International Journal of Modern Physics D

Observations of the cosmic microwave background (CMB), especially of its frequency spectrum and its anisotropies, both in temperature and in polarization, have played a key role in the development of modern cosmology and of our understanding of the very early universe. We review the underlying physics of the CMB and how the primordial temperature and polarization anisotropies were imprinted. Possibilities for distinguishing competing cosmological models are emphasized. The current status of CMB experiments and experimental techniques with an emphasis toward future observations, particularly in polarization, is reviewed. The physics of foreground emissions, especially of polarized dust, is discussed in detail, since this area is likely to become crucial for measurements of the B modes of the CMB polarization at ever greater sensitivity.

Starting from Newton's gravitational theory, we give a general introduction into the spherically symmetric solution of Einstein's vacuum field equation, the Schwarzschild(–Droste) solution, and into one specific stationary axially symmetric solution, the Kerr solution. The Schwarzschild solution is unique and its metric can be interpreted as the exterior gravitational field of a spherically symmetric mass. The Kerr solution is only unique if the multipole moments of its mass and its angular momentum take on prescribed values. Its metric can be interpreted as the exterior gravitational field of a suitably rotating mass distribution. Both solutions describe objects exhibiting an event horizon, a frontier of no return. The corresponding notion of a black hole is explained to some extent. Eventually, we present some generalizations of the Kerr solution.

We derive the equations governing static, spherically symmetric vacuum solutions to the Einstein equations, as modified by the frame-dependent effective action (derived from trace dynamics) that gives an alternative explanation of the origin of "dark energy". We give analytic and numerical results for the solutions of these equations, first in polar coordinates, and then in isotropic coordinates. General features of the static case are that: (i) there is no horizon, since g₀₀ is nonvanishing for finite values of the polar radius, and only vanishes (in isotropic coordinates) at the internal singularity, (ii) the Ricci scalar R vanishes identically, and (iii) there is a physical singularity at cosmological distances. The large distance singularity may be an artifact of the static restriction, since we find that the behavior at large distances is altered in a time-dependent solution using the McVittie Ansatz.

In the framework of the most-studied doubly special relativity models the use of the naive formula v = dE/dp has been argued to lead to inconsistencies connected to different rules of transformation, under boosts, of particles with the same energy but with different masses. In this paper, we show that, at least in 1 + 1 dimensions, doubly special relativity can be formulated in such a way that the formula v = dE/dp is fully consistent with the invariance of the relative rest, easily fitting to the relativity principle. It is also argued that, always in 1 + 1 dimensions, is not necessary to renounce to the usual (commutative) Minkowski spacetime endowed with energy-independent boost transformations. The compatibility of the approach with superluminal propagation, with linear addition rule for energy, and possible extensions to 3 + 1 dimensions are also discussed.

In this paper, we address the well-known cosmic coincidence problem in the framework of the f(G) gravity. In order to achieve this, an interaction between dark energy and dark matter is considered. A set-up is designed and a constraint equation is obtained, which generates the f(G) models that do not suffer from the coincidence problem. Due to the absence of a universally accepted interaction term introduced by a fundamental theory, the study is conducted over three different forms of logically chosen interaction terms. To illustrate the set-up three widely known models of f(G) gravity are taken into consideration and the problem is studied under the designed set-up. The study reveals that the popular f(G) gravity models do not approve of a satisfactory solution of the long standing coincidence problem, thus proving to be a major setback for them as successful models of universe. Finally, two nonconventional models of f(G) gravity have been proposed and studied in the framework of the designed set-up. It is seen that a complete solution of the coincidence problem is achieved for these models. The study also reveals that the b-interaction term is much more preferable compared to the other interactions, due to its greater compliance with the recent observational data.

In this paper, we study a version of Szekeres spacetime which is radially moving with respect to the congruence of observers and is endowed with vorticity as pointed out by Herrera et al. [L. Herrera, A. Di Prisco and J. Ibáñez, Phys. Rev. D 86 (2012) 044003]. The dynamical variables as well as structure scalars associated with tilted and nontilted frames are explored. Moreover, an explicit expression for the super-Poynting vector has been investigated in this scenario. We confirm the fact that the vorticity in the tilted Szekeres spacetime is not linked with the circular flow of superenergy on the planes perpendicular to the vorticity vector indicating its kinematical nature [L. Herrera, A. Di Prisco and J. Ibáñez, Phys. Rev. D 87 (2011) 087503]. Finally, we explore the effect of cosmological constant on the structure scalars for nontilted geometry.

We investigate an effective torsion curvature in a second-order formalism underlying a two-form world-volume dynamics in a D₅-brane. In particular, we consider the two form in presence of a background (open string) metric in a U(1) gauge theory. Interestingly the formalism may be viewed via a noncoincident pair of -brane with a global Nereu–Schwarz (NS) two form on an anti-brane and a local two form on a brane. The energy–momentum tensor is computed in the six-dimensional (6D) conformal field theory (CFT). It is shown to source a metric fluctuation on a vacuum created pair of -brane at a cosmological horizon by the two-form quanta in the gauge theory. The emergent gravity scenario is shown to describe a low-energy (perturbative) string vacuum in 6D with a nonperturbative (NP) quantum correction by a lower (p < 5) dimensional D_p-brane or an anti-brane in the formalism. A closed string exchange between a pair of -brane, underlying a closed/open string duality, is argued to describe the Einstein vacuum in a low-energy limit. We obtain topological de Sitter (TdS) and Schwarzschild brane universe in six dimensions. The brane/anti-brane geometries are analyzed to explore some of their characteristic and thermal behaviors in presence of the quantum effects. They reveal an underlying nine-dimensional type IIA and IIB superstring theories on S¹.

In this paper, we exactly solve the (2 + 1)-dimensional Dirac equation in a constant magnetic field in the presence of a minimal length. Using a proper ansatz for the wave function, we transform the Dirac Hamiltonian into two two-dimensional nonrelativistic harmonic oscillator and obtain the solutions without directly solving the corresponding differential equations which are presented by Menculini et al. [Phys. Rev. D 87 (2013) 065017]. We also show that Menculini et al. solution is a subset of the general solution which is related to the even quantum numbers.

In the present paper, the role of modified chaplygin gas models in relation with the Bianchi type VI₀ universe is examined. For obtaining complete solution of Einstein field equations, it is assumed that expansion scalar in the model is proportional to shear scalar and equation of state of this modified model is valid from the radiation era to the Lambda cold dark matter (ΛCDM) model. State finder and various physical, geometrical properties have also been discussed.

The Galilean (and more generally Milne) invariance of Newtonian theory allows for Killing vector fields of a general kind, whereby the Lie derivative of a field is not required to vanish but only to be cancellable by some infinitesimal Galilean (respectively Milne) gauge transformation. In this paper, it is shown that both the Killing–Milne vector fields, which preserve the background Newtonian spacetime structure and the Killing–Cartan vector fields, which in addition preserve the gravitational field, form a Lie subalgebra.

In this paper, Bianchi type VI₀ magnetized anisotropic dark energy models with constant deceleration parameter have been studied by solving the Rosen's field equations in Bimetric theory of gravitation. The models corresponding to power law expansion and exponential law expansion have been evaluated and studied their nature geometrically and physically. It is seen that there is real visible matter (baryonic matter) suddenly appeared only for small interval of time 0.7 ≤ t < 0.7598 and for the remaining whole range of time t, there is dark energy matter in the universe. Our investigations are supported to the observational fact that the usual matter described by known particle theory is about 4% and the dark energy cause the accelerating expansion of the universe and several high precision observational experiments, especially the Wilkinson Microwave Anisotropic Probe (WMAP) satellite experiment (see [C. L. Bennett et al., Astrophys. J. Suppl. Ser. 148 (2003) 1; WMAP Collab. (D. N. Spergel et al.), Astrophys. J. Suppl. Ser. 148 (2003) 175; D. N. Spergel et al., Astrophys. J. Suppl. 170 (2007) 377; WMAP Collab. (E. Komastu et al.), Astrophys. J. Suppl. 180 (2009) 330; WMAP Collab. (G. Hinshaw et al.), Astrophys. J. Suppl. 208 (2013) 19; Plank Collab. (P. A. R. Ade), arXiv:1303.5076; arXiv:1303.5082]) conclude that the dark energy occupies near about 73% of the energy of the universe and dark matter is about 23%. In exponential law of expansion, our model is fully occupied by real visible matter and there is no chance of dark energy and dark matter.

In this paper, we study circular geodesic motion of test particles and photons in the Bardeen and Ayon–Beato–Garcia (ABG) geometry describing spherically symmetric regular black-hole or no-horizon spacetimes. While the Bardeen geometry is not exact solution of Einstein's equations, the ABG spacetime is related to self-gravitating charged sources governed by Einstein's gravity and nonlinear electrodynamics. They both are characterized by the mass parameter m and the charge parameter g. We demonstrate that in similarity to the Reissner–Nordstrom (RN) naked singularity spacetimes an antigravity static sphere should exist in all the no-horizon Bardeen and ABG solutions that can be surrounded by a Keplerian accretion disc. However, contrary to the RN naked singularity spacetimes, the ABG no-horizon spacetimes with parameter g/m > 2 can contain also an additional inner Keplerian disc hidden under the static antigravity sphere. Properties of the geodesic structure are reflected by simple observationally relevant optical phenomena. We give silhouette of the regular black-hole and no-horizon spacetimes, and profiled spectral lines generated by Keplerian rings radiating at a fixed frequency and located in strong gravity region at or nearby the marginally stable circular geodesics. We demonstrate that the profiled spectral lines related to the regular black-holes are qualitatively similar to those of the Schwarzschild black-holes, giving only small quantitative differences. On the other hand, the regular no-horizon spacetimes give clear qualitative signatures of their presence while compared to the Schwarschild spacetimes. Moreover, it is possible to distinguish the Bardeen and ABG no-horizon spacetimes, if the inclination angle to the observer is known.

In this paper, we have studied nonstationary radiative spherically symmetric spacetime, in general covariant theory (U(1) extension) of the Hořava–Lifshitz gravity with the minimum coupling, in the parameterized post-Newtonian (PPN) approximation, without the projectability condition and in the infrared limit. The Newtonian prepotential φ was assumed null. We have shown that there is not the analog of the Vaidya's solution in the Hořava–Lifshitz Theory with the minimum coupling, as we know in the General Relativity Theory (GRT).