Abstract
The collapsing dynamics of relativistic fluid are explored in f(R) gravity in detailed systematic manner for the non-static spherically symmetric spacetime satisfying the equation of the conformal Killing vector. With quasi-homologous condition and diminishing complexity factor condition, exact solutions for dissipative as well as for non-dissipative system are found and the astrophysical applications of these exact solutions are discussed. Furthermore, it is demonstrated that \(f(R)=R,\) which is the extensive restriction of f(R) gravity, prior solutions of the collapsing fluid in general relativity, can be retrieved.
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1 Introduction
The evolution of the universe has always been an interesting topic for scientists. Different models were proposed by different scientists to explain the initial state, evolution, and final fate of our universe. After a tremendous explosion referred to as the “Big Bang” [1], our universe began as a incredible small dot that quickly began to expand due to its extreme density and heat. Many physicists accepted that the Big Bang theory, which describes how the universe grew from a denser, hotter childhood, accurately captured the nature of the universe [2,3,4]. The celestial objects, including planets, all shining matter, and stars, are made of matter (referred to as baryonic matter), which constitutes a small part of this dark cosmos, and other minor contributor is electromagnetic radiation. Dark energy, which has controlled the current energy balance, is more frequent than dark matter, which has not yet been found in a laboratory.
In the same way, celestial objects (stars, planets, and galaxies) also undergo changes in their life cycles. The structure and formation of celestial objects in the universe are a result of gravitational collapse [5], which is a marvelous issue in the theory of gravitation and relativistic astrophysics and attracted the attention of the researchers. The factor that balances the star is the equilibrium between the outward-directed pressure and the gravitational pull that directs it inward. In gravitational collapse, the celestial object contracts towards its center under the dominant action of its own gravity. The result of this collapse is the emergence of compact (dense) objects that includes black holes, white dwarfs, and neutron stars. If the star has a mass that is just a few times that of the sun, it consumes its nuclear fuel and collapses. According to the general relativity theory (GR), the result of this collapse is either a black hole or a naked singularity. Spherically symmetric spacetime has been taken into consideration by many researchers when discussing the majority of gravitational collapse-related issues.
Oppenheimer and Snyder [6] studied the collapse of dust. They found the solutions of Einstein’s field equations (EFEs) describing this process. Friedman and Schutz [7] looked into how gravitational radiation and viscosity affected the evolution of stars. In some cases, viscosity keeps rotating stars stable, whereas gravitational radiation makes them unstable. Lake and Hellaby [8] confirmed the presence of the naked singularity after examining the radiating spherical collapse. Joshi and Singh [9] studied the spherically symmetric dust collapse encapsulating irregular distribution of energy density. They came to the conclusion that the energy density of star and its radius have a significant impact on the outcome. The inhomogeneity factor that may result in naked singularities was identified by Herrera et al. [10]. Chan [11] analyzed the collapse of the relativistic star model and figured out that the shear viscosity of the fluid contributes to the increase in anisotropic pressure. Wang [12] in his investigation of the cylindrical collapse, discovered a limitation that may cause a collapse to produce black holes. Herrera et al. [13] discovered a link between anisotropic pressure, shear, and Weyl scalars and found a constraint on the density irregularity of a diffusing star. Sharif and Yousaf [14] investigated the collapse of perfect fluid. They investigated that the involvement of the constant curvature terms slows down the rate of collapse. Cipolletta and Giambó [15] studied the effects of electromagnetic field on the gravitational collapse of the spherical relativistic geometries. It turns out that in this anisotropic scenario, electromagnetic charge entirely altered the final state of the imploding star.
The GR is a successful theory of cosmology that agrees with many solar system tests in the presence of a weak gravitational field. At large scales or in strong gravitational fields, this theory may need some modifications to unveil various hidden cosmological aspects, like dark energy problem. The modifications in GR can help to explain the cosmic accelerating expansion. These modifications are referred to as modified gravity theories (MGTs) [16,17,18,19,20,21,22,23,24,25,26]. The MGTs include f(R) [27,28,29], f(G) [30, 31], f(R, T) [32, 33] (where G represents Gauss Bonnet-invariant, the symbol R stands for Ricci-scalar and the quantity T symbolizes trace of stress-energy tensor) etc. Out of these MGTs, the simplest one is f(R) theory. The idea of f(R) theory has fascinated great attention because it may provide the simplest description of DE. This theory is obtained when we have replaced the Ricci scalar R by a generic function of R i.e., f(R), in the Einstein–Hilbert action. The f(R) gravity is presented by Nojiri and Odintsov [34] and they analyzed that this theory can explain the accelerating expansion of universe in an effective way.
Many researchers investigated the spherical star collapsing phenomenon using various fluid configurations in f(R) gravity. Yousaf [35] analyzed the collapse of the perfect fluid. They deduced that current curvature corrections produce repulsion effects to reduce the collapse rate. Sebastiani et al. [36] used a special f(R) formalism to study the formulation of Nariai black holes and figured out that the choice of f(R) model affects the stages at which a collapsing system occurs. Astashenok et al. [37, 38] discussed a few features of stellar systems in various modified gravity models. They described some eras of model parameters that support the formation of massive stellar structure in a better way. A set of structure scalars can be used explicitly to write all static anisotropic cylindrical solutions [39]. Bhatti and Yousaf [40] studied the collapse of electrically charged Lemaître–Tolman–Bondi spacetime and evaluated the effect of f(R) theory on this geometry. They investigated a group of solutions to the f(R) field equations in the presence of an electromagnetic field and with a constant curvature scalar. Malik et al. [41] studied the static non-rotating stellar models in f(R) gravity. They deduced the corresponding field equations for some specific star models and discovered the connection between the mass variables and radius. Mustafa et al. [42] derived wormhole solutions by utilizing the Karmarkar condition [43] and described the possibility for the formation of traversable wormholes meeting the energy requirements. Oikonomou [44] described a to understand gravitational waves phenomenon in f(R) gravity and studied the impact of post-inflationary era. In f(R) gravity, spherically symmetric static solutions associated with electromagnetic fields were generated by Nashed and Nojiri [45]. They demonstrated that the curvature singularity in GR is substantially softer due to the higher-derivative terms than it is in charged black holes. Recently, Oikonomou [46, 47] performed a powerful numerical simulations to analyze the existence of neutrons stars corresponding to particular equations of state and attractors.
We require the analytical solutions of the nonlinear field equations to investigate the internal geometry of celestial objects, represented by the equation of state (EoS). The EoS gives the relationship between the pressure and the matter density. The degree of nonlinearity of EFEs is one of the major challenges in connecting the unique characteristics of GR to actual physical issues. Therefore, it becomes very challenging to solve these Eq.s without imposing specific symmetry constraints on a space-time metric. These isometries are called Killing vectors (KVs) and give rise to conservational laws. In spite of nonlinearity of these partial differential equations, numerous researchers found exact, astrophysical and cosmological solutions of field equations. The field equations for an isotropic sphere in vacuum were first solved by Schwarzschild [48]. Rahaman et al. [49] studies solutions of compact stars by considering the barotropic equation of state. Malaver [50] considered the equation of state to determine the distinct forms for the gravitational potential. Two solutions of field equations for compact objects like neutron or quark stars were presented by Zubairi et al. [51]. In order to solve field equations for spherically symmetric mass configurations (for a limited value of the cosmological constant), they investigated the composition of distorted (non-spherical) dense objects. For various configurations of conformal Killing vectors (CKV), Herrera et al. [52] discovered a variety of exact analytical solutions, in dissipative as well as in adiabatic regimes. In order to find particular solutions, they imposed some restrictions on the system such as diminishing complexity factor [53,54,55] and quasi-homologous evolution [56].
In this paper, we study a spherically symmetric fluid configurations that is enclosed in a surface \((\Sigma ).\) The source is an anisotropic fluid with dissipation freedom. We find a few exact, non-static solutions satisfying a one parameter group of conformal motions. We take the two cases: either four velocity is parallel to the Killing vector field \((\chi ^{\nu }\Vert V^{\nu })\) or four velocity is perpendicular to the Killing vector field \((\chi ^{\nu }\bot V^{\nu }).\) Each case is further discussed in both dissipative \((q\ne 0)\) and non-dissipative \((q=0)\) regimes. To find particular solutions, we impose some restrictions, like vanishing complexity factor conditions, quasi-homologous conditions, etc. In Sect. 2, the metric, source and kinematical variables are defined and relevant equations are calculated. The expression of complexity factor in the form of metric coefficients is given. In Sect. 3, the expression of transport equation is given, which is useful to analyze the distribution and evolution of temperature. In Sect. 4, the homologous and quasi-homologous conditions are defined. In Sect. 5, we start by assuming the line element of interior spacetime satisfying the CKV equation and we found a number of solutions of the given system. Section 6 includes findings and a analysis of the physical applicability of obtained solutions. Finally, Appendices are given that include helpful formulas.
2 The metric, energy momentum tensor, physical variables and related equations
The action of f(R) gravity minimally coupled with matter Lagrangian density \({\mathbb {L}}_{m}\) is described as
where \(\kappa \) is a coupling constant, the symbol \(\varphi _{m}\) specifies the matter field and the scalar g is the determinant of the metric tensor \(g_{\rho \nu }.\) Upon varying Eq. (1) in regard to \(g_{\rho \nu },\) fourth-order partial differential equations are obtained as under
where \(f_{R}\equiv \frac{df}{dR},\) \(R_{\rho \nu }\) is the Ricci tensor, the notation of covariant derivative is \(\nabla _{\rho }\) and \({{\mathbb {T}}^{({\mathfrak {M}})}_{\rho \nu }}\) is the energy momentum tensor and is given by
with
where \(\mu \) is energy density, \(P_{r}\) and \(P_{\bot }\) are stress components along radial and tangential directions, respectively. \(V^{\rho },\) \(q^{\rho },\) and \(K^{\rho }\) are four velocity, the heat flux, and a unit four vector along radial direction, respectively. Equation (2) can be rearranged as
where
where \(\Box \) is d’Alembert operator. We model our system in such a way that the geometry of the interior region is described through spherically symmetric line element as
where A, B and C depends upon the coordinates t and r and are positive. Here the coordinates are represented as \({\textbf{x}}^{0}=t,\) \({\textbf{x}}^{1}=r,\) \({\textbf{x}}^{2}=\theta \) and \({\textbf{x}}^{4}=\phi .\) Also, A and B have no dimension, while dimension of R is length. Since we have assumed that the observers are co-moving, we have
These terms fulfill the identities
The acceleration, expansion scalar, and shear of the fluid are given, respectively, as
From the above expressions, we can write
here the notations dot and prime show that the derivatives are evaluated with regard to t and r, respectively. The non vanishing components of shear are
where \(\sigma ^{\rho \nu }\sigma _{\rho \nu }=\frac{2}{3}\sigma ^{2},\) with
is the shear scalar. The expression for Misner–Sharp mass [57] is
The proper time derivative \(D_{T}\) is defined as \(D_{T}=\frac{1}{A}\frac{\partial }{\partial t}\) Through this operator, we define the velocity \({\mathbb {V}}\) of the fluid, interpreted as \({\mathbb {V}}=D_{T}C,\) which is assumed to be negative here. Thus, the Eq. (13) takes the form
Utilizing Eq. (14), (A6) can be written as
with
The mass variation with the help of previously defined operators can be written as
and
Equation (20) gives
which satisfies the condition \(m(0)=0.\)
2.1 The Weyl tensor and the complexity factor
It is notable [58] that the complexity function is a scalar entity related to the so-called structural scalars [59] that is used to find the level of complexity of a particular fluid configuration. The condition of the diminishing complexity factor led to some of the solutions that will be shown in the next section. The magnetic component of the Weyl tensor \(({\mathcal {C}}^{\nu }_{\alpha \beta \mu })\) vanishes for spherically symmetric spacetime; as a result, it is described by its “electric” component \({\mathbb {E}}_{\rho \gamma }\) only, given by
whose non zero components are
where
The Weyl tensor’s electric part can also be written as
Introduce Weyl tensor \(Y_{\rho \gamma }\) in a better way
which is further written as follows
The trace and trace free parts of Eq. (25), i.e., scalar functions \(Y_{T}\) and \(Y_{TF}\) can be written as
The scalar \(Y_{TF}\) has been recognized as a complexity factor by many researchers [60,61,62,63,64,65,66,67]. Finally, the complexity factor in the form of metric functions reads as
2.2 The exterior spacetime and matching conditions
We assume that the fluid is enclosed by the surface \(\Sigma .\) To deal with such scenario, we shall compute junction conditions [68]. We assume that the exterior metric to \(\Sigma \) is
where \(M(\nu )\) signifies the total mass and the symbol \(\nu \) is used to represent retarded time. The continuity of the first and second fundamental forms across \(\Sigma \) is necessary for the matching of the interior spacetime to the exterior metric, on the surface \(r=r_{\Sigma }= constant\) [69], which provide us
and
where \(\overset{\Sigma }{=}\) indicates that both sides of the equation are examined on \(\Sigma .\)
3 The transport equation
In order to find the temperature distribution and evolution in the dissipative case, we require a transport equation. The general expression of transport equation reads
where the notation \({\mathcal {K}}\) is for the thermal conductivity, and T is for temperature and \(\tau \) simply configures the relaxation time. If the spacetime is spherically symmetric, then their exists only one independent component of transport equation, which is obtained from Eq. (31), when we contract Eq. (31) with the vector \(K^{\alpha },\) giving us
When we neglect the last term, Eq. (32) reads as
This equation is called truncated transport equation (which is comparatively easy to solve).
4 Homologous and the quasi-homologous condition
We will impose the diminishing complexity factor constraint in order to evaluate models. It is also necessary to clarify what is the most basic technique that defines the collapse of the system. The notion of homologous evolution was presented in Ref. [70]. Thus, Eq. (A3) takes the form
whose integration gives
where \(\tilde{a}(t)\) is an integration function, or
If the integral terms of Eqs. (35) and (36) disappear, then we obtain
The homologous evolution may take place if the integral terms of above equations cancel each other. Later, the homologous condition was relaxed resulting in what was termed quasi-homologous evolution. The condition in Eq. (37) suggests
This equation informs us about the simplest evolution of the fluid in metric f(R) gravity.
5 Exact solutions
We explore a spherically symmetric metric that satisfy the conformal Killing vector, i.e., satisfying the equation
where \(\varphi \) is a function of coordinates t and r and \({\mathcal {L}}_{\chi }\) symbolizes the Lie derivative with regard to the vector field \(\chi ,\) whose general expression is
where \(\zeta \) and \(\lambda \) are functions of independent variables t and r. The case \(\varphi =constant\) corresponds to a homothetic Killing vector. Our aim is to find exact solutions that admit a one-parameter group of conformal motions. We define it in terms of fundamental functions and initiate it by examining the case \(\chi ^{\upsilon }\) orthogonal to \(V^{\upsilon }\) and \(q=0.\)
5.1 Case of \(\chi _{\upsilon }V^{\upsilon }=q=0\)
In this context, Eq. (39) provides
which further implies
and
From Eqs. (41) and (43), we obtain
here n is integration function. We can take it 1, if we reparameterize t. Therefore, we can write
where \(\omega \) is unit constant. The time derivatives of (42) and (43) along with (44) produce
where N(r) is arbitrary integration function which can be taken equal to 1 and \( N_{1}(t)\) is an integration function which is dimensionless. Thus, we get
and
Using the above values in Eq. (A3) with \(q=0,\) we obtain
where k, g and I are arbitrary functions of coordinates t and r and
Thus, any model can be determined by specifying the four arbitrary functions F(t), k(t), g(r) and I(r, t). The field equations in the form of arbitrary functions F(t), g(r), k(t), and I(r, t) can be written as
Imposing the junction conditions on the surface \(r=r_{\Sigma }=constant,\) we obtain from Eqs. (29) and (30), respectively
and
where \(\alpha \equiv g'^{2}(r_{\Sigma }).\) Equation (55) can also be written as
where
which can further be written as
with \(z\equiv \frac{C_{\Sigma }}{M}.\) Equation (57) can be manipulated as under
The integration of Eq. (60) gives the following two solutions (with \(\alpha =\frac{1}{27M^{2}}\))
The first solution shows that the areal radius \(C_{\Sigma }^{(1)}\) expands from 0 to 3M as \(t\rightarrow \infty .\) It represents a white whole structure. However, in the second solution, the areal radius \(C_{\Sigma }^{(2)}\) contracts from \(\infty \) to 3M as \(t\rightarrow \infty .\) The entity k(t) can be calculated from the expression \(C_{\Sigma }.\) While we impose quasi-homologous conditions and vanishing complexity factor conditions in order to determine other arbitrary functions. In the case \(q=0,\) the quasi-homologous condition implies \((\sigma =0),\) which further implies
In this case, the metric functions take the form
Now, instead of four arbitrary functions, we have to evaluate only three functions. One of these arbitrary functions can be fixed by imposing the vanishing complexity factor condition.
Using Eq. (63) in Eq. (64), we obtain
From Eq. (61) and, we obtain
and
The physical variables corresponding to \(C^{(1)}_{\Sigma }\) are mentioned in Appendix B. The expansion scalar for this model is found as under
thereby showing its homogenous and positive nature. The physical variables corresponding to \(C^{(2)}_{\Sigma }\) are described in Appendix B. Now, we consider \(\alpha =0.\) For this case, the expression of \(C_{\Sigma }\) takes the form
also
Thus, the metric coefficients take the form
where \(f(t)=f^{(3)}(t)\) is
The physical variables for this model are
Now we presume that the outside metric is Minkowski, implying that \(M=0.\) Thus, the solutions of Eq. (55) are as
From the above solutions, we obtain
The physical variables corresponding to \(C^{(4)}(t)\) read as
The physical variables corresponding to \(C^{(5)}(t)\) read as
In model 4 and model 5, we have used the relation \(F_{0}=\omega r_{\Sigma }.\) The symmetry assumed in Sect. 5.1 lowers the metric variables (two functions of t, one function of r, and one function of t and r) to four functions. The matching requirements (33) and (34) are then reduced to a single differential equation (62), the solution of which gives one of the four functions characterizing the metric. We made assumptions about the values of the equation’s parameters to arrive at a solution defined in respect of fundamental functions. The remaining functions are determined by imposing other restrictions on the system. In these models, the extra dark source terms that come from theory are represented as the last terms on the right side of the equations wrapped in square brackets.
5.2 Case of \(\chi _{\upsilon }V^{\upsilon }=0;q\ne 0\)
In this subsection, we investigate a dissipative case when \(\chi _{\upsilon }V^{\upsilon }=0.\) From Eq. (40), we acquire
where \(\omega \) is a unit constant whose dimension is \(\frac{1}{length}.\) And
where F(t) is an function appeared in integration process and
Using the above equations in (A3) with \((q\ne 0),\) we get
The integration of the above equation produces
where k(t), g(r) and \(I_2(r,t)\) are functions of integration. To find an exact solution, we impose vanishing complexity factor condition \((Y_{TF}=0),\) which implies
For simplification, we consider
and
The integration of Eq. (90) leads to
where \(\delta (t)\) and \(\eta (t)\) are arbitrary integration functions. The dimension of \(\delta (t)\) is \(\frac{1}{length}\) and \(\eta (t)\) is dimensionless. The differentiation of Eq. (89) gives \(\eta =\frac{\delta }{\omega }.\) Therefore, we can write Eq. (91) as
From Eqs. (89) and (92), we obtain
where \(c_{3}\) and \(c_{4}\) are constants of integration. From the above Eq. (93), we can write
The comparison of Eqs. (87) and (92) gives
In the form of arbitrary function \(\delta (t),\) the field equations (A1)–(A4) take the form as
The arbitrary function \(\delta (t)\) can be found by imposing the quasi-homologous condition (41) and the junction condition (30). By utilizing the quasi-homologous condition (38), we get
Thus, the function \(\delta (t)\) can be written as
where \(c_{5}\) is an integration constant whose dimension is length. Resultantly, the function B is formulated as
To solve set of equations we consider shear free case under which \(\sigma =0.\) Now, the shear-free condition \((\sigma =0)\) implies that \(\dot{F}=0\) which further implies that \(c_{4}=0.\) Thus, the metric coefficients are calculated as
The physical variables in the form of arbitrary function \(\delta (t)\) take the form
To specify the function \(\delta (t),\) we impose junction condition (31). Using the Eqs. (104)–(107) in Eq. (30), we obtain
with \(\omega _{1}\equiv \frac{\omega c_{3}}{\omega r_{\Sigma }+1}.\) To find the solution of Eq. (108), let \(u=\frac{\dot{\delta }}{\delta }.\) In this case, the above Eq. (108) becomes Riccati equation, formulated as
The solution of this equation is found as under
from which we can find \(\delta \) as
where \(\omega _{2}\) an is integration constant, whose dimension is the same as of \(\delta .\) The expression of temperature is found by utilizing the transport Eq. (36) as
where \(c_{3}\) and \({\mathcal {T}}_{0}(t)\) are arbitrary entities appeared due to integration process. In this instance, we discovered a model that evolves quasi-homologously (38), the diminishing complexity factor \((Y_{TF}=0),\) and the matching conditions (29) and (30), which together formulate all of the metric coefficients. Expressions (104)–(107) and expression (112), which calculates temperature using the truncated transport equation (36), define this model (model 6). The additional dark source terms resulting from theory are shown as the last terms on the right side of the equations enclosed in square brackets in this model. This model does not meet the regularity requirements at the center.
5.3 Case of \(\chi ^{\upsilon }\Vert V^{\upsilon };q=0\)
In this subsection, we study the non-dissipative case with \(\chi ^{\upsilon }\Vert V^{\upsilon }.\) In this respect, Eq. (40) gives
where l(r) is an integration function. Resultantly, we can write the line element ,mentioned in Eq. (6), as
Again utilizing (113), Eq. (A3) takes the form
whose solution is found as under
which implies
here g(r) and k(t) are arbitrary integration functions. To find the solution, we have to specify four arbitrary functions k(t), g(r), l(r) and \(I_{2}(r,t).\) The function k(t) shall be evaluated by using the matching conditions Eqs. (29) and (30). The first condition reads
where \(\omega ^{2}\equiv \frac{l^{2}_{\Sigma }}{r^{2}_{\Sigma }},\) \(\varepsilon \equiv (g')^{2}_{\Sigma } l^{2}_{\Sigma },\) and
with \(a_{1}\equiv \frac{l'_{\Sigma }r_{\Sigma }}{l_{\Sigma }}.\)
Now, we have to find the solution of Eq. (120) in order to specify the arbitrary function k(t). We consider a particular case \(a_{1}=0,\) which implies
which is same as Eq. (57), so have the same solutions. The Eq. (30) produces
here Eq. (122) is just the derivative of Eq. (121). Thus, we will just consider Eq. (122) for further calculation. Now, we will imply vanishing complexity factor condition (27) to specify the other arbitrary functions g(r) and l(r), which implies
or
where \(u\equiv g'+I'.\) The solution of Eq. (124) is
giving us \(g+I_{2}=b_{4}(t)\int \frac{r}{l^{2}}dr+b_{5}(t),\) with \(b_{4}(t)\) and \( b_{5}(t)\) as integration functions. On selecting \(l(r)=b_{6}r,\) we get \(a_{1}=1\) and \(g+I_{2}=b_{7}(t) \ln (r)+b_{5}(t),\) where \(b_{6}\) and \(b_{7}(t)\) are integration entities. The function k(t) can be determined by imposing junction condition (29), whose solution is given by
The physical variables that corresponds to this model are
Here, we have used the relation \(\omega ^{2}=c^{2}_{6}.\) Now, we consider another case \(\varepsilon =0\) and \(a_{1}=\frac{1}{2},\) which implies
The integration of Eq. (130) gives
where \(T\equiv \frac{\sqrt{3}\omega }{2}(t-t_{0}).\) To determine other arbitrary functions, we shall vanish the complexity factor (31). For \(a_{1}=\frac{1}{2},\) we obtain
Since \(\varepsilon =0 \Rightarrow b_2(t)=0,\) therefore \(g(r)+I_{2}(r,t)=b_3(t).\) For this case, the physical variables are
Now, we examine the case \(M=0,\) then Eq. (118) becomes
By considering the case \(a_{1}=0,\) the two solutions of above Eq. in the form of elementary functions are expressed as
The physical variables corresponding to \(C^{(9)}_{\Sigma }\) are calculated as
The physical variables corresponding to \(C^{(10)}_{\Sigma }\) are given as
Here, we have used the relation \(\omega ^{2}=c^{2}_{6}.\) These solutions corresponds to the situation where the CKV is parallel to the four-velocity and the heat flux \(q=0.\) The function k(t) is formulated when the matching conditions (29) and (30) are satisfied. The differential Eq. (121), which is obtained by the fulfilment of junction conditions, is integrated by introducing the different values of the parameters into it. The function h(r) is assumed and the other functions are specified by imposing the diminishing complexity factor condition (27). In this subsection, we have constructed four models. These models show the additional dark source terms as the last terms on the right side of the Equations, surrounded in square brackets.
5.4 Case of \(\chi ^{\upsilon }\Vert V^{\upsilon };q\ne 0\)
Ultimately, we explore the system which is dissipative and for which 4-velocity is \(\Vert \) to CKV. For this case, the metric coefficients take the form as given in Eqs. (83)–(85). Utilizing these results in Eq. A(3), we find
The integration of the Eq. (144) produces
where k(t) and g(r) are integration entities. Here, the expression of \(I_2(r,t)\) is mentioned in Appendix A. To find these arbitrary functions, we apply vanishing complexity factor condition \((Y_{TF}=0).\) By utilizing Eqs. (145)–(147), Eq. (144) takes the form
which produces
with \(\lambda (t)\) as an arbitrary function. The differentiation of Eq. (148) with regard to time gives
Using this result, the metric coefficients may be formulated as
In order to specify other functions k(t), \(\lambda (t)\) we will impose junction condition \((q=P_{r}+\frac{f-Rf_{R}}{16\pi })_{\Sigma },\) which produces
where
To find the solution of Eq. (152), we suppose
which gives
Equation (154) can be written as
To find the solution of Eq. (156), we propose \(v=\frac{\dot{H}_{\Sigma }}{H_{\Sigma }}.\) Thus, the Eq. (156) reads
This equation is known as Riccati equation. A particular solution of this equation is
And the general solution of this Riccati equation can be found by substituting \(y=v-v_0\) in Eq. (157), giving us
The solution of Eq. (159) is
here b is integration constant and \(\nu \equiv \mu (1-b_{1})-v_0.\) The final expression \(H_{\Sigma }\) is
where a is integration constant. The physical variables in the form of functions h(r), and H(t, r) read as
To find a particular model, we suppose that \(b_{1}=2,\) which gives \(v_0=-\eta \) and \(\nu =0,\) thus \(X_{\Sigma }\) read \(X_{\Sigma }=c e^{-\eta t},\)where \(c=\frac{a}{(1+b)^{2}}.\) Further, we assume that \(h(r)=c_{2}r^{2},\) with \(c_{2}\) is a constant having dimension \(\frac{1}{[length]^{2}},\) giving us \(\eta =c_{2}r_{\Sigma }.\) Using these relations, we obtain
Thus, the function k(t) is formulated as \(k(t)=\frac{ce^{-\eta t}}{2}.\) The final expression for H(t, r) is
The physical variables in the form of \(H^{(11)}(t,r)\) read
The corresponding total matter quantity (m) and temperature turn out to be
The metric coefficients for this case as mentioned in Eqs. (162)–(164), which, after implying the diminishing complexity factor condition \((Y_{TF}=0),\) transforms as given in Eqs. (167)–(170). Thus, the spacetime is determined up to four functions (two functions of t, one function of r and one function of t and r). The function of t is acquired from the integration of the matching conditions (33) and (34), whereas the function g(r) is presumed along-with the choice \(a_{1}=2.\) This produces the model 11. The additional dark source terms resulting from theory are shown as the last terms on the right side of the resulted field equations enclosed in square brackets in this model.
6 Conclusions
The field equations for general spherically symmetric fluid configuration can be solved in many ways as a result of the acceptance of CKV, as we have seen earlier. The present study is recognized as a kind of the generalization of Herrera’s work [52]. We have found a number of solutions of field equations in f(R) gravity. These solutions can be used to solve a wide range of astrophysical issues or as test theoretical models for the study of hypothetical concepts like white holes and wormholes. We added more restrictions to the fluid distribution in order to identify solutions that could be stated in regard of elementary functions. Some of these were imposed solely to create models represented by simple functions, while others (such as the diminishing complexity factor or the quasi-homologous condition) are implied with a clear physical meaning.
In Sect. 5.1, we found five models. The first two models are obtained by choosing \(\alpha =\frac{1}{27M^{2}}\) giving us the values of areal radius \(C^{(1)}_{\Sigma }\) and \(C^{(2)}_{\Sigma },\) describing the expansion and contraction of the fluid from 0 to 3M and from infinity to 3M, respectively. These models show that the energy densities of the system are positive and the first model has only one singularity at \(t=t_{0}.\) The third model is obtained by choosing \(\alpha =0,\) representing that the areal radius fluctuates between 0 and 2M. This model’s radial pressure only depends on f(R) curvature terms. In addition, we considered the case \(M=0,\) showing that the fluid distribution has no gravitational effect outside the boundary surface. These models represent a kind of “ghost stars” but they demonstrate pathologies, both topological and physical. Hence their physical uses are unclear. These types of distributions have been studied in the past [71].
In Sect. 5.2 for the dissipative case, we have obtained the model 6. The corresponding equations of motion modify in f(R) gravity analysis. The symmetry assumed in Sect. 5.3, again lowers the metric variables (two functions of t, one function of r, and one function of t and r) to four functions. The model 7 is obtained by taking the particular value of constant \(a_{1}\) (\(a_{1}=1\)) and taking \(h(r)=c_{6}r.\) The expression of \(C^{(7)}_{\Sigma }\) shows that the areal radius expands from 0 to 3M. For this model, the valve of energy density is positive and is non-singular at \(t=t_{0}.\) The model 8 is obtained by taking the particular value of constants \(a_{1}\) \((a_{1}=\frac{1}{2})\) and \(\varepsilon \) \((\varepsilon =0).\) The expression of \(C^{(8)}_{\Sigma }\) shows that the areal radius oscillates between 0 to \(\frac{8M}{3}.\) The value of \(\mu \) is positive but the fluid configuration has a singularity at \(r=0.\) Models 9 and 10 are obtained for \(M=0\) and \(a_{1}=1.\) They represent the type of “ghost stars” that was previously discussed.
Finally, in Sect. 5.4, we studied the dissipative case for the CKV parallel to the four-velocity. The model 11 is found by taking \(a_{1}=2.\) In this model, the value of areal radius compensates for the decline in energy density and heat flux by rising exponentially. In general, all the obtained solutions (or some of them) may be helpful to mark out the evolution of some phases of the collapsing fluid in the framework of metric f(R) gravity theory. The pertinent parameters are given particular values according to each particular case. It is important to remember that in any actual scenario involving a collapse, we cannot anticipate that the same equation of state would hold true throughout the evolution and for the entire fluid configuration. Secondly, the regularity conditions are not fulfilled at the center of the fluid configuration. Thirdly, if we take \(f(R)=R,\) all the obtained results are reduced to GR.
Data Availability Statement
All data generated or analyzed during this study are included in this published article.
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Acknowledgements
The work of KB was supported in part by the JSPS KAKENHI Grant number JP21K03547.
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Appendices
Appendix A: Field equations
For interior line element, the field equations in metric f(R) gravity are as
and its non zero components of field equations are
Using expansion and shear scalars, the component (A3) is written as
The arbitrary function appearing in the value of metric coefficients A and B of Eq. (50) is found as under
The value of the temperature function \({\mathcal {T}}(r,t)\) under the conditions \(\chi _{\upsilon }V^{\upsilon }=0\) and \(q\ne 0\) as shown in Eq. (112). Its value is found as under
The metric coefficients A under the conditions \(\chi _{\upsilon }V^{\upsilon }=0\) and \(q\ne 0\) contains \(I_2\) as an arbitrary function as shown in Eq. (87). Its value is found as under
The temperature function within the background of \(\chi ^{\upsilon }\Vert V^{\upsilon }\) and \(q\ne 0\) contains \(Z_{2}(r,t)\) as shown in Eq. (172). Its value is found as under
Appendix B
The physical variables corresponding to \(C^{(1)}_{\Sigma }\) described in Eq. (66) are found as under
The above mentioned results are obtained in the background of \(\chi _{\upsilon }V^{\upsilon }=0\) constraint for the adiabatic spherical structure.
The physical variables corresponding to \(C^{(2)}_{\Sigma }\) described in Eq. (67) are
The above mentioned model described the matter variables of the non-radiating spherical anisotropic objects in an environment of \(\chi _{\upsilon }V^{\upsilon }=0.\)
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Bamba, K., Yousaf, Z., Bhatti, M.Z. et al. Collapsing dynamics of relativistic fluid in modified gravity admitting a conformal Killing vector. Eur. Phys. J. C 83, 739 (2023). https://doi.org/10.1140/epjc/s10052-023-11911-2
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DOI: https://doi.org/10.1140/epjc/s10052-023-11911-2