String Cloud Research Articles

Homogeneous and anisotropic universe models with bouncing deceleration parameter in $f(R,T)$ theory

In this study, we have investigated the cloud of string with perfect fluid (CSPF) matter distribution for the homogeneous and anisotropic (Bianchi I, III, and Kantowski-Sachs) universe models in the $f(R,T)$ theory. We have used the\, bouncing\, deceleration parameter, anisotropy parameter, and equation of state to obtain the exact solutions of field equations. The obtained physical and kinematic quantities were analyzed with the help of graphics. If the anisotropy parameter is zero, then our model becomes an isotropic model of the universe and the string tension vanishes in LRS Bianchi I metric. Finally, we discuss the need for further research.

Quasinormal modes of Bardeen black holes with a cloud of strings* *Supported by the National Natural Science Foundation of China (12275087), and the Fundamental Research Funds for the Central Universities

We investigate the quasinormal mode and greybody factor of Bardeen black holes with a cloud of strings via the WKB approximation and verify them using the Prony algorithm. We find that the imaginary part of the quasinormal mode spectra is always negative, and the perturbation does not increase with time, indicating that the system is stable under scalar field perturbation. Moreover, the string parameter a has a dramatic impact on the frequency and decay rate of the waveforms. In addition, the greybody factor increases when a and λ increase and when q and l decrease. The parameters λ and l have a significant effect on the tails. In particular, when , a de Sitter phase appears at the tail.

Continuous phase transition of the de Sitter spacetime with charged black holes and cloud of strings and quintessence* *Supported by the Natural Science Foundation of China (11705106, 12075143), the Scientific Innovation Foundation of the Higher Education Institutions of Shanxi Province (2020L0471, 2020L0472), the Science Technology Plan Project of Datong City, China (2020153), the Science Foundation of Shanxi Datong University (2022Q1,

Recently, some meaningful results have been obtained by studying the phase transition, critical exponents, and other thermodynamical properties of different black holes. Especially for the Anti-de Sitter (AdS) black holes, their thermodynamical properties nearby the critical point have attracted considerable attention. However, there exists little work on the thermodynamic properties of the de Sitter (dS) spacetime with black holes. In this paper, based on the effective thermodynamical quantities and the method of the Maxwell's equal-area law, we explore the phase equilibrium for the de Sitter spacetime with the charged black holes and the cloud of string and quintessence (i.e., C-dSSQ spacetime). The boundaries of the two-phase coexistence region in both and diagrams are obtained. The coexistent curve and the latent heat of phase transition for this system are also investigated. Furthermore, we analyze the effect of parameters (the state parameter ω and the ratio of two horizon radii /) on the two-phase coexistence region boundary. The results indicate that the phase transition in C-dSSQ spacetime is analogous to that in a van der Waals fluid (vdw) system, which is determined by the electrical potential at the horizon. These results are helpful for understanding the basic properties of black holes and are also of great value for the establishment of quantum gravity.

Splitting the Nariai cosmology

At certain time the Nariai spacetime is split into two parts. The resulting cosmology consists of the original Nariai and a new component with topology (flat)2 × S 2, that is equivalent to a cloud of strings. We explore the properties of our hybrid cosmological model.

The phase transition of Rastall AdS black hole with cloud of strings and quintessence

In this paper, we introduce the black hole solution in Rastall theory of gravity in the presence of quintessence and the cloud of strings. Our investigations show that this model meets only second-order phase transition in four dimensions. While both the first- and second-order phase transitions are seen in five dimensions. Therefore, according to the AdS/CFT duality, the confinement–deconfinement phase transition only occurs in five dimensions for this model.

WGC and WCC for charged black holes with quintessence and cloud of strings

One of the important problems with the existence of weak gravity conjecture (WGC) is the violation of the cosmic censorship. Such a cosmic phenomena is important and consistent in general relativity. It means that for a charged black hole in four dimensions and in the normal state, the WGC cannot hold due to the violation of the weak cosmic censorship conjecture (WCCC). But in this paper, in order to eliminate such a violation, we try to consider RN black hole at two cases, with quintessence, quintessence and cloud of strings in asymptotically flat and non-asymptotically flat. By using such metric background and applying restrictions on its parameters, we show that there will be a good compatibility between WGC and WCCC. Here, we can also see that quintessence case has less freedom in establishing compatibility than quintessence and cloud of strings. Because in the second case, the number of parameters is more and there is a degree of freedom to better establish the compatibility of two theories. Also, here we explain charged black hole in presence of quintessence and cloud of string for Q>M and use some interesting figures we show the corresponding the black hole has an event horizon.

Shadows and quasinormal modes of the Bardeen black hole in cloud of strings

Shadows and quasinormal modes of the Bardeen black hole in cloud of strings

Traversable wormhole models supported by a string cloud in rainbow gravity

The traversable wormholes are fascinating as the shortcuts in space–time. This work discusses the geometry of a traversable wormhole with a string cloud as a source in rainbow gravity. The field equations are developed and solved to obtain the wormhole’s shape function, cloud’s mass density, and string tension. We have calculated the shape function and made it specific to study the dynamics for toy model. Based on the three well-known pairs of rainbow functions (mentioned by Ali and Khalil), the string cloud’s dynamical variables including mass density and string tension are graphically presented. The corresponding energy conditions are also visually depicted. It is found that the source matter (string cloud) of the traversable wormhole must be exotic. However, the positive values of the string tension led to the presence of Casimir and dark energy effects. It is found that the toy model respects the null energy condition for a single pair of rainbow functions, i.e., Rainbow Function Type II. We have concluded that in rainbow gravity theory, the traversable wormhole solutions originating from a string cloud requires the presence of exotic matter to achieve a stable structure.

Effect of a cloud of strings and quintessence on a phase transition of charged rotating AdS black holes

Effect of a cloud of strings and quintessence on a phase transition of charged rotating AdS black holes

Thermodynamical study of black hole with cloud of strings and quintessence in the 4D Einstein–Gauss–Bonnet context

This paper explores the effects of cloud of strings and quintessence on the thermodynamic features of the 4D Einstein–Gauss–Bonnet (EGB) Black Hole (BH). For this purpose, we have evaluated temperature, entropy, Gibb’s free energy and heat capacity of the 4D EGB BH in the presence of cloud of strings and quintessence. We have also shown the graphical behavior of all these physical quantities for various values of Gauss–Bonnet term [Formula: see text], quintessence [Formula: see text], equation of state parameter [Formula: see text] and cloud of string [Formula: see text].

Optical appearance of the Schwarzschild black hole in the string cloud context* *Supported by the National Natural Science Foundation of China (11903025) and the Natural Science Foundation of Sichuan Province, China (2022NSFSC1833)

The image of a black hole (BH) consists of direct and secondary images that depend on the observer position. We investigate the optical appearance of a Schwarzschild BH in the context of a string cloud to reveal how the BH's observable characteristics are influenced by the inclination angle, string cloud parameter, and impact parameter. Following Luminet's work [Astron. Astrophys. 75, 228 (1979)], we adopt a semi-analytic method to calculate the total bending angle of the light ray and derive the direct and secondary images of the Schwarzschild string cloud BH. Our results show that an increase in the inclination angle leads to a more pronounced separation of the images. We consider the gravitational redshift and present the redshift distribution of the direct image while illustrating the flux distribution. We observe that the direct image exhibits blueshift and redshift simultaneously, and the asymmetry of the flux distribution increases with the inclination angle. Finally, we obtain the Schwarzschild string cloud BH image via a numerical simulation, which provides an approximate illustration of the EHT resolution.

WGC and WCCC of black holes with quintessence and cloud strings in RPS space

In this paper, we first introduce Riesner-Nordstrom AdS (RN-AdS) black hole in presence of quintessence and cloud of strings. And then we use the Restrict Phase Space (RPS) formulism in the corresponding black holes at the critical point with equation state ωq=−13,−23,−1. We take advantage from central charge and Newton's constant with form of variable and prove the compatibility between weak gravity conjecture (WGC) and weak cosmic censorship conjecture (WCCC). Here, we show that when we use usual RN-AdS black hole with QM>G condition, we have weak gravity conjecture. But, in that case the WCCC is violated by WGC conjecture. In order to avoid such violation, we consider the Riesner-Nordstrom AdS (RN-AdS) black hole in the presence of quintessence, quintessence and cloud strings. Here, also one can say that, by using the special central charge, chemical potential, appropriate quintessence, cloud strings parameters and the effective Newton's constant with suitable conditions one can arrive the compatibility between weak gravity conjecture and weak cosmic censorship conjecture.

Weak Gravitational Lensing around Bardeen Black Hole with a String Cloud in the Presence of Plasma

The effect of spacetime curvature on optical properties may provide an opportunity to suggest new tests for gravity theories. In this paper, we investigated gravitational weak lensing around a Bardeen black hole with the string clouds parameter. First, we examined the horizon structure in the presence of string clouds around the gravitational compact object defined by Bardeen spacetime. The effect of gravitational weak lensing in a plasma medium is also discussed. According to the findings, the influence of the string cloud parameter on the circular orbits of a light ray around the black hole is greater than that in the Schwarzschild case, while the influence of the charge is reversed. The deflection angle of light rays in weak lensing is also used to study how much the image is magnified.

Ringing and echoes from black bounces surrounded by the string cloud

In the string theory, the fundamental blocks of nature are not particles but one-dimensional strings. Therefore, a generalization of this idea is to think of it as a cloud of strings. Rodrigues et al. embedded the black bounces spacetime into the string cloud, which demonstrates that the existence of the string cloud makes the Bardeen black hole singular, while the black bounces spacetime remains regular. On the other hand, the echoes are the correction to the late stage of the quasinormal ringing for a black hole, which is caused by the deviation of the spacetime relative to the initial black hole spacetime geometry in the near-horizon region. In this work, we study the gravitational wave echoes of black bounces spacetime surrounded by a cloud of strings under scalar field and electromagnetic field perturbation to explore the effects caused by a string cloud in the near-horizon region. The ringing of the regular black hole and traversable wormhole with string cloud are presented. Our results demonstrate that the black bounce spacetime with strings cloud is characterized by gravitational wave echoes as it transitions from regular black holes to wormholes, i.e. the echoes signal will facilitate us to distinguish between black holes and the wormholes in black bounces surrounded by the string cloud.

Criticality and phase transition of Kerr–anti-de Sitter black hole with quintessence and cloud of strings

In this paper, a thermodynamical study of the Kerr–anti-de Sitter (KAdS) black hole (BH) encircled by quintessence matter and the cloud of strings is presented. We check the effects of the cloud parameter on the thermodynamical variables including Hawking temperature, entropy, Helmholtz free energy (HFE), Gibbs free energy (GFE), and specific heat capacity. These thermodynamic variables hold the Smarr–Gibbs–Dehum (SGD) relation in the appearance of quintessence and the cloud of the string parameter. Then we calculate the critical expressions of the developed variables in two different ways that are “Van-der-Waals (vdW)-type equation of state” (EoS) and the “Maxwell equal-area law” (MEAL). The latter technique is found to be more reasonable to examine the criticality of the complex BHs. Using MEAL, we plot the phase diagram in a temperature–entropy plane and obtain an isobar, which represents the co-existence region of liquid–gas phases. It is concluded that the uncharged BHs exhibit the same phase transition (PT) to vdW fluid as the temperature drops down its critical value. Also, we discuss the influence of thermal fluctuations (TF) on the stability of KAdS BH.

On the Radial Solutions of the Dirac Equation in the Kerr-Newman Black Hole Surrounded by a Cloud of Strings

In this paper, we obtain the metric of the space-time generated by a charged and rotating gravitational body surrounded by a loud of strings, namely, the Kerr–Newman black hole space-time with the addition of a cloud of strings. In this background, we find the radial solutions of the Dirac equation for massive particles and show that they are given in terms of the Generalized Heun functions. The dependence of these solutions on the parameter that codifies the presence of the cloud of strings is pointed out.

Tideless traversable wormholes surrounded by cloud of strings in f(R) gravity

We study the tideless traversable wormholes in the f(R) gravity metric formalism. First we consider three shape functions of wormholes and study their viabilities and structures. The connection between the f(R) gravity model and wormhole shape function has been studied and the dependency of the f(R) gravity model with the shape function is shown. We also obtain a wormhole solution in the f(R) gravity Starobinsky model surrounded by a cloud of strings. In this case, the wormhole shape function depends on both the Starobinsky model parameter and the cloud of strings parameter. The structure and height of the wormhole is highly affected by the cloud of strings parameter, while it is less sensitive to the Starobinsky model parameter. The energy conditions have been studied and we found the ranges of the null energy condition violation for all wormhole structures. The quasinormal modes from these wormhole structures for the scalar and Dirac perturbations are studied using higher order WKB approximation methods. The quasinormal modes for the toy shape functions depend highly on the model parameters. In case of the Starobinsky model's wormhole the quasinormal frequencies and the damping rate increase with an increase in the Starobinsky model parameter in scalar perturbation. Whereas in Dirac perturbation, with an increase in the Starobinsky model parameter the quasinormal frequencies decrease and the damping rate increases. The cloud of strings parameter also impacts prominently and differently the quasinormal modes from the wormhole in the Starobinsky model.

Five-dimensional strange quark matter cosmological model with string cloud

The work in this paper is devoted to the study of Kaluza–Klein (KK) cosmological model with strange quark matter attached to string cloud in the framework of metric version of [Formula: see text] gravity. By utilizing hybrid expansion law for average scale factor of the model i.e. [Formula: see text], here [Formula: see text] and [Formula: see text] are positive constants, we derive the solution of the field equations of KK model in the [Formula: see text] gravity. Various dynamical parameters of the model are obtained and their behavior is analyzed through graphical representation. We observe that the analysis of deceleration parameter exhibits a smooth transition of the universe from early decelerated phase to current accelerated epoch. Also, we discussed well-known state-finder parameters [Formula: see text] and [Formula: see text] planes which are analyzed by graphical representation.

Spherical photon orbits around a rotating black hole with quintessence and cloud of strings

In this paper, we calculate the analytical solutions for the radii of planar and polar spherical photon orbits around a rotating black hole that is associated with quintessential field and cloud of strings. This includes a full analytical treatment of a quintic that describes orbits on the equatorial plane. Furthermore, The radial profile of the impact parameters is studied and the radii corresponding to the extreme cases are derived. For the more general cases, we also discuss the photon regions that form around this black hole. To simulate the orbits that appear in different inclinations, we analytically solve the latitudinal and azimuth equations of motion in terms of the Weierstrassian elliptic functions, by considering the radii of spherical orbits, in their general form, as the initial conditions. The period and the stability conditions of the orbits are also obtained analytically.

Geometrization of string cloud spacetime in general relativity

<abstract><p>The purpose of the article is to analyze the behavior of spacetime using a string cloud energy-momentum tensor $ \mathcal{T} $ having string cloud fluid density $ \rho $ and string tension $ \lambda $, named <italic>relativistic string cloud spacetime</italic>. We obtain some results for string cloud spacetime with a divergence-free matter tensor and a diminishing space matter tensor. Next, we discuss some curvature characteristics, such as conformally flat, Ricci semi-symmetric and pseudo-Ricci-symmetric, for relativistic string cloud spacetime. In addition, we gain a condition that coincides with the equation of state for the cloud of geometric strings in Ricci semi-symmetric string cloud spacetime.</p></abstract>