Gravity: A Brief Review
In this section we review how to derive the field equations associated with cosmic string system in the context of modified theories of gravity following Azadi et al. .
The field equations in vacuum are given by
Taking the trace of Equation (11), we have
Now, since we are interested in obtaining static solutions with cylindrical symmetry in vacuum, we will work with a general metric in Weyl coordinates given by
where and are functions of r only.
The non-zero components of the Ricci tensor are:
where prime (') indicates derivative with respect to r. Therefore, the scalar curvature is
Replacing of Equation (12) in Equation (11), finally we get:
Defining (which is the equivalent of in the absence of matter) as
we can easily see that
which is a scalar quantity. This means that for any , which implies that we can replace Equation (11) by , and . After some straightforward calculation, we finally get, respectively
Therefore, any group of functions and which satisfies the equations above is a solution of the modified gravity equations in vacuum.
Since these equations are highly nonlinear, we will consider the particular case where . This is justified by the fact that we are interested in cosmic string solution and the external metric of a cosmic string is locally flat.
Field Equations Solutions for the Special Case R=0
Let us start deriving Equation (11) with respect to r,
In the particular case where R= constant, Equation (22) implies that . As a consequence, Equations (17-19) become
In order to solve this non-linear equations system, let us first sum up Equations (22) and (23). In doing this, we get
Defining in Equation (26), we get
which can be rewritten as
Integrating Equation (28), we obtain
where is a constant to be determined later.
Now, subtracting Equation (23) from Equation (22), we get
and we find
where is a constant to be determined later.
Replacing Equations (29) and (31) in Equation (21), we get a differential equation for the function , which is
In order to solve our equations, we will make the hypothesis that is a linear function of r. This is justified by cosmic string solutions in either General Relativity or Scalar-tensor gravities. Therefore, . Hence, we have
In order to satisfy Equation (32), the constants and must obey the following relations
We can easily see that the metric functions (33-35) satisfy Equation (32) in the particular case where .
Redefining the quantities and and making without any loss of generality, we can write down the metric in Weyl coordinates as
Defining the quantities and and defining new coordinates such as
In doing this, the metric reduces to a very simple form1
Applying the complex transformation and , we get a well known metric 
which, apart from the sigh is pretty much the same as the Levi-Civita static cylindrically symmetric solution in General Relativity .
When , the spacetime (41) becomes2
It is very easy to see that this spacetime is locally flat but not globally Euclidean. This spacetime is conical, as we can see in Figure 1, with a deficit angle equal to
as long as which imposes some constraints on the constants and .
4. Formation and Evolution of Wakes in the Spacetime of a Cosmic String
In this section we will study the formation and evolution of some structures when a cosmic string moves through a region containing baryonic matter.
4.1. The Formation of Wakes
Let us suppose that the cosmic string moves with a constant velocity in the x-axis. Since is constant in (42), the string does not exert any gravitational force on test particles. However, test particles do suffer a perturbation when passing through a cosmic string. To see that, let us consider that one is in a comoving frame in which the string is at rest and, by a Lorentz transformation, the baryonic matter is approaching the string with the velocity . From Figure 2, we can see how the velocity of the test particles are perturbed and we can calculate this perturbation. Of course, it depends on the deficit angle and, hence, on the parameters of the theory
where , considering .
In this way, particles which move in regions where may collide with particles which move in regions where after passing through the string and form stable structures called “wakes”, see Figure 3.
4.2. The Evolution of Wakes: The Zel’dovich Approximation
Let us now make a quantitative description of the accretion problem using the Zel’dovich approximation , which consists in considering the Newtonian
Figure 1. The Conical Spacetime.
Figure 2. (a) A cosmic string moving with constant velocity vc. (b) Particles moving with constant velocity vc in the comoving string frame.
Figure 3. Particles collision with impact parameters equal to R and R'.
accretion problem in an expanding Universe by means of the method of linear approximation.
Suppose that the wake is formed at , where is the time in which matter starts to dominate over radiation. The physical trajectory of a particle can be written as
where is the unperturbed comoving position and is the comoving displacement developed as a consequence of the gravitational attraction induced by the wake on the particle.
The equation of motion in the Newtonian limit is given by
where is the Newtonian potential which obeys the Poisson equation3
The matter density is determined in terms of the background density as 
where and are the radial components of these quantities.
Replacing Equation (49) in the Poisson Equation (47), we obtain
If we replace the relation in the equation above, we get the linearized equation for
Since we are working in the matter era, . Hence, Equation (51) becomes
The equation above is the well-known Euler equation. Applying appropriate conditions such as and the solution is
The comoving coordinate can be calculated using the fact that , which means that, eventually, the particle stops expanding with the Hubble flow and starts to collapse onto the wake. Therefore, we get
Hence, we are now able to compute the wake’s thickness and surface density 
Finally, we obtain
where , which means that the wake’s thickness and density depend on the parameters of the theory.
5. Final Remarks
In this work we considered cylindrically symmetric solutions in the framework of theory of gravity. These solutions were obtained in vacuum regime and in the particular case where . A cosmic string solution was of special interest and we studied the formation and evolution of wakes in this spacetime. Comparing our results with those obtained previously in the literature  , both in General Relativity and in Scalar-Tensor theories, respectively, we can see that they resemble the GR wakes instead of the scalar-tensor ones, as we would expect since is constant and there is no gravitational force exerted by the cosmic string in the same way as the GR cosmic string. However, for a precise comparison and further numerical evaluation, we must consider the internal cosmic string matter configuration because we need to determine the metric constants . In particular, they must obey the GUT cosmic string order of magnitude for all parameters.
As we expected, all wake’s physical quantities depended on the parameters of the particular theory of gravity under consideration. But, again, in order to make a quantitative evaluation, we need to consider not a vacuum solution but a full energy-momentum tensor for the internal cosmic string configuration in the same way as  . This is under consideration now.
As a future perspective of this work, we plan to study the cosmic string as a generator of the rotational curves in galaxies . This work will come as a forthcoming paper.
P. O. Mesquita and M. E. X. Guimarães would like to thank PIBIC/CNPq (Conselho Nacional de Desenvolvimento Cientfico e Tecnológico) for a support during the preparation of this work.
1The calculations are long but straightforward.
2We will suppress the symbol from now on because it will not cause any confusion.
3In order to avoid confusion, we have changed the radial coordinate for r instead of which represents now the matter density.
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