International Journal of Modern Physics D

After reviewing the motivations for cosmological inflation formulated in the formalism of supersymmetry, we argue that the appropriate framework is that of no-scale supergravity. We then show how to construct within this framework inflationary models whose predictions for the tilt in the spectrum of scalar perturbations, $n_s$, and the ratio, $r$, of tensor and scalar perturbations coincide with those of the $R+R^2$ model of inflation proposed by Starobinsky. A more detailed study of no-scale supergravity reveals a structure that is closely related to that of $R^2$ modifications of the minimal Einstein–Hilbert action for general relativity, opening avenues for constructing no-scale de Sitter and anti-de Sitter models by combining pairs of Minkowski models, as well as generalizations of the original no-scale Starobinsky models of inflation. We then discuss the phenomenology of no-scale models of inflation, including inflaton decay and reheating, and then the construction of explicit scenarios based on SU(5), SO(10) and string-motivated flipped SU(5)×U(1) GUT models. The latter provides a possible model of almost everything below the Planck scale, including neutrino masses and oscillations, the cosmological baryon asymmetry and cold dark matter, as well as $n_s$ and $r$.

We start by looking at why we believe that black holes have entropy. According to Boltzmann, the entropy is a measure of the number of microstates of a system. We suggest here that the entropy arises from a holographic conformal field theory on the black hole horizon. Finally, we discuss some of the implications for the information paradox.

In this paper, we study some features of the Kerr metric both from an analytic and a visual point of view by performing accurate raytracing in various situations. We focus on features that are unique to the maximal analytic extension of the Kerr metric as compared to that of the Schwarzschild or even the Reissner–Nordström one. A large number of new, yet underexplored phenomena appear, especially regarding the structure of bounded null geodesics and the aspect of the negative gravity regions whose visual characteristics are shown both from outside and inside it.

The paper studies the uniqueness and the monotonicity of the mass-spin relation of black holes (BHs) the X-ray power density spectra of which contain twin high-frequency quasi-periodic oscillations in 3:2 ratio. Some of the results are valid for a relatively large class of models — the geodesic models, which attribute the observed pair of HF QPOs to the frequencies of epicyclic motion of inhomogeneities in the accretion disk surrounding the black hole, treated as point particles, or to simple linear combinations of them. It is found that for geodesic models the properties of the mass-spin relation are independent of the observed frequencies, i.e. they are independent of the particular object. For concreteness two of the most commonly used geodesic models are studied here – the 3:1 nonlinear epicyclic resonance model and its Keplerian version.

A three-dimensional slowly rotating black hole solution in the presence of negative cosmological constant in the Einstein-power-Maxwell theory is studied. It is shown that in the small rotation limit, the electric field, diagonal metric function and thermodynamic properties are the same as for static case, whereas the small rotation gives in addition a nondiagonal metric function and magnetic field which are also small. For these functions cased by rotation of black hole, exact integral solution and analytic asymptotic solution were obtained.

The effective anisotropic stresses induced by the scalar modes of the geometry depend on the coordinate system so that the comparison of the competing results is ultimately determined by the evolution of the pivotal variables in each particular gauge. After arguing that the only reasonable physical coordinate systems for this problem are the ones where the gauge freedom is completely fixed (like the longitudinal and the uniform curvature gauges), we propose a novel gauge-invariant strategy for the comparison of gauge-dependent results. Instead of employing the pivotal variables of a given coordinate system, the effective anisotropic stress is solely expressed in terms of the gravitating normal modes of the plasma and in terms of their conformal time derivatives. The new approach is explicitly gauge-invariant and when the wavelengths of the normal modes are either shorter or larger than the sound horizon, the physical limits of the anisotropic stresses are determined without relying on the specific details of the background evolution. The relevance of the proposed strategy is discussed in the general situation where the scalar anisotropic stress and the nonadiabatic pressure fluctuations are simultaneously present. We finally argue that the anisotropic stress can be most efficiently obtained from the second-order effective action of the curvature inhomogeneities.

We study some consequences of a wave packet resulting as a solution in a class of Supersymmetric Wheeler–DeWitt equation in Kantowski–Sachs minisuperspace. As a result of our analysis, we show that the Supersymmetric noncommutative proposal can modify significantly the universe evolution. The volume expansion, the shear and the characteristic volume are calculated explicit.

We study an $SO(1,3)$ pure connection formulation in four dimensions for real-valued fields, inspired by the Capovilla, Dell and Jacobson complex self-dual approach. By considering the CMPR BF action, also, taking into account a more general class of the Cartan–Killing form for the Lie algebra $𝔰𝔬(1,3)$ and by refining the structure of the Lagrange multipliers, we integrate out the metric variables in order to obtain the pure connection action. Once we have obtained this action, we impose certain restrictions on the Lagrange multipliers, in such a way that the equations of motion led us to a family of torsionless conformally flat Einstein manifolds, parametrized by two numbers. Finally, we show that, by a suitable choice of parameters, self-dual spaces (Anti-) de Sitter can be obtained.

In this paper, we give a physical explanation to the accelerated expansion of the universe, alleviating the tension between the discrepancy of Hubble constant measurements. By the EulerCauchy stress principle, we identify a controversy on the lack of consideration of the surface forces contemplated in the study of the expansion of the universe. We distinguish a new general relativistic effect that modifies the spacetime fabric by means of the energy conservation equation. The resulting dynamical equations from the proposed hypothesis are contrasted to several testable astrophysical predictions. This paper also explains why we have not found any particle or fluid responsible for dark energy and clarifies the Cosmological Coincidence Problem. These explanations are achieved without assuming the existence of exotic matter of unphysical meaning or having to modify Einstein’s Field Equations.

In this paper, we have studied the propagation of axial gravitational waves in Bianchi I universe using the Regge–Wheeler gauge. In this gauge, there are only two nonzero components of $h_{\mu \nu }$ in the case of axial waves: $h_0(t,r)$ and $h_1(t,r)$. The field equations in absence of matter have been derived both for the unperturbed as well as axially perturbed metric. These field equations are solved simultaneously by assuming the expansion scalar $\mathrm{\Theta }$ to be proportional to the shear scalar $\sigma$ (so that $a=b^n$, where $a$, $b$ are the metric coefficients and $n$ is an arbitrary constant), and the wave equation for the perturbation parameter $h_0(t,r)$ has been derived. We used the method of separation of variables to solve for this parameter, and have subsequently determined $h_1(t,r)$. We then discuss a few special cases to interpret the results. We find that the anisotropy of the background spacetime is responsible for the damping of the gravitational waves as they propagate through this spacetime. The perturbations depend on the values of the angular momentum $l$. The field equations in the presence of matter reveal that the axially perturbed spacetime leads to perturbations only in the azimuthal velocity of the fluid leaving the matter field undisturbed.

This paper is devoted to discuss the attractor solutions of inflationary Chaplygin gas models such as generalized Chaplygin gas, modified Chaplygin gas and generalized cosmic Chaplygin gas in the framework of Randall–Sundrum type II braneworld scenario. We investigate the inflationary parameters like scalar spectral index $n_s$, tensor to scalar ratio $r$, and the running of scalar index $\frac{d{n}_s}{dln\kappa }$ as a function of e-folding numbers $N$ in the presence of attractor: $n_s-1\propto \frac{1}{N}$. We evaluate and reformulate these parameters under high energy condition. In this inflationary scenario, we develop $n_s-N$, $r-N$, $r-n_s$ and $\frac{d{n}_s}{dln\kappa }-N$ planes. We also found that these cosmological parameters and perturbation strongly agree with the recent Planck data 2018 for considered Chaplygin gas models instead of $\frac{d{n}_s}{dln\kappa }$ in case of generalized cosmic Chaplygin gas.

In this work, we study cosmological evolution in the mini creation event with anisotropic fluid under the Bianchi-I spacetime. After providing the basic mathematical formalism of the model, we find out the exact solutions in its particular form to the field equations. In order to get physical validity, we have presented elaborate discussions on the graphical results, especially nonsingular behaviour of the model. It is shown that the model under mini bang can successfully exhibit several interesting cosmological features which have specific signatures to tally with the observational evidences.

In this paper, by the use of entangled and nonentangled coherent and squeezed state formalism of two-mode nonclassical states, we studied the chaotic inflationary model of a massive scalar field with quadratic potential in the semiclassical gravity, derived from canonical quantum gravity. It was found that the semiclassical quantum gravity leads to the same power-law expansion of the universe as that of the matter-dominated era ${\tau }^{\frac{2}{3}}$ in an oscillatory phase of the scalar field in all the nonclassical quantum states considered. The coherently oscillating scalar field in the expanding universe suffers from the phenomenon of particle creation which restricts the duration of stable coherent oscillations of the scalar field dependent on the parameters of the states considered and affect in a certain way the abundant particle production owing to the parametric resonance of bosonic fields coupled to this coherently oscillating scalar field.

In this paper, the scalar–tensor theory is applied to the study of perturbations in a multifluid universe, using the $1+3$ covariant approach. Both scalar and harmonic decompositions are instituted on the perturbation equations. In particular, as an application, we study perturbations on a background Friedmann-Robertson-Walker (FRW) cosmology consisting of both radiation and dust in the presence of a scalar field. We consider both radiation-dominated and dust-dominated epochs, respectively, and study the results. During the analysis, quasi-static approximation is instituted. It is observed that the fluctuations of the energy density decrease with increasing redshift, for different values of $n$ of a power-law $R^n$ model.

The circular orbits of a spinning test particle moving around a charged Hayward black hole is investigated by using the Mathisson–Papapetrou–Dixon equations together with the Tulczyjew spin-supplementary condition. By writing the equations of motion, the effective potential for the description of the test particle is obtained to study the properties of the Innermost Stable Circular Orbit (ISCO). The results show that the ISCO radii for spinning particles moving in the charged Hayward background differ from those obtained in the corresponding Schwarzschild or Reissner–Nordstrom spacetimes, depending on the values of the electric charge and the length-scale parameter of the metric. When the spin of the particle and its orbital angular momentum are aligned, an increase in the spin produces a decrease in the ISCO radius, while in the case in which the spin of the particle and its orbital angular momentum are anti-aligned, an increase in the spin results in an increase of the radius of the ISCO.