International Journal of Modern Physics D

The cosmological consequences of a slow-rolling scalar field with constant kinetic term in analogy to the vertical movement of a skydiver after reaching terminal velocity are investigated. In this approach, the scalar field potential is given by a quadratic function of the field. This model provides solutions in which the universe was dominated in the past by a mixture of baryons and dark matter, is currently accelerating (as indicated by type Ia supernovae data), but will be followed by a contraction phase. The theoretical predictions of this model are consistent with current observations, therefore, a terminal scalar field is a viable candidate to dark energy.

The cosmological constant $\mathrm{\Lambda }$ used to be a freedom in Einstein’s theory of general relativity (GR), where one had a proclivity to set it to zero purely for convenience. The signs of $\mathrm{\Lambda }$ or $\mathrm{\Lambda }$ being zero would describe universes with different properties. For instance, the conformal structure of spacetime directly depends on $\mathrm{\Lambda }$: null infinity ${ℐ}$ is a spacelike, null, or timelike hypersurface, if $\mathrm{\Lambda }>0$, $\mathrm{\Lambda }=0$, or $\mathrm{\Lambda }<0$, respectively. Recent observations of distant supernovae have taught us that our universe expands at an accelerated rate, and this can be accounted for by choosing $\mathrm{\Lambda }>0$ in Einstein’s theory of GR. A quantity that depends on the conformal structure of spacetime, especially on the nature of ${ℐ}$, is the Bondi mass which in turn dictates the mass loss of an isolated gravitating system due to energy carried away by gravitational waves. This problem of extending the Bondi mass to a universe with $\mathrm{\Lambda }>0$ has spawned intense research activity over the past several years. Some aspects include a closer inspection on the conformal properties, working with linearization, attempts using a Hamiltonian formulation based on “linearized” asymptotic symmetries, as well as obtaining the general asymptotic solutions of de Sitter-like spacetimes. We consolidate on the progress thus far from the various approaches that have been undertaken, as well as discuss the current open problems and possible directions in this area.

In this paper, we investigate static spherically symmetric wormhole solutions with galactic halo region in the background of $F(T,T_G)$ gravity. Here, $T$ represents torsion scalar and $T_G$ is teleparallel equivalent Gauss–Bonnet term. For this purpose, we consider a diagonal tetrad and two specific $F(T,T_G)$ models. We analyze the wormhole structure through shape function graphically for both models. We also investigate the behavior of null/weak energy conditions. Finally, we evaluate the equilibrium condition to check stability of the wormhole solutions. It is concluded that there exists physically viable wormhole solution only for the first model that turns out to be stable.

Recent studies relating the approximations for the equations-of-state for thin shells and their consequent perturbative evolution are extended to thin-shell wormholes in theories beyond general relativity and more than four spacetime dimensions. The assumption of equations-of-state of the same form for static and slowly evolving shells appears as a strong restriction excluding the possibility of oscillatory evolutions. Then the new results considerably differ from previous ones obtained within the usual linearized approach.

This paper explores the instability of gaseous masses for the radial oscillations in post-Newtonian correction of massive Brans–Dicke (BD) gravity. For this purpose, we derive linearized perturbed equation of motion through Lagrangian radial perturbation which leads to the condition of marginal stability. We discuss radius of instability of different polytropic structures in terms of the Schwarzschild radius. It is concluded that our results provide a wide range of difference with those in general relativity and BD gravity.

Dark energy models and modified gravity theories have been actively studied and the behaviors in the solar system have been also carefully investigated in a part of the models. However, the isotropic solutions of the field equations in the simple models of dark energy, e.g. quintessence model without matter coupling, have not been well investigated. One of the reason would be the nonlinearity of the field equations. In this paper, a method to evaluate the solution of the field equations is constructed, and it is shown that there is a model that can easily pass the solar system tests, whereas, there is also a model that is constrained from the solar system tests.

We analyze the radiative and nonradiative linearized variables in a gravity theory within the family of the nonprojectable Hořava theories, the Hořava theory at the kinetic-conformal point. There is no extra mode in this formulation, the theory shares the same number of degrees of freedom with general relativity. The large-distance effective action, which is the one we consider, can be given in a generally-covariant form under asymptotically flat boundary conditions, the Einstein-aether theory under the condition of hypersurface orthogonality on the aether vector. In the linearized theory, we find that only the transverse-traceless tensorial modes obey a sourced wave equation, as in general relativity. The rest of variables are nonradiative. The result is gauge-independent at the level of the linearized theory. For the case of a weak source, we find that the leading mode in the far zone is exactly Einstein’s quadrupole formula of general relativity, if some coupling constants are properly identified. There are no monopoles nor dipoles in this formulation, in distinction to the nonprojectable Horava theory outside the kinetic-conformal point. We also discuss some constraints on the theory arising from the observational bounds on Lorentz-violating theories.

We analytically describe the properties of the s-wave holographic superconductor with the exponential nonlinear electrodynamics in the Lifshitz black hole background in four-dimensions. Employing an assumption the scalar and gauge fields backreact on the background geometry, we calculate the critical temperature as well as the condensation operator. Based on Sturm–Liouville method, we show that the critical temperature decreases with increasing exponential nonlinear electrodynamics and Lifshitz dynamical exponent, $z$, indicating that condensation becomes difficult. Also we find that the effects of backreaction has a more important role on the critical temperature and condensation operator in small values of Lifshitz dynamical exponent, while $z$ is around one. In addition, the properties of the upper critical magnetic field in Lifshitz black hole background using Sturm–Liouville approach is investigated to describe the phase diagram of the corresponding holographic superconductor in the probe limit. We observe that the critical magnetic field decreases with increasing Lifshitz dynamical exponent, $z$, and it goes to zero at critical temperature, independent of the Lifshitz dynamical exponent, $z$.

We carry out a general study on axially symmetric, static fluids admitting a conformal Killing vector (CKV). The physical relevance of this kind of symmetry is emphasized. Next, we investigate all possible consequences derived from the imposition of such a symmetry. Special attention is paid to the problem of symmetry inheritance. Several families of solutions endowed with a CKV are exhibited.

We propose a new model for studying the dark constituents of the universe by regarding the dark energy as a $q$-deformed scalar field interacting with the dark matter, in the framework of standard general relativity. Here we assume that the number of particles in each mode of the $q$-deformed scalar field varies in time by the particle creation and annihilation. We first describe the $q$-deformed scalar field dark energy quantum-field theoretically, then construct the action and the dynamical structure of these interacting dark sectors, in order to study the dynamics of the model. We perform the phase space analysis of the model to confirm and interpret our proposal by searching the stable attractor solutions implying the late-time accelerating phase of the universe. We then obtain the result that when interaction and equation-of-state parameter of the dark matter evolve from the present day values into a particular value, the dark energy turns out to be a $q$-deformed scalar field.

By proposing a new cosmic fluid model of $-1\le \omega \le 0$ as an alternative to the generalized Chaplygin gas, we reexamine the role of Chaplygin gaslike fluid models in understanding dark energy and dark matter. Instead of as a unified dark matter, the fluid is suggested to be a mixture of unclustered dark energy and pressureless dark matter. Within such a scenario, the sub-horizon fluctuations of matter are stable and scale invariant, similar to those in standard $\mathrm{\Lambda }\text{CDM}$ model.

We report exotic orbital phenomena of spinning test particles orbiting around a Kerr black hole, i.e. some orbits of spinning particles are asymmetrical about the equatorial plane. When a nonspinning test particle orbits around a Kerr black hole in a strong field region, due to relativistic orbital precessions, the pattern of trajectories is symmetrical about the equatorial plane of the Kerr black hole. However, the patterns of the spinning particles’ orbit are no longer symmetrical about the equatorial plane for some orbital configurations and large spins. We argue that these asymmetrical patterns come from the spin–spin interactions between spinning particles and Kerr black holes, because the directions of spin–spin forces can be arbitrary, and distribute asymmetrically about the equatorial plane.

We investigate the Carter-like constant in the case of a particle moving in a nonrelativistic dipolar potential. This special case is a missing link between the Carter constant in stationary and axially symmetric spacetimes (SASS) such as Kerr solution and its possible Newtonian counterpart. We use this system to carry over the definition of angular momentum from the Newtonian mechanics to the relativistic SASS.

In this paper, we have analyzed the dynamical stability of shearing viscous anisotropic fluid with cylindrical symmetry in $f(R,T)$ theory. We have chosen two viable $f(R,T)$ models for dynamical analysis, and explored their nature and role for stable stellar configuration. Modified field equations and corresponding dynamical equations have been constructed, perturbation approach is adopted to deal with complexity of these equations. With the help of perturbed dynamical equations, the evolution equation has been established to analyze the role of shear viscosity and pressure anisotropy on dynamics of cylindrical system. The adiabatic index $\mathrm{\Gamma }$ is used to investigate the instabilities appearing in Newtonian (N) and post-Newtonian (pN) approximations. Some conditions are found for material variables that are required to meet the stability criterion. We compare the outcomes of our analysis with the results of various models available in literature to reach at more comprehensive conclusion.

This paper investigates static spherically symmetric traversable wormhole (WH) solutions in $f({𝒢},T)$ gravity (${𝒢}$ and $T$ represent the Gauss–Bonnet invariant and trace of the energy–momentum tensor, respectively). We construct explicit expressions for ordinary matter by taking specific form of redshift function and $f({𝒢},T)$ model. To analyze the possible existence of wormholes, we consider anisotropic, isotropic, as well as barotropic matter distributions. The graphical analysis shows the violation of null energy condition for the effective energy–momentum tensor throughout the evolution while ordinary matter meets energy constraints in certain regions for each case of matter distribution. It is concluded that traversable WH solutions are physically acceptable in this theory.

It is shown that the structures in the universe can be interpreted to show a closed wheel of time, rather than a straight arrow. An analysis in $f(R)$ gravity model has been carried out to show that due to local observations, a small arc at any given spacetime point would invariably indicate an arrow of time from past to future, though on a quantum scale it is not a linear flow but a closed loop, a fact that can be examined through future observations.

We analyze the magnetic field evolution in dense quark matter with unbroken chiral symmetry, which can be found inside quark and hybrid stars. The magnetic field evolves owing to the chiral magnetic effect in the presence of the electroweak interaction between quarks. In our study, we also take into account the magnetohydrodynamic turbulence effects in dense quark matter. We derive the kinetic equations for the spectra of the magnetic helicity density and the magnetic energy density as well as for the chiral imbalances. On the basis of the numerical solution of these equations, we find that turbulence effects are important for the behavior of small scale magnetic fields. It is revealed that, under certain initial conditions, these magnetic fields behave similarly to the electromagnetic flashes of some magnetars. We suggest that fluctuations of magnetic fields, described in frames of our model, which are created in the central regions of a magnetized compact star, can initiate magnetar bursts.

We propose a new model of $f(R)$ gravity containing Arctan function in the Lagrangian. We show here that this model satisfies fifth force constraint unlike a similar model in 2013 by Kruglov. In addition to this, we carry out the fixed point analysis as well as comment on the existence of curvature singularity in this model. The cosmological evolution for this $f(R)$ gravity model is also analyzed in the Friedmann–Robertson–Walker (FRW) background. To understand observational significance of the model, cosmological parameters are obtained numerically and compared with those of Lambda cold dark matter ($\mathrm{\Lambda }$CDM) model. We also scrutinize the model with supernova data. We apply $Om$ diagnostic given by Sahni et al. in 2008 to the model. Using this diagnostic, we detect the distinction between cosmic evolution caused by the $f(R)$ model and $\mathrm{\Lambda }$CDM. We find best-fit parameter values of the model using baryon acoustic oscillations data.

We study the Starobinsky or $R^2$ model of $f(R)=R+\alpha R^2$ for neutron stars with the structure equations represented by the coupled differential equations and the polytropic type of the matter equation-of-state (EoS). The junction conditions of $f(R)$ gravity are used as the boundary conditions to match the Schwarzchild solution at the surface of the star. Based on these the conditions, we demonstrate that the coupled differential equations can be solved directly. In particular, from the dimensionless EoS $\bar{\rho }=\bar{k}{\bar{p}}^{\gamma }$ with $\bar{k}\sim 5.0$ and $\gamma \sim 0.75$ and the constraint of $\alpha \lesssim 1.47722\times 10^7{\text{m}}^2$, we obtain the minimal mass of the NS to be around 1.44 $M_{\odot }$. In addition, if $\bar{k}$ is larger than 5.0, the mass and radius of the NS would be smaller.

The theory of universal extra dimensions involves Kaluza–Klein (KK) particles. The lightest KK particle (LKP) is one of the good candidates for cold dark matter. Annihilation of LKP dark matter in the Galactic halo produces high-energy gamma-rays. The gamma-ray spectrum shows a characteristic peak structure around the LKP mass. This paper investigates the observability of this peak structure by near-future detectors taking account of their energy resolution and calculates the expected energy spectrum of the gamma-ray signal. Then, by using the High-Energy Stereoscopic System (HESS) data, we set some constraints on the boost factor, which is a product of the annihilation cross-section relative to the thermal one and an uncertain factor dependent on the substructure of the LKP distribution in the Galactic halo, for each LKP mass. The resulting upper limit on the boost factor is in the range from 1 to 30. The constraints can be regarded as comparable with the results of previous work for gamma-ray and electron–positron observation. However, the observational data for the TeV or higher energy region are still limited, and the possible LKP signal is not conclusive. Thus, we expect near-future missions with better sensitivity will clarify whether the LKP dark matter should exist or not.