arXiv subjects

Kazuharu Bamba

Publications and source records attributed to Kazuharu Bamba.

At least 109 records · Page 6Linked to original sources

arXiv2020

A Generalized Interacting Tsallis Holographic Dark Energy Model and its thermodynamic implications

The paper deals with a theoretical model for interacting Tsallis holographic dark energy (THDE) whose infrared (IR) cut-off scale is set by the Hubble length. The interaction $Q$ between the dark sectors (dark energy and pressureless dark matter) of the universe has been assumed to be non-gravitational in nature. The functional form of $Q$ is chosen in such a way that it reproduces well known and most used interactions as special cases. We then study the nature of the THDE density parameter, the equation of state parameter, the deceleration parameter and the jerk parameter for this interacting THDE model. Our study shows that the universe exhibits the usual thermal history, namely the successive sequence of radiation, dark matter and dark energy epochs, before resulting in a complete dark energy domination in the far future. It is shown the evolution of the Hubble parameter for our model and compared that with the latest Hubble parameter data. Finally, we also investigate both the stability and thermodynamic nature of this model in the present context.

gr-qc↗

arXiv2020

Stability Analysis of Charged Rotating Black Ring

We study the electromagnetic field equation along with the WKB approximation. The boson tunneling phenomenon from charged rotating black ring(CRBR) is analyzed. It is examined that reserve radiation consistent with CRBR can be computed in general by neglecting back reaction and self-gravitational of the radiated boson particle. The calculated temperature depends upon quantum gravity and CRBR geometry. We also examine the corrected tunneling rate/probability of boson particles by assuming charge as well as energy conservation laws and the quantum gravity. Furthermore, we study the graphical behavior of the temperature and check the stability and instability of CRBR.

gr-qc↗

arXiv2020

Traversable Wormholes with Exponential Shape Function in Modified Gravity and in General Relativity: A Comparative Study

We have proposed a novel shape function on which the metric that models traversable wormholes is dependent. Using this shape function, the energy conditions, equation of state and anisotropy parameter are analyzed in $f(R)$ gravity, $f(R,T)$ gravity and general relativity. Furthermore, the consequences obtained with respect to these theories are compared. In addition, the existence of wormhole geometries is investigated.

gr-qc↗

arXiv2020

The possibility of a stable flat dark energy-dominated Swiss-cheese Brane-world universe with a deceleration-acceleration transition

In this paper, we study the possibility of obtaining a stable flat dark energy-dominated universe in a good agreement with observations in the framework of Swiss-cheese Brane-world cosmology. Two different Brane-world cosmologies with black strings have been introduced for any cosmological constant $Λ$ using two empirical forms of the scale factor. In both models, we have performed a fine-tuning between the brane tension and the cosmological constant so that the EoS parameter $ω(t)\rightarrow -1$ for the current epoch where the redshift $z\simeq 0$. We then used these fine-tuned values to calculate and plot all parameters and energy conditions. The deceleration-acceleration cosmic transition is allowed in both models, and the jerk parameter $j\rightarrow 1$ at late-times. Both solutions predict a future dark energy-dominated universe in which $ω=-1$ with no crossing to the phantom divide line. While the pressure in the first solution is always negative, the second solution predicts a better behavior of cosmic pressure where the pressure is negative only in the late-time accelerating era but positive in the early-time decelerating era. Since black strings have been proved to be unstable by some authors, this instability can actually reflect doubts on the stability of cosmological models with black strings (Swiss-cheese type brane-worlds cosmological models). For this reason, we have carefully investigated the stability through energy conditions and sound speed. Because of the presence of quadratic energy terms in Swiss-cheese type brane-world cosmology, we have tested the new nonlinear energy conditions in addition to the classical energy conditions. We have also found that constructing non-singular and cyclic solutions through certain ansatze in Swiss-cheese Brane-worlds are not possible.

gr-qc↗

arXiv2020

Charged (A)dS black hole solutions in conformal teleparallel equivalent of general relativity

We continue our study in 4-dimension to derive non-charged and charged (Anti)-de Sitter black hole solutions in conformal teleparallel equivalent of general relativity. The non-charged and charged equations of motion are applied to a spherically symmetric tetrad and the non-linear differential equations are derived. It is shown that the output solutions of the two cases are identical to those obtained in teleparallel equivalent theory to general relativity. As a result, it is found that in the conformal teleparallel equivalent theory to general relativity, the scalar field cannot influence on the manifold of spherical symmetry, i.e. the scalar field must equal 1 in order to have a well known asymptote spacetime.

gr-qc↗

arXiv2020

Charged spherically symmetric Taub-NUT black hole solutions in $f(R)$ gravity

$f(R)$ theory is a modification of Einstein general relativity which has many interesting results in cosmology and astrophysics. To derive black hole solution in this theory is difficult due to the fact that it is fourth order differential equations. In this study, we use the first reliable deviation from general relativity which is given by the quadratic form of $f(R)=R+βR^2$, where $β$ is a dimensional parameter. We calculate the energy conditions of the charged black holes and show that all of them are satisfied for the Taub-NUT spacetime. Finally, we study some thermodynamic quantities such as entropy, temperature, specific heat and Gibbs free energy. The calculations of heat capacity and free energy show that the charged Taub-NUT black hole have positive values which means that it has thermal stability.

gr-qc↗

arXiv2020

Reconstruction of $f(R)$ lagrangian from a massive scalar field

We present a reconstruction of the Lagrangian for $f(R)$ gravity by using a massive scalar field. The scalar field is minimally coupled to the action of $f(R)$ gravity. We demonstrate the use of a theorem based on invertible point transformation of anharmonic oscillator equation that has only recently been applied to gravitational physics. This avenue gives us a direct way to solve the field equations for a few well-known self-interaction potential of the scalar field. A few cases which do not fall in the regime of the theorem are also discussed. The $f(R)$ models are discussed in the context of recent cosmological observations and viability issues.

gr-qc↗

arXiv2020

Neutral compact spherically symmetric stars in teleparallel gravity

We present novel neutral and uncharged solutions that describe the cluster of Einstein in the teleparallel equivalent of general relativity (TEGR). To this end, we use a tetrad field with non-diagonal spherical symmetry which gives the vanishing of the off-diagonal components for the gravitational field equations in the TEGR theory. The clusters are calculated by using an anisotropic energy-momentum tensor. We solve the field equations of TEGR theory, using two assumptions: the first one is by using an equation of state that relates density with tangential pressure while the second postulate is to assume a specific form of one of the two unknown functions that appear in the non-diagonal tetrad field. Among many things presented in this study, we investigate the static stability specification. We also study the Tolman-Oppenheimer-Volkoff equation of these solutions in addition to the conditions of energy. The causality constraints with the adiabatic index in terms of the limit of stability are discussed.

gr-qc↗

arXiv2020

Hairy black holes and holographic heat engine

By considering AdS charged black hole in the context of extended thermodynamic as the working substance we use it as a heat engine. We investigate the effect of hairy charge on the evolution of efficiency and Carnot efficiency along with electric charge. Because of interesting thermodynamic behavior of hairy black holes it would be natural to know their effects when we use black hole as a heat engine. We show that the hairy charge increases the efficiency, and so maximum temperature would be happened for bigger Maxwell charge when this hairy charge grows. For the fixed electric charges, the efficiency has a minimum value. In fact all critical points describe physical states except when the charge removed. If the electric charge takes a zero value then the hairy charge must be negative. We also seek behavior of the system for large charges which is provided a model with low-temperature thermodynamics.

hep-th↗

arXiv2019

The Lambert $W$ function: A newcomer in the Cosmology class?

We propose a novel equation of state (EoS) which explains the evolutionary history of a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe. The uniqueness of this EoS lies in the fact that it incorporates the Lambert $W$ function in a special fashion. It is explicitly demonstrated that with observationally relevant values of the unknown parameters $\vartheta_\mathrm{1}$ and $\vartheta_\mathrm{2}$, all the evolutionary phases of the Universe can be reproduced. Moreover, it also shows that the initial singularity is unavoidable and asserts that the late-time acceleration of the Universe would continue forever.

gr-qc↗

arXiv2019

Charged Gravastars in Modified Gravity

In this paper, we investigate the effects of electromagnetic field on the isotropic spherical gravastar models in metric $f(R,T)$ gravity. For this purpose, we have explored singularity-free exact models of relativistic spheres with a specific equation of state. After considering Reissner Nordström spacetime as an exterior region, the interior charged manifold is matched at the junction interface. Several viable realistic characteristics of the spherical gravastar model are studied in the presence of electromagnetic field through graphical representations. It is concluded that the electric charge has a substantial role in the modeling of proper length, energy contents, entropy and equation of state parameter of the stellar system. We have also explored the stable regions of the charged gravastar structures.

gr-qc↗

arXiv2019

Parametrizations of Dark Energy Models in the Background of General Non-canonical Scalar Field in $D$-dimensional Fractal Universe

We explore non-canonical scalar field model in the background of non-flat $D$-dimensional fractal Universe with cosmological constant $Λ$ on the condition that the matter and scalar field are separately conserved. The potential $V$, scalar field $ϕ$, function $f$, densities, Hubble parameter and deceleration parameter can be expressed in terms of the redshift $z$ and these depend on the equation of state parameter $w_ϕ$. We also investigate four kinds of well known parametrization models and graphically we have analyzed the natures of potential, scalar field, function $f$, densities, the Hubble parameter and deceleration parameter. As a result, the best fitted values of the unknown parameters ($w_{0},w_{1}$) of the parametrizations models due to the joint data analysis (SNIa+BAO+CMB+Hubble) are found. Furthermore, the minimum values of $χ^{2}$ function are obtained. Also we have plotted the graphs for different confidence levels 66\%, 90\% and 99\% contours for ($w_{0},~w_{1}$) by fixing the other parameters.

gr-qc↗

arXiv2019

Dissipative Self-Gravitating Systems in Modified Gravity

We discuss the gravitational collapse of spherical compact objects in the background of $f(R,T,Q)$ theory, where $R$ represent the Ricci scalar, $T$ is the trace of energy momentum tensor while $Q\equiv R_{μν}T^{μν}$, and investigate the influence of anisotropy and heat dissipation in this scenario. We provide an analysis on the role of distinct material terms considered while studying the dynamical equation. The dynamical equation is coupled with a heat transport equation and discussed in the background of $f(R,T,Q)$ theory of gravity. The reduction element in the density of inertial mass, is re-acquired which is based on the internal position of thermodynamics. In collation with the equivalence relation, the reduction quantity in density is similar as appeared with gravitational force. We formulate the connection of Weyl tensor with different matter variables to see the non-identical outcomes. The inhomogeneous nature of energy density is also analyzed in the framework of modified gravity.

gr-qc↗

arXiv2019

`Hidden' symmetry of linearized gravity in de Sitter space

We demonstrate that the linearized Einstein gravity in de Sitter (dS) spacetime besides the evident symmetries also possesses the additional (local) symmetry $h_{μν}\rightarrow h_{μν} + {\cal E}_{μν}χ$, where ${\cal E}_{μν}$ is a spin-two projector tensor and $χ$ is an arbitrary constant function. We argue that an anomalous symmetry associated with this hitherto `hidden' property of the existing physics is indeed at the origin of `dS breaking' in linearized quantum gravity.

gr-qc↗

arXiv2019

Axially magnetized Dark Energy cosmological model

We investigate the behaviour of the skewness parameters for an anisotropic universe in the framework of General Relativity. Non interacting dark energy is considered in presence of electromagnetic field. A time varying deceleration parameter simulated by a hybrid scale factor is considered. The dynamics of the universe is investigated in presence and absence of magnetic field. The equation of state parameter of dark energy evolves within the range predicted by the observations. Magnetic field is observed to have a substantial effect on the cosmic dynamics and the skewness parameters. The models discussed here end in a big rip and become isotropic at finite time.

physics.gen-ph↗

arXiv2019

On Reconstruction of Extended Teleparallel Gravity from the Cosmological Jerk Parameter

Teleparallel gravity offers a path to resolve a number of longstanding issues in general relativity by re-interpreting gravitation as an artifact of torsion rather than curvature. The present work deals with cosmological solutions in an extension of teleparallel gravity. A reconstruction scheme of the theory has been proposed based on the cosmological jerk parameter. The work contains analysis of ensuing cosmological parameters for different viable models and the stability of the models against cosmic time through an investigation of perturbation of matter overdensity and the hubble parameter.

gr-qc↗

arXiv2019

Cosmological Consequences of New Dark Energy Models in Einstein-Aether Gravity

In this paper, we reconstruct various solutions for the accelerated universe in the Einstein-Aether theory of gravity. For this purpose, we obtain the effective density and pressure for Einstein-Aether theory. We reconstruct the Einstein-Aether models by comparing its energy density with various newly proposed holographic dark energy models such as Tsallis, R$\acute{e}$nyi and Sharma-Mittal. For this reconstruction, we use two forms of scale factor, power-law and exponential forms. The cosmological analysis of underlying scenario has been done by exploring different cosmological parameters. This includes equation of state parameter, squared speed of sound and evolutionary equation of state parameter via graphical representation. We obtain some favorable results for some values of model parameters.

physics.gen-ph↗

arXiv2019

Dynamical properties of Bianchi-I spacetimes in $f(R)$ gravity

We present a dynamical analysis in terms of new expansion-normalized variables for homogeneous and anisotropic Bianchi-I spacetimes in $f(R)$ gravity in the presence of anisotropic matter. With a suitable choice of the evolution parameter, the Einstein's equations are reduced to an autonomous 5-dimensional system of ordinary differential equations for the new variables. Further restrictions lead to considerable simplifications. For instance, we show that for a large class of functions $f(R)$, which includes several cases commonly considered in the literature, all the fixed points are polynomial roots, and hence they can be determined with good accuracy and classified for stability. Moreover, typically for these cases, any fixed point corresponding to isotropic solutions in the presence of anisotropic matter will be unstable. The assumption of a perfect fluid as source and or the vacuum cases imply some dimensional reductions and even more simplifications. In particular, we find that the vacuum solutions of $f(R) = R^{1+δ}$, with $δ$ a constant, are governed by an effective bi-dimensional phase space which can be analytically constructed, leading to an exactly soluble dynamics. Finally, we demonstrate that several results already reported in the literature can be re-obtained in a more direct and easy way by exploring our dynamical formulation.

gr-qc↗