International Journal of Modern Physics D

The general exact solution to the Einstein matter field equations describing spherically symmetric shells satisfying an equation of state in closed form is discussed under general assumptions of physical reasonableness. The solutions split into two classes: one describes the behavior of "ordinary" matter satisfying the weak energy condition, while the other describes "exotic" matter for which this condition can be violated. In both cases all those solutions which are already known are recovered as particular cases.

Higher-dimensional black holes have long been considered within the context of brane worlds. Recently, it was shown that the brane-world ethos also permits the consideration of higher-dimensional wormholes. When such a wormhole, pre-existing in the bulk, impinges upon our universe, taken to be a positive-tension three-brane, it can induce the creation in our universe of a wormhole of ordinary dimensionality. The throat of this wormhole might fully constrict, pinch off, and thus birth a baby universe. Alternatively, the induced wormhole might persist. I show that persistence is more likely and note that the persistent wormhole manifests itself as a particle-like object whose interaction with cosmic matter is purely gravitational. I consider the viability of this object as a dark matter candidate.

When one is solving the equations of general relativity in a symmetric sector, it is natural to consider the same symmetry for the geometry and stress–energy. This implies that for static and isotropic space–times, the most general natural stress–energy tensor is a sum of a perfect fluid and a radially imperfect fluid component. In the special situations where the perfect fluid component vanishes or is a space–time constant, the solutions to Einstein's equations can be thought of as modified Schwarzschild and Schwarzschild–de Sitter spaces. Exact solutions of this type are derived and it is shown that whereas deviations from the unmodified solutions can be made small, among the manifestations of the imperfect fluid component is a shift in angular momentum scaling for orbiting test bodies at large radius. Based on this effect, the question of whether the imperfect fluid component can feasibly describe dark matter phenomenology is addressed.

Based on the ϕ mapping topological current theory, the proton vortex clusters in the core of a neutron star are investigated. We derive rigorously the London equation with topological structure for superconducting protons. We also show that the proton vortices can only stem from the zero points of the vector order parameter. The evolution of the proton vortices is discussed, the proton vortices are found generating or annihilating at the limit points and splitting or merging at the bifurcation points of the vector order parameter, and the total topological charges remain invariant during the evolution.

We consider in this paper a variable modified Chaplygin gas (VMCG) model for describing the unification of dark energy and dark matter, in which dark energy interacts with dark matter. Concretely, the evolution of the VMCG model with interaction is discussed and the statefinder diagnostic for the model is performed. By analysis, we find that the effective state parameter of dark energy can cross the phantom divide w_Λ = -1 and our universe will not end up with a Big Rip in the future. Furthermore, we perform a statefinder analysis on this scenario and show the discrimination between this scenario and other dark energy models.

Well-known causality arguments show that events occurring during or at the end of inflation, associated with reheating or preheating, could contribute a blue component to the spectrum of primordial curvature perturbations, with the dependence k³. We explore the possibility that they could be observably large in CMB, LSS, and Lyman-α data. We find that a k³ component with a cutoff at some maximum k can modestly improve the fits (Δχ² = 2.0, 5.4) of the low multipoles (ℓ ~ 10–50) or the second peak (ℓ ~ 540) of the CMB angular spectrum when the three-year WMAP data are used. Moreover, the results from WMAP are consistent with the CBI, ACBAR, 2dFGRS, and SDSS data when they are included in the analysis. Including the SDSS galaxy clustering power spectrum, we find weak positive evidence for the k³ component at the level of Δχ^2′ = 2.4, with the caveat that the nonlinear evolution of the power spectrum may not be properly treated in the presence of the k³ distortion. To investigate the high-k regime, we use the Lyman-α forest data (LUQAS, Croft et al., and SDSS Lyman-α); here we find evidence at the level Δχ^2′ = 3.8. Considering that there are two additional free parameters in the model, the above results do not give a strong evidence for features; however, they show that surprisingly large bumps are not ruled out. We give constraints on the ratio between the k³ component and the nearly scale-invariant component, r₃ < 1.5, over the range of wave numbers 2.3 × 10^-3Mpc^-1 < k < 8.2 Mpc^-1. We also discuss theoretical models which could lead to the k³ effect, including ordinary hybrid inflation and double D-term inflation models. We show that the well-motivated k³ component is also a good representative of the generic spikelike feature in the primordial perturbation power spectrum.

We present here the transformations required to recast the Robertson–Walker metric and Friedmann–Robertson–Walker equations in terms of observer-dependent coordinates for several commonly assumed cosmologies. The overriding motivation is the derivation of explicit expressions for the radius R_h of our cosmic horizon in terms of measurable quantities for each of the cases we consider. We show that the cosmological time dt diverges for any finite interval ds associated with a process at R → R_h, which therefore represents a physical limit to our observations. This is a key component required for a complete interpretation of the data, particularly as they pertain to the nature of dark energy. With these results, we affirm the conclusion drawn in our earlier work that the identification of dark energy as a cosmological constant does not appear to be consistent with the data.

We study the capture of galactic dark matter by the solar system. The effect is due to the gravitational three-body interaction between the sun, one of the planets, and a dark matter particle. The analytical estimate for the capture cross-section is derived and the upper and lower bounds for the total mass of the captured dark matter particles are found. The estimates for their density are less reliable. The most optimistic of them gives an enhancement of dark matter density by about three orders of magnitudes compared to its value in our galaxy. However, even this optimistic value remains below the best present observational upper limits by about two orders of magnitude.

We attempt to construct the braneworld analog of the cheese slice universe, an inhomogeneous cosmology constructed from alternating layers of Kasner and FLRW space–times. This construction is possible in four dimensions and we find that the energy conditions can be satisfied in the braneworld context. However, an extension into the bulk becomes more problematic. We use a 3 + 1 + 1 decomposition inspired by the ADM decompositions to show that structure is required in the bulk to support an inhomogeneous brane.