International Journal of Modern Physics D

The origin of gamma-ray burst (GRB) prompt emission, bursts of γ-rays lasting from shorter than one second to thousands of seconds, remains not fully understood after more than 40 years of observations. The uncertainties lie in several open questions in the GRB physics, including jet composition, energy dissipation mechanism, particle acceleration mechanism and radiation mechanism. Recent broad-band observations of prompt emission with Fermi sharpen the debates in these areas, which stimulated intense theoretical investigations invoking very different ideas. I will review these debates, and argue that the current data suggest the following picture: A quasi-thermal spectral component originating from the photosphere of the relativistic ejecta has been detected in some GRBs. Even though in some cases (e.g. GRB 090902B) this component dominates the spectrum, in most GRBs, this component either forms a sub-dominant "shoulder" spectral component in the low energy spectral regime of the more dominant "Band" component, or is not detectable at all. The main "Band" spectral component likely originates from the optically thin region due to synchrotron radiation. The diverse magnetization in the GRB central engine is likely the origin of the observed diverse prompt emission properties among bursts.

We explore, in the context of AdS/CFT correspondence, the causality constraints on the Noncritical Einstein–Weyl (NEW) gravity model in five dimensions. The scalar and shear channels are considered as small metric perturbations around an AdS black brane background. Our results show that the causality analysis on the propagation of these two channels imposes a new bound on the coupling of the Weyl-squared term in the NEW gravity. This new bound imposes more stringent restrictions than those by the tachyon-free condition, improving the predictive power of the theory.

The problem of the cosmic acceleration is here revisited by using the fact that the adiabatic speed of sound can be assumed to be negligible small. Within the context of general relativity, the total energy budget is recovered under the hypothesis of a vanishing speed of sound by assuming the existence of one fluid only. We find a cosmological model which reproduces the main results of the ΛCDM paradigm at late-times, showing an emergent cosmological constant, which is not at all related with the vacuum energy term. As a consequence, the model presented here behaves as a unified dark energy (DE) model.

In this paper, we investigate the physics of a radiating star undergoing dissipative collapse in the form of a radial heat flux. Our treatment clearly demonstrates how the presence of shear affects the collapse process; we are in a position to contrast the physical features of the collapsing sphere in the presence of shear with the shear-free case. By employing a causal heat transport equation of the Maxwell–Cattaneo form we show that the shear leads to an enhancement of the core temperature thus emphasizing that relaxational effects cannot be ignored when the star leaves hydrostatic equilibrium.

Relational particle mechanics models bolster the relational side of the absolute versus relational motion debate. They are additionally toy models for the dynamical formulation of general relativity (GR) and its problem of time (PoT). They cover two aspects that the more commonly studied minisuperspace GR models do not: (1) by having a nontrivial notion of structure and thus of cosmological structure formation and of localized records. (2) They have linear as well as quadratic constraints, which is crucial as regards modeling many PoT facets. I previously solved relational triangleland classically, quantum mechanically and as regards a local resolution of the PoT. This rested on triangleland's shape space being 𝕊² with isometry group SO(3), allowing for use of widely-known geometry, methods and atomic/molecular physics analogies. I now extend this work to the relational quadrilateral, which is far more typical of the general N-a-gon, represents a "diagonal to nondiagonal Bianchi IX minisuperspace" step-up in complexity, and encodes further PoT subtleties. The shape space now being ℂℙ² with isometry group SU(3)/ℤ₃, I now need to draw on geometry, shape statistics and particle physics to solve this model; this is therefore an interdisciplinary paper. This Paper treats quadrilateralland at the classical level, and then paper II provides a quantum treatment.

We investigate holographic dark energy (HDE) correspondence of interacting Generalized Chaplygin Gas (GCG) in the framework of compact Kaluza–Klein (KK) cosmology. The evolution of the modified HDE with corresponding equation of state is obtained here. Considering the present value of the density parameter a stable configuration is found which accommodates Dark Energy (DE). We note a connection between DE and Phantom fields. It reveals that the DE might have evolved from a Phantom state in the past.

In this paper, we first deduce the Tolman–Oppenheimer–Volkoff (TOV) equations and Schwarzschild–de Sitter (SdS) constant-density interior solutions of perfect fluid spheres in hydrostatic equilibrium by the Einstein equations with a nonzero cosmological constant. The TOV equations and the spacetime properties of exact solutions inside uniform perfect fluid spheres with different spatial curvature and cosmological constants will be respectively analyzed in detail. Moreover, a brief comparison between the internal static solutions of the SdS type and the dynamical Einstein–Strauss–de Sitter (ESdS) vacuole spacetime is obtained.

Jacobi polynomials appear to play a very important role in describing all the spin field (s = 0, 1/2, 1, 2) perturbation of the Friedmann–Robertson–Walker (FRW) spacetime. The formulation becomes very transparent when done in Newman–Penrose (NP) formalism. All the variables are separable, and the spatial eigenfunctions turn out to be Jacobian polynomials in different forms. In particular, the angular ones are expressible as spin weighted spherical harmonics which are just the spherical harmonics formed with Jacobi polynomials. The radial eigenfunctions are also Jacobi polynomials but with unconventional parameters. Various properties of these polynomials are used to describe the scalar, vector and tensor modes of the perturbation. The Green's function of the scalar perturbations and also its Lienard–Wiechert type potentials are derived, and are shown to reduce to the familiar ones in the limit to flat FRW case. Some of the components of the perturbed metric tensor h^μν have also been calculated.

We discuss the consequences of a duality-invariant Einstein–Planck (DIEP) relation on the equation of state of micro black holes. The results are analogous to those obtained from the "world-crystal" model, but with some significative differences, as for instance a limiting vanishing value for temperature for very small black holes. The model leads to a total evaporation of micro black holes but with the final stage being very slow.

It is believed that soon after the Planck era, spacetime should have a semi-classical nature. Therefore, it is unavoidable to modify the theory of general relativity or look for alternative theories of gravitation. An interesting possibility found in the literature considers two geometric counter-terms to regularize the divergences of the effective action. These counter-terms are responsible for a higher-order derivative metric theory of gravitation. In the present paper, we investigate how isotropization occurs. For this reason a single solution is chosen throughout this paper. We obtain perturbatively, by two different methods, that the tensor and scalar components emerge naturally during the isotropization process. In this sense our result provides a numerical example to Stelle's well-known result on classical gravity with higher derivates. Our entire analysis is restricted to the particular Bianchi type I case.

In this paper, we study interacting modified Chaplygin gas (MCG) which has shear and bulk viscosities. We consider sign-changeable interaction between MCG and matter, then investigate the effects of shear and bulk viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter.

We perform a phase space analysis of a nonminimally coupled modified gravity theory with the Lagrangian density of the form , where f₁(R) and f₂(R) are arbitrary functions of the curvature scalar R and is the matter Lagrangian density. We apply the dynamical system approach to this scenario in two particular models. In the first model we assume f₁(R) = 2R with a general form for f₂(R) and set favorable values for effective equation of state parameter which is related to the several epochs of the cosmic evolution and study the critical points and their stability in each cosmic eras. In the second case, we allow the f₁(R) to be an arbitrary function of R and set f₂(R) = 2R. We find the late-time attractor solution for the model and show that this model has a late-time accelerating epoch and an acceptable matter era.