International Journal of Modern Physics D

A class of solutions to Einstein field equations is studied, which represents gravitational collapse of thick spherical shells made of self-similar and shear-free fluid with heat flow. It is shown that such shells satisfy all the energy conditions, and the corresponding collapse always forms naked singularities.

The relativistic Friedmann Lemaitre cosmology model (FLCM) is very sucessful in describing the evolution history of the Universe from the "First three Minutes". Any alternative model should be consistent with the FLCM explanations to the later stage evolutions of the Universe at certain points. A noncompact extra dimension model was recently proposed by Randall and Sundrum. Binetruy et al. obtained the modified Friedmann equation, in which the energy density of the brane appears quadratically in contrast with the linear behavior of the standard Friedmann equation. We investigate various kinds of classical cosmological effects of the new models and get a general solution of the cosmic evolution for this extended model, with more detail discussions of the brane tension parameter on these cosmological tests.

Taking into account a torsion field gives rise to a negative pressure contribution in cosmological dynamics and then to an accelerated behaviour of Hubble fluid. The presence of torsion has the same effect as a Λ-term. We obtain a general exact solution which well fits data coming from high redshift supernovae and Sunyaev–Zeldovich/X-ray method for the determination of cosmological parameters. On the other hand, it is possible to obtain observational constraints on the amount of torsion density. A dust dominated Friedmann behaviour is recovered as soon as torsion effects are not relevant.

Years ago Einstein and Straus (ES) showed that it is possible to match a static Schwarzschild region to an external expanding Friedmann universe. This model is extended in this work to a spacetime of arbitrary dimensions. Frequency shift of radiation coming from the boundary of the two spacetimes is calculated. Depending on the relative magnitude of gravitational and doppler effects our model gives both blue shift and red shift. The dynamical behaviour of the boundary is investigated and it is found that like the ES case our model is also unstable against small perturbation.

A new approach to the concept of particles and their production in quantum field theory is developed. A local operator describing the current of particle density is constructed for scalar and spinor fields in arbitrary gravitational and electromagnetic backgrounds. This enables one to describe particles in a local, general-covariant and gauge-invariant way. However, the current depends on the choice of a 2-point function. There is a choice that leads to the local non-conservation of the current in a gravitational or an electromagnetic background, which describes local particle production consistent with the usual global description based on the Bogoliubov transformation. The most natural choice based on the Green function calculated using the Schwinger–DeWitt method leads to the local conservation of the current, provided that interactions with quantum fields are absent. Interactions with quantum fields lead to the local non-conservation of the current which describes local particle production consistent with the usual global description based on the interaction picture.

Recent cosmological observations reveal that we are living in a flat accelerated expanding universe. In this work we have investigated the nature of the potential compatible with the power law expansion of the universe in a self interacting Brans–Dicke cosmology with a perfect fluid background and have analyzed whether this potential supports the accelerated expansion. It is found that positive power law potential is relevant in this scenario and can drive accelerated expansion for negative Brans–Dicke coupling parameter ω. The evolution of the density perturbation is also analyzed in this scenario and is seen that the model allows growing modes for negative ω.

We study the perturbations of two classes of static black ellipsoid solutions of four-dimensional vacuum Einstein equations. Such solutions are described by generic off-diagonal metrics which are generated by anholonomic transforms of diagonal metrics. The analysis is performed in the approximation of small eccentricity deformations of the Schwarzschild solution. We conclude that such anisotropic black hole objects may be stable with respect to the perturbations parametrized by the Schrodinger equations in the framework of the one-dimensional inverse scattering theory.

We analyze the horizon and geodesic structure of a class of 4D off–diagonal metrics with deformed spherical symmetries, which are exact solutions of the vacuum Einstein equations with anholonomic variables. The maximal analytic extension of the ellipsoid type metrics are constructed and the Penrose diagrams are analyzed with respect to the adapted frames. We prove that for small deformations (small eccentricities) there are such metrics that the geodesic behaviour is similar to the Schwarzcshild one. We conclude that some vacuum static and stationary ellipsoid configurations^1,2 may describe black ellipsoid objects.

We present the equation of state (EOS) of quark-diquark matter in the quark mass-density-dependent model. The region of the 2-D parameter space inside which this quark-diquark matter is stable against diquark disassembling and hadronization is determined. Motivated by observational data suggesting a high compactness of some neutron stars (NS) we present models based on the present EOS and compare the results with those obtained with previous works addressing a quark-diquark composition based on the MIT Bag model. We show that very compact self-bound stars (yet having ≥ 1M_⊙) are allowed by our EOS even if the diquark itself is unbound.

Scaling properties of the cosmic microwave background (CMB) radiation are studied using satellite (COBE-DMR maps), balloon-borne and ground-based (combined QMASK map) data. Quantitative consistency is found between the multiscaling properties of the COBE-DMR and QMASK CMB maps. Surprisingly, it is found that the observed CMB temperature multiscaling quantitatively resembles the multiscaling properties of fluid turbulence, that indicates primordial plasma turbulence as an origin of the CMB temperature space anisotropy.

Recent observational data suggests a high compacticity (the quotient M/R) of some "neutron" stars. Motivated by these works we revisit models based on quark–diquark degrees of freedom and address the question of whether that matter is stable against diquark disassembling and hadronization within the different models. We find that equations of state modeled as effective λϕ⁴ theories do not generally produce stable self-bound matter and are not suitable for constructing very compact star models, that is the matter would decay into neutron matter. We also discuss some insights obtained by including hard sphere terms in the equation of state to model repulsive interactions. We finally compare the resulting equations of state with previous models and emphasize the role of the boundary conditions at the surface of compact self-bound stars, features of a possible normal crust of the latter and related topics.

Minimal Spanning Tree (MST) statistics and associated reducing procedures have been used to establish the level of filamentary structure, if any in Pisces-Perseus, a sample containing 4501 galaxies. The procedure has been applied to subsets of galaxies with 247 galaxies that we call the Most Conspicuous sample which contain the larger intrinsic linear diameters and the brighter galaxies, and to the Least Conspicuous samples with 253 galaxies containing the smaller intrinsic linear diameter and the fainter members as well as the entire sample. The results show that there are longer filaments in the Most Conspicuous samples with less coiling as opposed to the Least Conspicuous sample. Pruning and separating also highlight the key structures in this region and not only the filamentary ones. Voids are highlighted, also indicating that MST's can be used to identify voids in the large scale structure of the Universe.