International Journal of Modern Physics D

Viscous generalized Chaplygin gas (GCG) cosmology is discussed, assuming that there is bulk viscosity in the linear barotropic fluid and GCG. w_γ = γ - 1 and w_g represent the state equation parameters for barotropic fluid and GCG, respectively, in which γ = 1 or corresponds to ordinary matter or radiation. The dynamical analysis indicates that the phase is a dynamical attractor and the equation of state of GCG approaches it from either w_g > -1 or w_g < -1 depending on the choice of its initial cosmic density parameter and the ratio of pressure to critical energy density, where τ_g and τ_γ are viscosity parameters. Therefore, the equation of state w_g will cross the boundary w_g = -1 if we choose initial value w_g < -1. Furthermore, we show that bulk viscosity coefficients should satisfy inequalities from the point of view of dynamics.

We consider a cosmological model with a variable gravitational constant, G, based on a scalar–tensor theory. Using the recent observational data for the Hubble diagram of type Ia supernovae (SNeIa), we find a phenomenological expression describing the variation of G. The corresponding variation of the fine structure constant α within multidimensional theories is also computed and is shown not to support known constraints on Δα/α.

The objective of this paper is to find out the suitability of an ansatz similar to that suggested by Vaidya–Tikekar, but prescribing paraboloidal geometry for the 3-space of the interior space–time of a relativistic spherical star in describing a family of physically viable models of superdense stars like Her X-1, SAX, and X-ray brust.

We study Doppler effects in curved space–time, i.e. the frequency shifts induced on electromagnetic signals propagating in the gravitational field. In particular, we focus on the frequency shift due to the bending of light rays in weak gravitational fields. We consider, using the PPN formalism, the gravitational field of an axially symmetric distribution of mass. The zeroth order, i.e. the sphere, is studied then passing to the contribution of the quadrupole moment, and finally to the case of a rotating source. We give numerical estimates for situations of physical interest, and by a very preliminary analysis, we argue that analyzing the Doppler effect could lead, in principle, in the foreseeable future, to the measurement of the quadrupole moment of the giant planets of the Solar System.

We study the common relationships that exist between the various structures in the Universe, and show that a unifying description appears when these are considered as emerging from dynamical critical phenomena characterized by complex exponents in the two-point correlation function of matter density fluctuations. Since gravity drives their formation, structures are more likely to form where there is maximal correlation in the matter density. Applying this simple principle of maximal correlation to the two-point correlation function in a scaling regime with complex exponents leads to a hierarchy of structures where: (1) the structures can be classified according to an integer and (2) there is a common real exponent for the two-point correlation function across the range of structures. This in turn implies the existence of both universal size and mass hierarchy-order relationships. We show that these relationships are in good agreement with observations, and that sizes and masses for the known structures, from Globules in the Interstellar Medium to Clusters of Galaxies, can be classified (essentially to within one order of magnitude out of more than 10 orders of magnitude) in terms of just three constants.

The metric of some Lorentzian wormholes in the background of the Friedman–Robertson–Walker (FRW) universe is obtained. It is shown that for a de Sitter space–time, the new solution is supported by phantom energy. The wave equation for a scalar field in such backgrounds is separable. The form of the potential for the Schrödinger type one-dimensional wave equation is found.

Gravitational collapse of FRW braneworld embedded in a conformally flat bulk is considered for matter cloud consisting of dark matter and dark energy with equation of state p = ∊ρ (∊ < -1/3). The effect of dark matter and dark energy is being considered first separately and then a combination of them with and without interaction. In some cases the collapse leads to black hole, in some other cases naked singularity appears.

We consider the scenario where our observable universe is devised as a dynamical four-dimensional hypersurface embedded in a five-dimensional bulk space–time, with a large extra dimension, which is the generalization of the flat FRW cosmological metric to five dimensions. This scenario generates a simple analytical model where different stages of the evolution of the universe are approximated by distinct parametrizations of the same space–time. In this model the evolution from decelerated to accelerated expansion can be interpreted as a "first-order" phase transition between two successive stages. The dominant energy condition allows different parts of the universe to evolve, from deceleration to acceleration, at different redshifts within a narrow era. This picture corresponds to the creation of bubbles of new phase, in the middle of the old one, typical of first-order phase transitions. Taking Ω_m = 0.3 today, we find that the cross-over from deceleration to acceleration occurs at z ~ 1 - 1.5, regardless of the equation of state in the very early universe. In the case of primordial radiation, the model predicts that the deceleration parameter "jumps" from q ~ +1.5 to q ~ -0.4 at z ~ 1.17. At the present time q = -0.55 and the equation of state of the universe is w = p/ρ ~ -0.7, in agreement with observations and some theoretical predictions.

Using the "δN-formalism," we obtain the expression of the non-Gaussianity of multiple-field inflationary models with the non-trivial field-space metric. Further, we have rewritten the result by using the slow-rolling approximation.

We extend the statistical mechanical theory of cosmological many-body problem to a system consisting of two kinds of galaxies with different masses. The general partition function for such a two-component system, in the grand canonical ensemble, is developed. From this partition function various thermodynamical quantities and distribution functions are evaluated. A quantity b_m, called the clustering parameter for the two-component system, which inherently takes into account the ratio of the number of particles (galaxies) of each kind and their masses, emerges directly from the calculations. Various clustering phenomena can be explained on the basis of this parameter. We find these results in agreement with the available N-body simulation results as well as the observational distribution function determined from the 2MASS survey. Besides, our distribution function, for the two-component system, makes these comparisons more objective and easily comprehensible.

The issue of non-Gaussianity is not only related to distinguishing the theories of the origin of primordial fluctuations, but also crucial for the determination of cosmological parameters in the framework of inflation paradigm. We present a method for testing non-Gaussianity on the whole-sky cosmic microwave background (CMB) anisotropies. This method is based on the Kuiper's statistic to probe the two-dimensional uniformity on a periodic mapping square associating phases: return mapping of phases of the derived CMB (similar to auto-correlation) and cross-correlations between phases of the derived CMB and foregrounds. Since phases reflect morphology, detection of cross-correlation of phases signifies the contamination of foreground signals in the derived CMB map. The advantage of this method is that one can cross-check the auto- and cross-correlation of phases of the derived maps and foregrounds, and mark off those multipoles in which the non-Gaussianity results from the foreground contaminations. We apply this statistic on the derived signals from the 1-year WMAP data. The auto-correlations of phases from the internal linear combination map show the significance above 95% C.L. against the random phase hypothesis on 17 spherical harmonic multipoles, among which some have pronounced cross-correlations with the foreground maps. We find that most of the non-Gaussianity found in the derived maps are from foreground contaminations. With this method we are better equipped to approach the issue of non-Gaussianity of primordial origin for the upcoming Planck mission.

We develop a generalization of semiclassical field theory for the case of non-Hermitian Hamiltonians with CPT symmetry and construct a classical cosmological, scalar-field based model describing a smooth transition from ordinary dark energy to the phantom one. Our model arises from a Lagrangian with a complex potential leading to a non-trivial vacuum with real vacuum energy. Equivalence with models involving two scalar fields one of which is phantom-like is discussed.

We present solutions to the Einstein–Maxwell system of equations in spherically symmetric gravitational fields for static interior space–times with a specified form of the electric field intensity. The condition of pressure isotropy yields three category of solutions. The first category is expressible in terms of elementary functions and does not have an uncharged limit. The second category is given in terms of Bessel functions of half-integer order. These charged solutions satisfy a barotropic equation of state and contain Finch–Skea uncharged stars. The third category is obtained in terms of modified Bessel functions of half-integer order and does not have an uncharged limit. The physical features of the charged analogue of the Finch–Skea stars are studied in detail. In particular, the condition of causality is satisfied and the speed of sound does not exceed the speed of light. The physical analysis indicates that this analogue is a realistic model for static charged relativistic perfect fluid spheres.