International Journal of Modern Physics D

A UV/IR complete theory of cosmological inflation is constructed in a braneworld setup that the inflaton field evolves on a DGP brane and the curvature effect is incorporated via the Gauss–Bonnet term in the bulk action. While the Induced Gravity on the DGP brane is a manifestation of the IR limit, the stringy Gauss–Bonnet effect in the bulk action realizes the UV sector of the scenario. This scenario generally supports a red power spectrum of perturbations, and for a large field regime requires a smaller number of e-folds than other braneworld inflation scenarios given the same assumptions. For the tensor-to-scalar ratio, this inflation scenario gives a better fit to observation than alternative scenarios.

A recent study of dissipative collapse considered a contracting sphere in which the areal and proper radii are equal throughout its evolution. The interior space–time was matched to the exterior Vaidya space–time which generated a temporal evolution equation at the boundary of the collapsing sphere. We present a solution of the boundary condition which allows the study of the gravitational and thermodynamical behavior of this particular radiating model.

Starting with any stationary axisymmetric vacuum metric, we build anisotropic fluids. With the help of the Ernst method, the basic equations are derived together with the expression for the energy–momentum tensor and with the equation of state compatible with the field equations. The method is presented by using different coordinate systems: the cylindrical coordinates ρ, z and the oblate spheroidal ones. A class of interior solutions matching with stationary axisymmetric asymptotically flat vacuum solutions is found in oblate spheroidal coordinates. The solutions presented satisfy the three energy conditions.

A new model is proposed to collapsing stars consisting of an anisotropic fluid with bulk viscosity, radial heat flow and outgoing radiation. In a previous paper one of us has introduced a time-dependent function into the g_rr, besides the time-dependent metric functions g_θθ and g_ϕϕ. The aim of this work is to generalize this previous model by introducing bulk viscosity and comparing it to the non-viscous collapse. The behavior of the density, pressure, mass, luminosity and the effective adiabatic index is analyzed. Our work is also compared to the case of a collapsing fluid with bulk viscosity of another previous model, for a star with 6 M_⊙. The pressure of the star, at the beginning of the collapse, is isotropic, but due to the presence of the bulk viscosity the pressure becomes more and more anisotropic. The black hole is never formed because the apparent horizon formation condition is never satisfied, in contrast to the previous model where a black hole is formed. An observer at infinity sees a radial point source radiating exponentially until it reaches the time of maximum luminosity, and suddenly the star turns off. This is in contrast to the former model where the luminosity also increases exponentially, reaching a maximum and decreases thereafter until the formation of the black hole. The effective adiabatic index diminishes due to the bulk viscosity, thus increasing the instability of the system, in both models, in the former paper as well as in this work.

For the description of the early inflation and acceleration expansion of the universe that are compatible with observational data, the 5D noncompact Kaluza–Klein cosmology is investigated. It is proposed that the 5D space is filled with a null perfect fluid, resulting in a perfect fluid in a 4D universe, plus one along the fifth dimension. By analyzing the reduced field equations for the flat FRW model, we show the early inflationary behavior and the current acceleration of the universe.

An entropy-corrected holographic dark energy (ECHDE) was recently proposed to explain the dark energy-dominated universe with the help of quantum corrections to the entropy–area relation in the setup of loop quantum cosmology. Using this new definition, we investigate its thermodynamical features including entropy and energy conservation. We describe the thermodynamical interpretation of the interaction between ECHDE and dark matter in a non-flat universe. We obtain a relation between the interaction term of the dark components and thermal fluctuation. Our study further generalizes the earlier works^{86, 87} in this direction.

We study the possibility of the vortex formation in axion condensates on the galactic scale. Such vortices can occur as a result of global rotation of the early universe. We study analytical models of vortices and calculate exemplary galaxy rotation curves. Depending on the setup it is possible to obtain a variety of shapes which give a good qualitative agreement with observational results. However, as we show, the extremely low velocity dispersions of the axion velocities are required to form the single vortex on the galactic scales. We find that the required velocity dispersion is of the order of σ≈10^-12 ms^-1. This is much smaller that predicted within the present understanding of the axion physics. The vortices in axion condensate can however be formed on the much smaller scales and give seeds to the galaxy formation and to their angular momenta. On the other hand, the vortices can be formed on the galactic scales, but only if the mass of the axion-like particles is of the order of 10^-30 eV. In this case, the particle de Broglie wavelength is comparable with the galactic diameter. This condition must be fulfilled in order to keep the coherence of the quantum condensate on galactic scales.

We study the interacting agegraphic dark energy (ADE) model in the non-flat universe by means of the statefinder diagnostic and an w–w′ analysis. First, the evolution of EoS parameter (w_d) and deceleration parameter (q) are calculated in terms of scale-factor for interacting ADE models in the non-flat universe. The dependency of w_d on the ADE model parameters n and α in different spatial curvatures is investigated. We show that the evolution of q is dependent on the type of spatial curvature, besides dependening on parameters n and α. The accelerated expansion takes place sooner in the open universe and latter in the closed universe as compared with the flat universe. Then, we plot the evolutionary trajectories of the interacting ADE models for different values of the parameters n and α, as well as for different contributions of spatial curvature, in the plane of statefinder parameters. In addition to the statefinder, we also investigate the ADE model in the non-flat universe with w–w′ analysis.