Abstract: Neutron star mergers have been long considered as promising sites of heavy r-process nucleosynthesis. We overview the observational evidence supporting this scenario including: the total amount of ...

Abstract: We investigate the analog effect of the Aharonov–Bohm effect for bound states in two relativistic quantum systems in a spacetime with a spacelike dislocation. We assume that the topological defect has an internal magnetic flux. Then, we analyze the interaction of a charged particle with a uniform magnetic field in this topological defect spacetime, and thus, we extend this analysis to the confinement of a hard-wall potential and a linear scalar potential. Later, the interaction of the Klein–Gordon oscillator with a uniform magnetic field is analyzed. We first focus on the effects of torsion that stem from the spacetime with a spacelike dislocation and the geometric quantum phase. Then, we analyze the effects of torsion and the geometric quantum phase under the presence of a hard-wall potential and a linear scalar potential.

Abstract: The aim of this paper is to explore some physically viable aspects for the possible emergence of compact stars in f(G,T) theory of gravity with some particular models, where G and T are Gauss–Bonnet invariant and trace of stress–energy tensor, respectively. We present basic formalism of this modified theory in the presence of anisotropic source. We explore some realistic aspects using the energy conditions with physical parameters. Three distinct known star models namely, SAXJ1808.4−3658, Her X−1 and 4U1820−30, are used for this systematic investigation. The physical behavior of anisotropic stresses, energy density, energy conditions, measure of anisotropy and stability of compact stars are discussed through plots. We conclude that compactness at the core of a star model increases and energy conditions hold.

Abstract: Motivated by the substantial modifications of gravitational theories and by the models that come out of f(R), we apply the field equation of the charged f(R) = R + βR2 as well as a general vector p...

Abstract: We study the photon’s motion around a black hole in the presence of a plasma whose density is a function of the radius coordinate by a renewed ray-tracing algorithm and investigate the influence of the plasma on the shadow of the black hole. The presence of plasma affects not only the size but also the shape of the black hole shadow. Furthermore, the influence of plasma on trajectories of photons depends on the frequency of the photons. For the high-frequency photons, the influence is negligible, on the contrary, the trajectories of low-frequency photons are affected significantly by the plasma. Interestingly, it is also found that the black hole image would take on a multi-ring structure due to the presence of plasma.

Abstract: Screened modified gravity models evade solar system tests of relativistic gravitation but exhibit novel and interesting effects on scales between the solar system and the Hubble flow: astrophysical...

Abstract: We consider spherically symmetric configurations in general relativity (GR), supported by nonlinear electromagnetic fields with gauge-invariant Lagrangians depending on the single invariant f = FμνFμν. Static black hole (BH) and solitonic solutions are briefly described, both with only an electric or magnetic charge and with both nonzero charges (the dyonic ones). It is stressed that only pure magnetic solutions can be completely nonsingular. For dyonic systems, apart from a general scheme of obtaining solutions in quadratures for an arbitrary Lagrangian function L(f), an analytic solution is found for the truncated Born–Infeld theory (depending on the invariant f only). Furthermore, considering spherically symmetric metrics with two independent functions of time, we find a natural generalization of the class of wormholes found previously by Arellano and Lobo with a time-dependent conformal factor. Such wormholes are shown to be only possible for some particular choices of the function L(f), having no Max...

Abstract: The number of gamma-ray bursts (GRBs) detected at high energies (∼0.1 − 100GeV) has seen a rapid increase over the last decade, thanks to observations from the Fermi-Large Area Telescope. The improved statistics and quality of data resulted in a better characterization of the high-energy emission properties and in stronger constraints on theoretical models. In spite of the many achievements and progresses, several observational properties still represent a challenge for theoretical models, revealing how our understanding is far from being complete. This paper reviews the main spectral and temporal properties of ∼0.1 − 100GeV emission from GRBs and summarizes the most promising theoretical models proposed to interpret the observations. Since a boost for the understanding of GeV radiation might come from observations at even higher energies, the present status and future prospects for observations at very-high energies (above ∼100GeV) are also discussed. The improved sensitivity of upcoming facilities, coup...

Abstract: We investigate particle motion and collisions in the vicinity of rotating black holes immersed in combined cosmological quintessential scalar field and external magnetic field. The quintessential d...

Abstract: In this paper, we shall analyze the f(𝒢) gravity phase space, in the case that the corresponding dynamical system is autonomous. In order to make the dynamical system autonomous, we shall appropriately choose the independent variables, and we shall analyze the evolution of the variables numerically, emphasizing on the inflationary attractors. As we demonstrate, the dynamical system has only one de Sitter fixed point, which is unstable, with the instability being traced in one of the independent variables. This result holds true both in the presence and in the absence of matter and radiation perfect fluids. We argue that this instability could loosely be viewed as an indication of graceful exit in the f(𝒢) theory of gravity.

Abstract: Wormholes are tunnels connecting different regions in spacetime. They were obtained originally as a solution for Einstein’s General Relativity theory and according to this theory, they need to be f...

Abstract: A promising theory in modifying general relativity (GR) by violating the ordinary energy–momentum conservation law in curved spacetime is the Rastall theory of gravity. In this theory, geometry and matter fields are coupled to each other in a nonminimal way. Here, we study thermodynamic properties of some black hole (BH) solutions in this framework, and compare our results with those of GR. We demonstrate how the presence of these matter sources amplifies the effects caused by the Rastall parameter in thermodynamic quantities. Our investigation also shows that BHs with radius smaller than a certain amount ( ≡ r0) have negative heat capacity in the Rastall framework. In fact, it is a lower bound for the possible values of horizon radius satisfied by the stable BHs.

Abstract: Growing evidence connects the progenitor systems of the short-hard subclass of gamma-ray bursts (GRBs) to the merger of compact object binaries composed of two neutron stars (NSs) or of an NS and a black hole (BH). The recent observation of the binary NS (BNS) merger event GW170817 associated with GRB 170817A brought a great deal of additional information and provided further support to the above connection, even though the identification of this burst as a canonical short GRB (SGRB) remains uncertain. Decades of observational constraints and theoretical models consolidated the idea of a jet origin for the GRB prompt emission, which can also explain the multiwavelength afterglow radiation observed in most of the events. However, the mechanisms through which a BNS or NS–BH merger remnant would power a collimated outflow are much less constrained. Understanding the properties of the remnant systems and whether they can provide the right conditions for jet production has been a main driver of the great effor...

Abstract: The recent analysis of low-redshift supernovae (SN) has increased the apparent tension between the value of H0 estimated from low and high redshift observations such as the cosmic microwave background (CMB) radiation. At the same time other observations have provided evidence of the existence of local radial inhomogeneities extending in different directions up to a redshift of about 0.07. About 40% of the Cepheids used for SN calibration are directly affected because they are located along the directions of these inhomogeneities. We compute with different methods the effects of these inhomogeneities on the low-redshift luminosity and angular diameter distance using an exact solution of the Einstein’s equations, linear perturbation theory and a low-redshift expansion. We confirm that at low redshift the dominant effect is the nonrelativistic Doppler redshift correction, which is proportional to the volume averaged density contrast and to the comoving distance from the center. We derive a new simple formula...

Abstract: We show that a generalized Dirac structure survives beyond the linear regime of the low-energy dispersion relations of graphene. A generalized uncertainty principle of the kind compatible with spec...

Abstract: The value of the cosmological constant is explained in terms of a noisy diffusion of energy from the low energy particle physics degrees of freedom to the fundamental Planckian granularity which is...

Abstract: We present a review on Lagrangian models admitting spherically symmetric regular black holes (RBHs), and cosmological bounce solutions. Nonlinear electrodynamics, nonpolynomial gravity, and fluid approaches are explained in details. They consist respectively in a gauge invariant generalization of the Maxwell–Lagrangian, in modifications of the Einstein–Hilbert action via nonpolynomial curvature invariants, and finally in the reconstruction of density profiles able to cure the central singularity of black holes. The nonpolynomial gravity curvature invariants have the special property to be second-order and polynomial in the metric field, in spherically symmetric spacetimes. Along the way, other models and results are discussed, and some general properties that RBHs should satisfy are mentioned. A covariant Sakharov criterion for the absence of singularities in dynamical spherically symmetric spacetimes is also proposed and checked for some examples of such regular metric fields.

Abstract: One of the most interesting astronomical objects is the Galactic Center. It is a subject of intensive astronomical observations in different spectral bands in recent years. We concentrate our discussion on a theoretical analysis of observational data of bright stars in the IR-band obtained with large telescopes. We also discuss the importance of VLBI observations of bright structures which could characterize the shadow at the Galactic Center. If we adopt general relativity (GR), there are a number of theoretical models for the Galactic Center, such as a cluster of neutron stars, boson stars, neutrino balls, etc. Some of these models were rejected or the range of their parameters is significantly constrained with consequent observations and theoretical analysis. In recent years, a number of alternative theories of gravity have been proposed because there are dark matter (DM) and dark energy (DE) problems. An alternative theory of gravity may be considered as one possible solution for such problems. Some of...

Abstract: We investigate the Kerr–Newman–NUT black hole solution obtained from Plebanski–Demianski solutions with several assumptions. The origin of the microscopic entropy of this black hole is investigated...

Abstract: We extend to nonstatic black holes our benchmarking scheme that allows for cross–comparison of the efficiencies of asymptotically AdS black holes used as working substances in heat engines. We use a circular cycle in the p–V plane as the benchmark cycle. We study Kerr black holes in four spacetime dimensions as an example. As in the static case, we find an exact formula for the benchmark efficiency in an ideal gas-like limit, which may serve as an upper bound for rotating black hole heat engines in a thermodynamic ensemble with fixed angular velocity. We use the benchmarking scheme to compare Kerr to static black holes charged under Maxwell and Born–Infeld sectors.

Abstract: The Higgs mechanism is one of the central pieces of the Standard Model of electroweak interactions and thanks to it we can generate the masses of the elementary particles. Its fundamental origin is...

Abstract: With the increasing wealth of high-quality astronomical and cosmological data and the manifold departures from General Relativity in principle conceivable, the development of generalized parametriz...

Abstract: A self-sustained traversable wormhole is obtained as a vacuum solution of a scale-dependent gravitational theory. Comparisons with other approaches towards wormhole self-sustainability are presente...

Abstract: Changes in the law of gravity have far-reaching implications for the formation and evolution of galaxy clusters, and appear as peculiar signatures in their mass-observable relations, structural pro...

Abstract: We present an elastic constitutive model of gravity where we identify physical space with the mid-hypersurface of an elastic hyperplate called the “cosmic fabric” and spacetime with the fabric’s wo...

Abstract: We discuss the future of gravitational theories in the framework of gravitational wave (GW) astronomy after the recent GW detections (the events GW150914, GW151226, GW170104, GW170814, GW170817 and GW170608). In particular, a calculation of the frequency and angular dependent response function that a GW detector would see if massive modes from f(R) theories or scalar–tensor gravity (STG) were present, allowing for sources incident from any direction on the sky, is shown. In addition, through separate theoretical results which do not involve the recent GW detections, we show that f(R) theories of gravity having a third massless mode are ultimately ruled out while there is still room for STG having a third (massive or massless) mode and for f(R) theories of gravity having a third massive mode.

Abstract: When a positively charged impurity is placed inside a cold metal, the resulting charge density around that object exhibits characteristic ripples to negative values, known as Friedel oscillations. ...

Abstract: In this short note we present the solution of Rastall gravity equations sourced by a Gaussian matter distribution. We find that the black hole metric shares all the common features of other regular, General Relativity BH solutions discussed in the literature: there is no curvature singularity and the Hawking radiation leaves a remnant at zero temperature in the form of a massive ordinary particle.

Abstract: The dark matter (DM) in neutron stars can exist from the lifetime of the progenitor or when captured by this compact object. The properties of DM that enter the neutron stars through each step coul...