International Journal of Modern Physics D

Short and long-duration gamma-ray bursts (GRBs) have been recently sub-classified into seven families according to the binary nature of their progenitors. For short GRBs, mergers of neutron star binaries (NS–NS) or neutron star-black hole binaries (NS-BH) are proposed. For long GRBs, the induced gravitational collapse (IGC) paradigm proposes a tight binary system composed of a carbon–oxygen core (CO${}_{\mathrm{\text{core}}}$) and a NS companion. The explosion of the CO${}_{\mathrm{\text{core}}}$ as supernova (SN) triggers a hypercritical accretion process onto the NS companion which might reach the critical mass for the gravitational collapse to a BH. Thus, this process can lead either to a NS-BH or to NS–NS depending on whether or not the accretion is sufficient to induce the collapse of the NS into a BH. We shall discuss for the above compact object binaries: (1) the role of the NS structure and the equation-of-state on their final fate; (2) their occurrence rates as inferred from the X and gamma-ray observations; (3) the expected number of detections of their gravitational wave (GW) emission by the Advanced LIGO interferometer.

We analyze structural parameters of the globular clusters belonging to the Milky Way system which were listed in the latest edition of the Harris Catalogue. We search for observational evidences of the effect of tidal forces induced by the Galaxy on the dynamical and thermodynamical evolution of a globular cluster. The behavior for the $W_0$ distribution exhibited by the globular cluster population seems to be in contrast with theoretical results in literature about gravothermal instability, and suggest a new limit value smaller than the previous one.

If the classical gravitational Lagrangian contains higher-derivative terms ${{ℛ}}^m$, where $m\ge 2$, then vacuum solutions of the Einstein–Hilbert theory $L_0=-R/2{\kappa }^2$ are subject to modification at sufficiently large spacetime curvatures. Previously, we have calculated the effective energy–momentum tensor $S_{ij}$ due to the quartic gravitational terms ${\alpha }^{{}^{\prime }3}{{ℛ}}^4$ of the heterotic superstring theory in the four-dimensional background spacetime of the Schwarzschild black hole, obtaining an expression which satisfies the strong energy condition, and thereby suggests that the ${{ℛ}}^m$ might not remove the central singularity. This conjecture was put forward from a different viewpoint by Horowitz and Myers, who argued that a non-singular black-hole interior resulting from the ${{ℛ}}^m$ would be unstable, necessitating reappraisal of the notion of a singular interior spacetime. Here, we show that the chief features of the solution can be simulated by a Bardeen-type ansatz, assuming the spherically symmetric line element $d{s}^2={\mathrm{\text{e}}}^{\lambda (r)}d{t}^2-{\mathrm{\text{e}}}^{-\lambda (r)}d{r}^2-r^2(d{𝜃}^2+{sin}^2𝜃d{\varphi }^2)$, where ${\mathrm{\text{e}}}^{\lambda (r)}=1-2M{r}^{n(r)}/{(r^2+r_0^2)}^{[n(r)+1]/2}$, which, when $S_i^j\sim r^{-12}$, can explain heuristically why $S_0^0,S_1^1<0$ in the shell region ${\kappa }^{4/3}{M}^{1/3}\ll r\ll {\kappa }^{2}M/4\pi$ of a macroscopic black hole for which $\kappa M\gg 1$, while $\sqrt{-g}{S}_i^j$ remains finite at $r=0$.

In this paper, we examine the cosmological dynamics of a slotheon field in a linear potential. The slotheon correction term $\frac{G^{\mu \nu }}{2M^2}{\pi }_{;\mu }{\pi }_{;\nu }$ respects the galileon symmetry in curved spacetime. We demonstrate the future evolution of universe in this model. We show that in this scenario, the universe ends with the Big Crunch singularity like the standard case. The difference being that the time at which the singularity occurs is delayed in the slotheon gravity. The delay crucially depends upon the strength of slotheon correction. We use observational data from Type Ia Supernovae, Baryon Acoustic Oscillations, and $H(z)$ measurements to constrain the parameters of the model for a viable cosmology, providing the corresponding likelihood contours.

In the present paper, we have constructed a new relativistic anisotropic compact star model having a spherically symmetric metric of embedding class one. Here we have assumed an arbitrary form of metric function $e^{\lambda }$ and solved the Einstein’s relativistic field equations with the help of Karmarkar condition for an anisotropic matter distribution. The physical properties of our model such as pressure, density, mass function, surface red-shift, gravitational redshift are investigated and the stability of the stellar configuration is discussed in details. Our model is free from central singularities and satisfies all energy conditions. The model we present here satisfy the static stability criterion, i.e. $dM/d{\rho }_c>0$ for $0\le {\rho }_c\le 4.04\times 10^{17}$g/cm³ (stable region) and for ${\rho }_c\ge 4.04\times 10^{17}$g/cm³, the region is unstable i.e. $dM/d{\rho }_c\le 0$.

This paper explores dynamics of particles in the combined gravitational and electromagnetic fields of the dyonic Reissner–Nordström background. We discuss possibilities for the particle escape to infinity at inner most stable circular orbit. We study the stability of orbit through Lyapunov exponent and the effective force on particle. The collision of particles is investigated through the center of mass energy near the horizon of black hole. Finally, we compare our results with the motion of particles around Schwarzschild and Reissner–Nordström black hole. We conclude that charge of the black hole has a strong effect on the motion of particles.

Spaniol and Andrade introduced gravitoelectromagnetism in TEGR by considering superpotentials, times the determinant of tetrads, as the gravitoelectromagnetic fields. However, since this defined gravitoelectromagnetic field strength does not give rise to a complete set of Maxwell-like equations, we propose an alternative definition of the gravitoelectromagnetic field strength: instead of superpotentials, torsions are taken as the gravitoelectromagnetic field strengths. Based on this new proposal, we are able to derive a complete set of Maxwell-like equations. We then apply them to obtain explicit expressions of the gravitoelectromagnetic fields both in Schwarzschilds spacetime and for gravitational waves.

The problem of cosmological production of gravitational waves (GWs) is discussed in the framework of an expanding, spatially homogeneous and isotropic FRW type universe with time-evolving vacuum energy density. The GW equation is established and its modified time-dependent part is analytically resolved for different epochs in the case of a flat geometry. Unlike the standard $\mathrm{\Lambda }$CDM cosmology (no interacting vacuum), we show that GWs are produced in the radiation era even in the context of general relativity. We also show that for all values of the free parameter, the high frequency modes are damped out even faster than in the standard cosmology both in the radiation and matter-vacuum dominated epoch. The formation of the stochastic background of gravitons and the remnant power spectrum generated at different cosmological eras are also explicitly evaluated. It is argued that measurements of the CMB polarization (B-modes) and its comparison with the rigid $\mathrm{\Lambda }$CDM model plus the inflationary paradigm may become a crucial test for dynamical dark energy models in the near future.

We propose a model describing the evolution of the free electron current density in graphene. Based on the concept of Mp-branes, we perform the analysis using the difference between curvatures of parallel and anti-parallel spins. In such a framework, an effective graviton emerges in the form of gauge field exchange between electrons. In a plain graphene system, the curvatures produced by both kinds of spins neutralize each other. However, in the presence of defects, the inequality between curvatures leads to the emergence of current density, modified gravity and conductivity. Depending on the type of the defects, the resulting current density can be negative or positive.

The lightest Kaluza–Klein particle (LKP), which appears in the theory of universal extra dimensions (UED), is one of the good candidates for cold dark matter (CDM). When LKP pairs annihilate around the center of the Galaxy where CDM is concentrated, there are some modes which produce electrons and positrons as final products, and we categorize them into two components. One of them is the “line” component, which directly annihilates into electron–positron pair. Another one is the “continuum” component, which consists of secondarily produced electrons and positrons via some decay modes. Before reaching Earth, directions of electrons and positrons are randomized by the Galactic magnetic field, and their energies are reduced by energy loss mechanisms. We assume the LKP is in the mass range from 300GeV to 1500GeV. We calculate the electron plus positron spectrum after propagation in the Galactic halo to Earth, and we analyze the resulting spectrum and positron fraction. We also point out that the energy dependence of observed positron fraction is well reproduced by the mixture of “line” and “continuum” components. We can fit the electron plus positron spectrum and the positron fraction by assuming appropriate boost factors describing dark matter concentration in the Galactic halo. However, it is difficult to explain both the electron plus positron spectrum and the positron fraction by a single boost factor if we take account of observational data obtained by AMS-02 only.

Frequently in Physics, insights and conclusions can be drawn from simple, idealized models. The discovery of critical behavior in the gravitational collapse of a massless scalar field leads to the simulation of binary black holes, from its coalescence to merging and ringdown. We refined a toy model to explore black hole formation as these events unfold to revisit the instability of a gravitational kink. We confirmed a conjecture related to a mass gap for critical behavior at the threshold of black hole formation. We find a critical exponent twice the standard value. Surprisingly, this larger critical exponent is also present in the multiple critical behavior for the black hole formation from a massless scalar field in asymptotically anti-de Sitter spacetimes. What is the meaning of this mass gap? Does it have physical relevance?

In this paper, we perform a cosmological model-independent test of the cosmic distance–duality relation (CDDR) in terms of the ratio of angular diameter distance (ADD) $D=D_{\mathrm{\text{A}}}^{\mathrm{\text{sl}}}/D_{\mathrm{\text{A}}}^{\mathrm{\text{s}}}$ from strong gravitational lensing (SGL) and the ratio of luminosity distance (LD) $D^{\ast }=D_{\mathrm{\text{L}}}^{\mathrm{\text{l}}}/D_{\mathrm{\text{L}}}^{\mathrm{\text{s}}}$ obtained from the joint of type Ia supernovae (SNIa) Union2.1 compilation and the latest Gamma-Ray Bursts (GRBs) data, where the superscripts s and l correspond to the redshifts $z_{\mathrm{\text{s}}}$ and $z_{\mathrm{\text{l}}}$ at the source and lens from SGL samples. The purpose of combining GRB data with SNIa compilation is to test CDDR in a wider redshift range. The LD associated with the redshifts of the observed ADD is obtained through two cosmological model-independent methods, namely, method A: binning the SNIa+GRBs data, and method B: reconstructing the function of DL by combining the Crossing Statistic with the smoothing method. We find that CDDR is compatible with the observations at $1\sigma$ confidence level for the power law model which is assumed to describe the mass distribution of lensing systems with method B in a wider redshift range.

We investigate the impacts of interaction between dark matter (DM) and dark energy (DE) in the context of two DE models, holographic (HDE) and ghost dark energy (GDE). In fact, using the dynamical system analysis, we obtain the cosmological consequence of several interactions, considering all relevant component of universe, i.e. matter (dark and luminous), radiation and DE. Studying the phase space for all interactions in detail, we show the existence of unstable matter-dominated and stable DE-dominated phases. We also show that linear interactions suffer from the absence of standard radiation-dominated epoch. Interestingly, this failure resolved by adding the nonlinear interactions to the models. We find an upper bound for the value of the coupling constant of the interaction between DM and DE as $b<0.57$in the case of holographic model, and $b<0.61$ in the case of GDE model, to result in a cosmological viable matter-dominated epoch. More specifically, this bound is vital to satisfy instability and deceleration of matter-dominated epoch.

We investigate the role of tilted and nontilted congruence in the dynamics of dissipative Lemaître–Tolman–Bondi spacetime in $f(R,T)$ gravity. We consider imperfect fluid with its congruences observed by tilted observer and dust fluid filled with LTB geometry observed by the nontilted observer. In order to elaborate the dynamical features of two congruences, we consider well-known $f(R,T)$ models and develop relationships between tilted and nontilted dynamical variables. We evaluate the nonzero divergence of energy–momentum tensor for tilted congruence and transport equation for the system in $f(R,T)$ gravity. We have also checked the instability regimes for nontilted congruence.

The main purpose of this paper is to study quasinormal modes (QNM) of a black hole in massive gravity with a positive cosmological constant due to electromagnetic perturbations. A detailed study of the QNM frequencies for the electromagnetic field is done by varying the parameters of the theory such as the mass, scalar charge, cosmological constant, and the spherical harmonic index. We have employed the sixth-order WKB approximation to calculate the QNM frequencies. The electromagnetic potential for the near extreme massive gravity de Sitter black hole is approximated with the Pöschl–Teller potential to obtain exact frequencies. The null geodesics of the black hole in massive gravity is employed to describe the absorption cross-sections at high-frequency limit.

We study the structure formation by investigating the spherical collapse model in the context of new agegraphic dark energy (NADE) model in flat FRW cosmology. We compute the perturbational quantities $g(a)$, ${\delta }_c(z_c)$, $\lambda (z_c)$, $\xi (z_c)$, ${\mathrm{\Delta }}_{\mathrm{\text{vir}}}(z_c)$, $log[\nu f(\nu )]$ and $log[n(k)]$ for the NADE model and compare the results with those of EdS and $\mathrm{\Lambda }\mathrm{\text{CDM}}$ models. We find that there is a dark energy-dominated universe at low redshifts and a matter-dominated universe at high redshifts in agreement with the observations. Also, the size of structures, the overdense spherical region, and the halo size in the NADE model are found to be smaller, denser, and larger than those of EdS and $\mathrm{\Lambda }\mathrm{\text{CDM}}$ models. We compare our results with the results of tachyon scalar field and holographic dark energy models.

Primordial black holes (PBHs) could account for variety of phenomena like dark matter, reionization of the universe, early quasars, coalescence of black holes registered through gravitational waves recently. Each phenomenon relates to PBH of a specific mass range. PBH mass spectra varies in a wide range depending on specific model. Earlier, we have shown that PBH with monochromatic mass distribution around $5\times 10^{16}$ g value allow to re-ionize the universe moderately. Here, we show that reionization effect and contribution to dark matter can be simultaneously enhanced with more natural extended mass distribution in the range around the same mass value.

The dynamical aspects of scaling solutions for the dark energy component in the theoretical framework of teleparallel gravity are considered, where dark energy is represented by a scalar field nonminimally coupled with the torsion and with a boundary term, where the boundary coupling term represents the divergence of the torsion vector. The behavior and stability of the scaling solutions are studied for scalar fields endowed with inverse power law potentials and with exponential potentials. It is shown that for scalar fields endowed with inverse power-law potentials, the stability conditions are not affected by the coupling coefficients. For the scalar fields endowed with exponential potentials, two cases are studied: at first, we have considered an infinitesimal deviation from the scaling solution in the corresponding Klein–Gordon equation, and the impact of distinct coupling coefficients on the stability of the solution are analyzed. Secondly, the potential-free case is considered where the dominance of the coupling terms over the potential term is analyzed, discussing the validity of the corresponding particular solution.

The aim of this paper is to study the dynamical instability of expansion-free spherically symmetric anisotropic fluid in the framework of $f({𝒢})$ gravity. We apply perturbation scheme of the first-order to the metric functions as well as matter variables and construct modified field equations for both static and perturbed configurations using power-law $f({𝒢})$ model. To discuss the instability dynamics, we use the contracted Bianchi identities to formulate the dynamical equations in both Newtonian and post-Newtonian regimes. It is found that the range of instability is independent of adiabatic index for expansion-free fluid but depends on anisotropic pressures, energy density and Gauss–Bonnet (GB) terms.