Stability of charged thin shell wormhole supported by polytropic gas

In the framework of Darmois-Israel formalism, the general equations describing the motion of thin shell wormhole with a general form of equation of state of a polytropic gas are derived. The mechanical stability analysis of thin shell wormhole with charge in Reissner–Nordstrom (RN) to linearized spherically symmetric perturbation about static equilibrium solution is carried out.     

Wormhole and its analogue in brane world

In Einstein gravity, for an inhomogeneous phantom energy distribution having linear equation of state (but anisotropic), there exists simple exact solution for spherically symmetric space time describing a wormhole. At infinity, the space time is not asymptotically flat and possesses a regular cosmological Killing horizon with an infinite area. In this work, we have shown that, this wormhole solution is also possible in brane world for various matter distribution, which are not necessarily phantom in nature.     

Effects of charge on the stability of thin-shell wormholes

In this paper, we use Visser’s cut and paste approach to construct thin-shell wormholes from charged black string. The dynamics of thin-shell wormholes is analyzed by taking Van der Waals quintessence fluid at the wormhole throat. We investigate the stability of these constructed thin-shell wormholes under linear perturbations preserving the cylindrical symmetry and also study the effects of charge on its stability. It turns out that there exist both unstable and stable wormhole solutions depending on different parameters involved in the equation of state.     

Time-dependent wormholes in an expanding universedominated by traceless matter

Time-dependent wormhole solutions are found which evolve in a cosmological background. Solutions are presented both for GR and Brans-Dicke field equations. Conditions are derived for the supporting matter to be non-exotic. The traceless energy-momentum tensor needed to support the geometry is in the form of an anisotropic fluid. Far from the wormhole, the equation of state rapidly approaches that of an isotropic perfect fluid with p = 1/3 ρ. For the BD wormholes we obtain ρ = 0everywhere, except for the π = const. limit, in which case the GR results are reproduced. This revised version was published online in July 2006 with corrections to the Cover Date.     

Static wormhole solutions in f(R) gravity

In this work, we study static spherically symmetric wormhole solutions in f(R) gravity. We explore wormhole solutions for anisotropic and isotropic fluids as well as barotropic equation of state with radial pressure. The behavior of weak and null energy conditions is investigated in each case. It is found that these energy conditions are violated for both the anisotropic and isotropic case but are satisfied for barotropic fluids in particular regions. This confirms the existence of wormholes obeying the energy conditions in these regions.     

Møller energy momentum distribution for higher dimensional Morris Thorne wormhole

In this study, Møller energy momentum distribution is investigated for the higher dimensional Morris Thorne wormhole (MTW) in general relativity theory (GR) and results are given for the MTW in (4+1) and (5+1) dimensions. In addition, using the MTW, Møller energy and momentum distributions were investigated for 4-dimensional Morris Thorne wormhole, Hyperbolic Morris Thorne wormhole, Zero Tidal wormhole, Zero Density wormhole, Visser–Kar–Dadhich wormhole and (2+1) dimensional Morris Thorne wormhole. Except for the Zero Tidal wormhole model, we obtained the Møller energy distribution as well defined and non-zero in all other wormhole models. Besides, our results are in agreement with Aygün and Yılmaz and support Lesnner’s idea for Møller energy momentum definition.     

Wormholes supported by two non-interacting fluids

We provide a new matter source that supplies fuel to construct wormhole spacetime. The exact wormhole solutions are found in the model having, besides real matter, an anisotropic dark energy. We have shown that the exotic matters that are the necessary ingredients for wormhole physics violate null and weak energy conditions but obey strong energy condition marginally. Though the wormhole comprises of exotic matters yet the effective mass remains positive. We have calculated the effective mass of the wormhole up to 8 km from the throat (assuming throat radius as 4 km) as 1.3559M _⊙. Some physical features are briefly discussed.     

Electromagnetic fields and charged particle motion around magnetized wormholes

We perform a study to describe motion of charged particles under the influence of electromagnetic and gravitational fields of a slowly rotating wormhole with nonvanishing magnetic moment. We present analytic expression for potentials of electromagnetic field for an axially symmetric slowly rotating magnetized wormholes. While addressing important issues regarding the subject, we compare our results of motion around black holes and wormholes in terms of the ratio of radii of event horizons of a black hole and of the throat of a wormhole. It is shown that both radial and circular motions of test bodies in the vicinity of a magnetized wormhole could give rise to a peculiar observational astrophysical phenomenon.     

Holographic dark energy with linearly varying deceleration parameter and escaping big rip singularity of the Bianchi type-V universe

The present work deals with the accretion of two minimally interacting fluids: dark matter and a hypothetical isotropic fluid as the holographic dark energy components onto black hole and wormhole in a spatially homogeneous and anisotropic Bianchi type-V universe. To obtain an exact solution of the Einstein’s field equations, we use the assumption of linearly varying deceleration parameter. Solution describes effectively the actual acceleration and indicates a big rip type future singularity of the universe. We have studied the evolution of the mass of black hole and the wormhole embedded in this anisotropic universe in order to reproduce a stable universe protected against future-time singularity. It is observed that the accretion of these dark components leads to a gradual decrease and increase of black hole and wormhole mass respectively. Finally, we have found that contrary to our previous case (Sarkar in Astrophys. Space. Sci. 341:651, 2014a), the big rip singularity of the universe with a divergent Hubble parameter of this dark energy model may be avoided by a big trip.     

Precession as a probe of the neutron star interior

Strong evidence that some neutron stars precess (nutate) with long periods (∼1 yr) challenges our current understanding of the neutron star interior. I describe how neutron star precession can be used to constrain the state of the interior in a new way. I argue that the standard picture of the outer core, in which superfluid neutrons coexist with type II, superconducting protons, requires revision. One possible resolution is that the protons are not type II, but type I. Another possibility is that the neutrons are normal in the outer core. I conclude with a brief discussion of the implications for detectable gravitational wave emission from millisecond pulsars. Much of the work described here was supported by the National Science Foundation under Grant AST-00098728.     

Some possible wormhole solutions on the brane

In this paper some wormhole solutions have been presented on the brane which are distinct from the presently available wormhole solution. The matter on the brane is chosen as perfect fluid and all physical variables are functions of the radial co-ordinate r. The solutions are obtained considering the trace of the resulting matter i.e. the Ricci scalar on the brane to be zero.     

Classical and quantum anisotropic wormholes in pure general relativity

In the homogeneous and isotropic Friedmann–Robertson–Walker minisuperspace model, it is known that there are no Euclidean wormhole solutions in the pure gravity system. Here it is demonstrated explicitly that in Taub cosmology, which is one of the simplest anisotropic cosmology models, wormhole solutions do exist in pure general relativity in both classical and quantum contexts. Indeed, it is realized that it is the nonvanishing momentum or the energy associated with the anisotropy change, that essentially renders the occurrence of both classical and quantum wormholes possible.     

Thin-shell wormholes in non-linear f(R) gravity with variable scalar curvature

The present paper explores a thin-shell wormhole (TSW) developed by employing the cut and paste method to two copies of the black hole. It develops TSW in modified $f\left(R\right)$ theory of gravity with variable scalar curvature. The effects of the model parameters on the wormhole solutions are tested, the regions of linear stability are analyzed and stable wormhole solutions have been obtained.     

Resonant Absorption as Mode Conversion? II. Temporal Ray Bundle

A fast-wave pulse in a simple, cold, inhomogeneous MHD model plasma is constructed by Fourier superposition over frequency of harmonic waves that are singular at their respective Alfvén resonances. The pulse partially reflects before reaching the resonance layer, but also partially tunnels through to it to convert to an Alfvén wave. The exact absorption/conversion coefficient for the pulse is shown to be given precisely by a function of transverse wavenumber tabulated in Paper I of this sequence, and to be independent of frequency and pulse width.     

Linear MHD Wave Propagation in Time-Dependent Flux Tube

The propagation of magnetohydrodynamic (MHD) waves is an area that has been thoroughly studied for idealised static and steady state magnetised plasma systems applied to numerous solar structures. By applying the generalisation of a temporally varying background density to an open magnetic flux tube, mimicking the observed slow evolution of such waveguides in the solar atmosphere, further investigations into the propagation of both fast and slow MHD waves can take place. The assumption of a zero-beta plasma (no gas pressure) was applied in Williamson and Erdélyi (Solar Phys. 2013, doi: 10.1007/s11207-013-0366-9 , Paper I) is now relaxed for further analysis here. Firstly, the introduction of a finite thermal pressure to the magnetic flux tube equilibrium modifies the existence of fast MHD waves which are directly comparable to their counterparts found in Paper I. Further, as a direct consequence of the non-zero kinetic plasma pressure, a slow MHD wave now exists, and is investigated. Analysis of the slow wave shows that, similar to the fast MHD wave, wave amplitude amplification takes place in time and height. The evolution of the wave amplitude is determined here analytically. We conclude that for a temporally slowly decreasing background density both propagating magnetosonic wave modes are amplified for over-dense magnetic flux tubes. This information can be very practical and useful for future solar magneto-seismology applications in the study of the amplitude and frequency properties of MHD waveguides, e.g. for diagnostic purposes, present in the solar atmosphere.     

Wormhole model with a hybrid shape function in f(R,T) gravity

In the present article we propose a new hybrid shape function for wormhole (WH)s in the modified f(R, T) gravity. The proposed shape function satisfied the conditions of WH geometry. Geometrical behavior of WH solutions are discussed in both anisotropic and isotropic cases respectively. Also, the stability of this model is obtained by determining the equilibrium condition. The radial null energy condition and weak energy condition are validated in the proposed shape function indicating the absence of exotic matter in modified f(R, T) gravity.     

Existence of static wormhole solutions in $$ gravity

This work investigates some feasible regions for the existence of traversable wormhole geometries in \(f(R,G)\) gravity, where \(R\) and \(G\) represent the Ricci scalar and the Gauss-Bonnet invariant respectively. Three different matter contents anisotropic fluid, isotropic fluid and barotropic fluid have been considered for the analysis. Moreover, we split \(f(R,G)\) gravity model into Strobinsky like \(f(R)\) model and a power law \(f(G)\) model to explore wormhole geometries. We select red-shift and shape functions which are suitable for the existence of wormhole solutions for the chosen \(f(R,G)\) gravity model. It has been analyzed with the graphical evolution that the null energy and weak energy conditions for the effective energy-momentum tensor are usually violated for the ordinary matter content. However, some small feasible regions for the existence of wormhole solutions have been found where the energy conditions are not violated. The overall analysis confirms the existence of the wormhole geometries in \(f(R,G)\) gravity under some reasonable circumstances.     

Thermal waves in stars

Conclusions In this paper I have set forth in detail the theory of thermal waves in inhomogeneous media; it has, I believe, independent theoretical interest. I wish also to point out the fact that the mechanism of separation of an envelope in a nova explosion has hitherto remained obscure, since it strongly depends on the nature of the energy source of the explosion. Thus, if this energy is liberated by thermonuclear reactions, it is more probable that the time of development of the phenomenon reaches hundreds or even thousands of seconds. In such a case, the ejection of an envelope of the star is the result of the total effect of an infinite series of acoustic and weak shock waves that, added together, give a powerful pressure wave [6]. But if the energy of the explosion is gravitational in nature, its liberation may be virtually instantaneous, and the mechanism that transports the energy to infinity could be either a shock wave or a thermal wave. Moreover, if the explosion is due to a rearrangement of only the outer shell of the star, a thermal wave is more probable. And although the velocities of the thermal waves themselves are high, the rate of expansion of the matter of the shell will be appreciably less, since the time (of the order of a few seconds) is too short for interaction between radiation and matter. This distribution of velocities of the matter behind the thermal front can be obtained by a numerical solution of the equations of gas dynamics with allowance for the effects of radiative thermal conductivity; I hope to find such a solution in the future.Astronomical Observatory, L'vov University. Translated from Astrofizika, Vol. 9, No. 2, pp. 307–317, April–June, 1973.     

Cylindrical thin-shell wormholes in f(R) gravity

In this paper, we employ cut and paste scheme to construct thin-shell wormhole of a charged black string with f(R) terms. We consider f(R) model as an exotic matter source at wormhole throat. The stability of the respective solutions are analyzed under radial perturbations in the context of R+δR ² model. It is concluded that both stable as well as unstable solutions do exist for different values of δ. In the limit δ→0, all our results reduce to general relativity.     

Universality in quasi-normal modes of neutron stars

We study universality in gravitational waves emitted from non-rotating neutron stars characterized by different equations of state (EOSs). We find that the quasi-normal mode frequencies of such waves, including the w -modes and the f -mode, display similar universal scaling behaviours that hold for most EOSs. Such behaviours are shown to stem from the mathematical structure of the axial and the polar gravitational wave equations, and the fact that the mass distribution function can be approximated by a cubic–quintic polynomial in the radius. As a benchmark for other realistic neutron stars, a simple model of neutron stars is adopted here to reproduce the pulsation frequencies and the generic scaling behaviours mentioned above with good accuracy.