arXiv subjects

Bobby Eka Gunara

Publications and source records attributed to Bobby Eka Gunara.

At least 37 records · Page 2Linked to original sources

arXiv2024

Isotropic and Anisotropic Radiating Gravastars with Various Matter Types of Thin Shell and Interior

In this paper, we investigate models of radiating gravastars with both isotropic and anisotropic interiors, incorporating various types of thin shell matter. For the isotropic interior case, we consider a thin spherical shell characterized by an equation of state in which its pressure is proportional to its mass density, enclosing a de Sitter spacetime and surrounded by Vaidya exterior spacetime. Our analysis reveals that stable gravastars can form under specific scenarios of radiative mechanisms and for certain thin shell matter types. In addition, we also show and discuss in brief the possibility of existence of stable radiating anti-de Sitter gravastar formation. For the anisotropic interior, we use an anisotropic dark energy model with a Tolman-Matese-Whitman (TMW) mass function. We explore several thin shell matter types: standard, dark energy, and repulsive phantom. Our findings indicate that stable gravastars can also emerge in this context, particularly with standard and repulsive phantom thin shells. Furthermore, our results suggest that the density of black holes is consistently higher than that of gravastars and normal stars, regardless of the type of matter in the thin shell. This observation supports the notion that gravastars and black holes are distinct entities, reinforcing the theoretical distinction between these two types of compact objects.

gr-qc↗

arXiv2024

Cosmic Inflation From Fluctuating Baby-Skyrme Brane

In this work, we explore the inflationary dynamics induced by small fluctuations on the Skyrme brane, characterized by a time-dependent perturbative function $\tildeϕ$. In the low-energy regime, the model successfully reproduces standard inflation, with a potential term dictated by the Skyrmion at the brane. Gravity localization is achieved at the brane, and the lowest energy scale is established at the asymptotic boundary. The model demonstrates the capability to emulate standard inflation dynamics, resembling $\tildeϕ^4$ potential characteristics under certain conditions. At higher energy levels, the behaviour of $\tildeϕ$ is contingent upon the Skyrme term coupling constant $λ$, influencing reheating phases. The wave-like nature of fluctuations allows for energy transfer, resulting in a possibly lower reheating temperature. We also discuss the prospect of $λ$ changing sign during inflation, presenting a non-standard coupling dependent on the matter field.

gr-qc↗

arXiv2024

Classical Solutions of Higher Dimensional Einstein-Maxwell-Higgs System With Nontrivial Potential: Global Existence and Completeness

We study the Cauchy problem of higher dimensional Einstein-Maxwell-Higgs system in the framework of Bondi coordinates. As a first step, the problem is reduced to a single first-order integro-differential equation by defining a generalized ansatz function. Then, we employ contraction mapping to show that there exists the unique fixed point of the problem. For a given small initial data, we prove the existence of a global classical solution. Finally, by introducing local mass and local charge functions in higher dimensions, we also show the completeness property of the spacetimes.

gr-qc↗

arXiv2024

Global Existence and Completeness of Classical Solutions in Higher Dimensional Einstein-Klein-Gordon System

In this paper we study the global existence and completeness of classical solutions of gravity coupled a scalar field system called Einstein-Klein-Gordon system in higher dimensions. We introduce a new ansatz function to reduce the problem into a single first-order integro-differential equation. Then, we employ the contraction mapping in the appropriate Banach space. Using Banach fixed theorem, we show that there exists a unique fixed point, which is the solution of the theory. For a given initial data, we prove the existence of both local and global classical solutions. We also study the completeness properties of the spacetime. Here, we introduce a mass-like function for $D\geq 4$ in Bondi coordinates. The completeness of spacetime along the future directed timelike lines outward to a region which resembles the event horizon of the black hole.

gr-qc↗

arXiv2023

Some Cosmological Consequences of Higher Dimensional Klein-Gordon-Rastall Theory

Using dynamical system analysis, we investigate some cosmological consequences of Rastall gravity coupled to a scalar field (called the Klein-Gordon-Rastall theory) with exponential scalar potential turned on in higher dimensions. From the critical points of the autonomous equations, we can determine the dominant components of the energy density in different cosmic eras. We obtain a fixed point representing a scalar field-matter-dominated era which corresponds to either a late-time or past-time attractor depending on the parameters used. According to this point, the inflationary phase, corresponding to past-time attractors, is given by unstable nodes, whilst the dark energy era, corresponding to late-time attractors, is represented by stable nodes. In the inflationary sector, power-law inflation can still occur in this Klein-Gordon-Rastall cosmological model. On the other hand, in the late-time sector, we find a nontrivial interplay between a scalar field with an exponential potential and the non-conservative energy-momentum tensor of the non-relativistic matter field (baryonic-dark matter) in curved spacetime plays a role as the dark energy. Based on such features, the Klein-Gordon-Rastall cosmology could be a promising candidate for describing both the early and late-time universe.

gr-qc↗

arXiv2023

Global Existence and Singularity Formation of Classical Solutions in Einstein-Skyrme System

In this paper, we elucidate the problem of gravitating Skyrmion governed by field equations of the Einstein-Skyrme system with no potential term in the Bondi coordinate. The spherical symmetry has to be assumed and both the metric functions and Skyrme ansatz depend on radial and retarded time coordinates which implies that the system is dynamic. We show that unique smooth solutions with arbitrary initial data exist for a certain time interval with the extra condition that the time interval must be finite in order to have a non-zero topological charge. Then, the strategy to show that global smooth solutions exist is to restrict the initial data and proceed to find suitable configurations for extending the time interval to infinity. We also discuss the possible configurations within The Einstein-Skyrme System which develop singularities in coordinate origin.

gr-qc↗

arXiv2023

Decay Estimate of Maxwell-Higgs System on Schwarzschild Black Holes

In this paper, we prove the decay estimate of Maxwell-Higgs system on four dimensional Schwarzschild spacetimes. We show that if the field equations support a Morawetz type estimate supported around the trapped surface, the uniform decay properties in the entire exterior of the Schwarzschild black holes can be obtained by using Sobolev inequalities and energy estimates. Our results also consider various forms of physical potential such as the mass terms, $ϕ^4$-theory, sine Gordon potential, and Toda potential.

math.AP↗

arXiv2023

Fast-forward adiabatic quantum dynamics of XY spin model on three spin system

We discussed a method to accelerate an adiabatic quantum dynamics of XY spin model by using the fast-forward method proposed by Masuda and Nakamura. The Accelerated scheme is constructed by adding the driving Hamiltonian to the original Hamiltonian and speeding it up with a large time-scaling factor and an adiabatic parameter that realizes adiabatic quantum dynamics in a shortened time. Accelerated adiabatic dynamics start by assuming the candidate of driving Hamiltonian consists of the pair-wise exchange interaction and magnetic field. The driving Hamiltonian terms multiplied by the velocity function together with the original Hamiltonian give fast-forward driving for adiabatic states. We apply our method to XY spin model by considering three spin systems on the Kagome lattice. In this model, we obtained the XY pair-wise exchange interaction of nearest neighbors and next-nearest neighbors should be added to the original Hamiltonian as a driving interaction to accelerate the adiabatic motion. This pair-wise driving interaction in the fast-forward scheme guarantees the complete fidelity of accelerated states.

quant-ph↗

arXiv2023

Global existence of classical static solutions of four dimensional Einstein-Klein-Gordon system

In this paper we prove the global existence of classical static solutions of Einstein gravitational theory coupled to a real scalar field where the spacetime admits spherically symmetry. The equations of motions can then be reduced into a single first-order integro-differential equation. First, we obtain the decay estimates of the solutions. Then, in order to prove the global existence, we use the contraction mapping theorem in the appropriate function spaces.

math-ph↗

arXiv2022

Electric-dual BPS Vortices in The Generalized Self-dual Maxwell-Chern-Simons-Higgs Model

In this paper we show how to derive the Bogomolny's equations of the generalized self-dual Maxwell-Chern-Simons-Higgs model presented in \cite{Bazeia:2012ux} by using the BPS Lagrangian method with a particular choice of the BPS Lagrangian density. We also show that the identification, potential terms, and Gauss's law constraint can be derived rigorously under the BPS Lagrangian method. In this method, we find that the potential terms are the most general form that could have the BPS vortex solutions. The Gauss's law constraint turns out to be the Euler-Lagrange equations of the BPS Lagrangian density. We also find another BPS vortex solutions by taking other identification between the neutral scalar field and the electric scalar potential field, $N=\pm A_0$, which is different by a relative sign to the identification in \cite{Bazeia:2012ux}, $N=\mp A_0$. Under this identification, $N=\pm A_0$, we obtain a slightly different potential terms and Bogomolny's equations compared to the ones in \cite{Bazeia:2012ux}. Furthermore we compute the solutions numerically, with the same configurations as in \cite{Bazeia:2012ux}, and find that only the resulting electric field plots differ by sign relative to the results in \cite{Bazeia:2012ux}. Therefore we conclude that these BPS vortices are electric-dual BPS vortices of the ones computed in \cite{Bazeia:2012ux}.

hep-th↗

arXiv2022

Bound States of Central Force System in Special Relativity

In this paper we consider the central force problem in the special theory of relativity. We derive the special relativistic version of the Binet equation describing the orbit of a body. Then, the motion of a planet in a solar-like system where the gravitational potential is modified by adding a term that is proportional to the inverse of the square of the radial coordinate is discussed. Using perturbative method, we obtain the explicit orbital function. At the end, the orbital precession of the planet and its relation to the result from general relativity are also discussed.

physics.class-ph↗

arXiv2022

BPS Skyrmions of Generalized Skyrme Model In Higher Dimensions

In this work we consider the higher dimensional Skyrme model, with spatial dimension $d > 3$, focusing on its BPS submodels and their corresponding features. To accommodate the cases with a higher topological degree, $B\geq 1$, a modified generalized hedgehog ansatz is used where we assign an integer $n_i$ for each rotational plane, resulting in a topological degree that proportional to product of these integers. It is found via BPS Lagrangian method that there are only two possible BPS submodels for this spherically symmetric ansatz which shall be called as BPS Skyrme model and scale-invariant model. The properties of the higher dimensional version of both submodels are studied and it is found that the BPS Skyrmions with $B\geq1$ exist in the first submodel but there is only $B=1$ BPS Skyrmion in the second submodel. We also study the higher dimensional version of self-duality conditions in terms of strain tensor eigenvalues and find that, in general, the scale-invariant model has a stronger self-duality condition than the BPS Skyrme model.

hep-th↗

arXiv2022

Static Black Holes of Higher Dimensional Einstein-Skyrme System with General Couplings

We construct hairy static black holes of higher dimensional general coupling Einstein-Skyrme theories with the scalar potential turned on and the cosmological constant is non-positive in which the scalar multiplets satisfy $O(d+1)$ model constraint where $d$ is the spatial dimension of the spacetime and $d \ge 3$. Some physical properties of solutions near the boundaries, namely, near the (event) horizon and in the asymptotic region are discussed. Then, we prove that these black hole solutions exist globally which may have finite energy for non-positive cosmological constant. Finally, we use perturbative method to perform a linear dynamical stability analysis and then, show the existence of stable and unstable solutions in the model.

gr-qc↗

arXiv2021

General Couplings of Four Dimensional Maxwell-Klein-Gordon System: Global Existence

In this paper, we consider the multi component fields interactions of the complex scalar fields and the electromagnetic fields (Maxwell-Klein-Gordon system) on four dimensional Minkowski spacetime with general gauge couplings and the scalar potential turned on. Moreover, the complex scalar fields span an internal manifold assumed to be Kähler. Then, by taking the Kähler potential to be bounded above $U(1)^N$ symmetric Kähler potential, the gauge couplings to be bounded functions, and the scalar potential to be the form of either polynomial, sine-Gordon, or Toda potential, we prove the global existence of the system.

math-ph↗

arXiv2021

Higher-Dimensional Minimal Theory of Mass-Varying Massive Gravity And Its Cosmological Consequences

In this paper we construct higher-dimensional minimal theory of mass-varying massive gravity (MTMVMG) where the masslike scalar potential is coupled to a vielbein potential, unlike in the previous literature where it is coupled to metric, such that the number of graviton degrees of freedom in the theory is the same as in general relativity. We then study the cosmological aspects of this theory and show that it has eight critical points: five in the massless sector and three in the massive sector. In contrast to the standard theory of mass-varying massive gravity where the graviton mass asymptotically approaches zero at late times, hence making the contribution of massive gravity to the late-time cosmic expansion minimum, the MTMVMG can provide good descriptions both in the massless and massive sectors. Especially, there are at least two interesting possible scenarios for the late-time cosmology in the theory: the dark energy is either due to the constant graviton mass which comes from the scalar field that becomes frozen after the reheating era, or due to the quintessence paradigm where the scalar field is dynamic. Therefore, if the accelerating expansion of the universe in the massless sector can be explained by standard quintessence paradigm, in the massive sector it has to be explained by the nontrivial interplay between quintessence and massive gravity.

gr-qc↗

arXiv2021

BPS Skyrme Submodels of The Five Dimensional Skyrme Model

In this paper, we search for the BPS skyrmions in some BPS submodels of the generalized Skyrme model in five-dimensional spacetime using the BPS Lagrangian method. We focus on the static solutions of the Bogomolny's equations and their corresponding energies with topological charge $B>0$ is an integer. We consider two main cases based on the symmetry of the effective Lagrangian of the BPS submodels, i.e. the spherically symmetric and non-spherically symmetric cases. For the spherically symmetric case, we find two BPS submodels. The first BPS submodels consist of a potential term and a term proportional to the square of the topological current. The second BPS submodels consist of only the Skyrme term. The second BPS submodel has BPS skyrmions with the same topological charge $B>1$, but with different energies, that we shall call "topological degenerate" BPS skyrmions. It also has the usual BPS skyrmions with equal energies, if the topological charge is a prime number. Another interesting feature of the BPS skyrmions, with $B>1$, in this BPS submodel, is that these BPS skyrmions have non-zero pressures in the angular direction. For the non-spherically symmetric case, there is only one BPS submodel, which is similar to the first BPS submodel in the spherically symmetric case. We find that the BPS skyrmions depend on a constant $k$ and for a particular value of $k$ we obtain the BPS skyrmions of the first BPS submodel in the spherically symmetric case. The total static energy and the topological charge of these BPS skyrmions also depend on this constant. We also show that all the results found in this paper satisfy the full field equations of motions of the corresponding BPS submodels.

hep-th↗

arXiv2020

Effective spacetime geometry of graviton condensates in $f({\mathcal R})$ gravity

We consider a model of Bose-Einstein condensate of weakly interacting off-shell gravitons in the regime that is far from the quantum critical point. Working in static spherically symmetric setup, recent study has demonstrated that the effective spacetime geometry of this condensate is a gravastar. In this paper we make three generalizations: introducing a composite of two sets of off-shell gravitons with different wavelength to enable richer geometries for the interior and exterior spacetimes, working in $f({\mathcal R})$ gravity, and extending the calculations to higher dimensions. We find that the effective spacetime geometry is again a gravastar, but now with a metric which strongly depends on the modified gravity function $f({\mathcal R})$. This implies that the interior of the gravastar can be de Sitter or anti-de Sitter and the exterior can be Schwarzschild, Schwarzschild-de Sitter, or Schwarzschild-anti-de Sitter, with a condition that the cosmological constant for the exterior must be smaller than the one for the interior. These geometries are determined by the function $f({\mathcal R})$, in contrast to previous works where they were selected by hand. We also presented a new possible value for the size of the gravastar provided a certain inequality is satisfied. This restriction can be seen manifested in the behavior of the interior graviton wavelength as a function of spacetime dimension.

gr-qc↗

arXiv2020

BPS Submodels of The Generalized Skyrme Model and How to Find Them

Using the BPS Lagrangian method we show that all known BPS submodels of the generalized Skyrme model, with a particular ansatz for the fields content, can be devided into three groups based on the (effective) number of derivative-terms in the BPS submodels. We are able to derive rigorously the Bogomolny's equations of those BPS submodels. The resulting Bogomolny's equations, along with possible constraint equations, are in general forms in which some of the known BPS submodels may contain other possible non-trivial (non-vacuum) solutions then the ones found in the literature. Furthermore, we derive some other new BPS submodels of the generalized Skyrme model for each of the groups and some of them yield new solutions.

hep-th↗