International Journal of Modern Physics D

We illustrate how a group of symmetry transformations, which preserve the form of the n-dimensional flat Friedmann–Robertson–Walker cosmologies satisfying Einstein's equations, act in any dimension. This group relates the energy density and the isotropic pressure of the cosmic fluid to the expansion rate. The freedom associated with the dimension of the space–time yields assisted inflation even when the energy density of the fluid is a dimensional invariant and enriches the set of duality transformations leading to phantom cosmologies.

The use of the coherent and squeezed state formalisms of quantum optics for a minimally coupled nonclassical inflaton in the FRW metric is studied, in the semiclassical theory of gravity. The leading order solutions to the semiclassical Einstein equations in the coherent, squeezed and squeezed vacuum states are obtained perturbatively and exhibit power law expansion behavior. The validity of the semiclassical theory is examined in the squeezed vacuum state in the oscillatory phase of the inflaton. The semiclassical theory in the oscillatory phase of the nonclassical inflaton holds only if the associated squeezing parameter is much smaller than unity. Quantum fluctuations of the inflaton are also examined in the coherent and squeezed state formalisms.

In this paper we consider a model of the scalar–tensor theory of gravitation, in which the scalar field ϕ determines the gravitational coupling G and has a Lagrangian of the form . We study the cosmological consequence of this theory in the matter-dominated era and show that it leads to a transition from an initial decelerated expansion to an accelerated expansion phase at the present epoch. Using observational constraints, we see that the effective equation of state today for the scalar field turns out to be p_ϕ = w_ϕρ_ϕ, with w_ϕ = -0.88, and that the transition to an accelerated phase happened at a redshift of about 0.3.

By studying a tachyon field on the DGP brane model, in order to embed the 4D standard Friedmann equation with a brane tachyon field in the 5D bulk, we present the metric of the 5D space–time. Then, adopting the inverse square potential of the tachyon field, we obtain an expanding universe with a power law on the brane and an exact 5D solution.

In this paper we investigate the constraints on the cosmological parameters, especially the equation of state of dynamical dark energy w_DE, the inflationary parameters n_s, α_s and r, the total neutrino mass ∑ m_ν and the curvature of the universe Ω_K, using the simulated data of future Planck measurement. First, we determine the cosmological parameters with the current observations, including ESSENCE (192 samples), Three-Year WMAP (WMAP3), Boomerang-2K2, CBI, VSA, ACBAR, SDSS LRG and 2dFGRS, and then we take the best-fit model as the fiducial model in the ensuing simulations. In the simulations we pay particular attention to the effects of the dynamical dark energy in the determination of the cosmological parameters. For this reason, in order to make our constraints more robust, we have added the simulated SNAP data to our simulations. Using the present data, we find that the Quintom dark energy model is mildly favored, while the ΛCDM model remains a good fit. In the framework of the dynamical dark energy model, the constraints on the inflationary parameters, ∑ m_ν and Ω_K, become weak, compared with the constraints in the ΛCDM model. Intriguingly, we find that the inflationary models with a "blue" tilt, which are excluded about 2σ in the ΛCDM model, are well within the 2σ region with the presence of the dynamics of dark energy. The upper limits of neutrino mass are weakened by a factor of 2 (95% CL) — say, ∑ m_ν < 1.59 eV and ∑ m_ν < 1.53 eV for two forms of parametrization of the equation of state of dark energy. The flat universe is a good fit to the current data, namely |Ω_K| < 0.03 (95% CL). With the simulated Planck and SNAP data, the dynamical dark energy model and the ΛCDM model might be distinguished at the 4σ confidence level. And the uncertainties of the inflationary parameters, ∑ m_ν and Ω_K, can be reduced significantly in the framework of the dynamical dark energy model. We also constrain the rotation angle Δα, denoting the possible CPT violation, from the simulated Planck and CMBpol data and find that our results are much more stringent than the current constraint and will be used to verify the CPT symmetry with a higher precision.

In this paper, we examine the efficiency of gravitational bremsstrahlung production in the process of head-on collision of two boosted Schwarzschild black holes. We construct initial data for the characteristic initial value problem in Robinson–Trautman space–times, which represent two instantaneously stationary Schwarzschild black holes in motion toward each other with the same velocity. The Robinson–Trautman equation is integrated for these initial data using a numerical code based on the Galerkin method. The resulting final configuration is a boosted black hole with Bondi mass greater than the sum of the individual masses of the individual initial black holes. Two relevant aspects of the process are presented. The first relates the efficiency Δ of the energy extraction by gravitational wave emission to the mass of the final black hole. This relation is fitted by a distribution function of nonextensive thermostatistics with entropic parameter q ≃ 1/2; the result extends and validates analysis based on the linearized theory of gravitational wave emission. The second aspect is a typical bremsstrahlung angular pattern in the early period of emission at the wave zone, a consequence of the deceleration of the black holes as they coalesce; this pattern evolves to a quadrupole form for later times.

The late-time evolution of massive Dirac fields in the backgrounds of brane-world black holes is investigated. We find that the dumping exponent depends on both the multiple number of the wave mode and the mass of the Dirac fields, but almost does not depend on the parameter ϒ of the brane-world black holes. We also find that the decay rate of the asymptotic late-time tail is t^-5/6. Our results show that the decay of massive Dirac hairs on brane-world black holes has the same behavior as that of the Schwarzschild black hole.

Motivated by the recent work of Kerner and Mann, we study Hawking tunneling radiation of fermions from the Garfinkle–Horne dilaton black hole on the brane. Because the electromagnetic field would couple with the gravity and matter fields, we introduce the Dirac equation of charged particles. We further discuss the correction spectrum of the radiant spin 1/2 particles by considering energy conservation and charge conservation. Our result shows that the tunneling rate of fermions also agrees with the underlying unitary theory as the unfixed background space–time is taken into account.

Pseudo-Newtonian gravitational potential describing the gravitational field of static and spherically symmetric black holes in the universe with a repulsive cosmological constant is introduced. In order to demonstrate the accuracy of the pseudo-Newtonian approach, the related effective potential for test particle motion is constructed and compared with its general-relativistic counterpart given by the Schwarzschild–de Sitter geometry. The results indicate that such an approach could be useful in applications of developed Newtonian theories of accretion disks in astrophysically interesting situations in large galactic structures for the Schwarzschild–de Sitter space–times with the cosmological parameter y = Λ M²/3 ≤ 10^-6.

The problem of anisotropic pressures arising from the spatial symmetry breaking introduced by an external magnetic field in quantum systems is discussed. The role of the conservation of energy and momentum of external fields as well as of systems providing boundary conditions in quantum statistics is considered. The vanishing of the average transverse momentum for an electron–positron system in its Landau ground state, i.e. the vanishing of its transverse pressure, is shown. The situation for the neutron case and strange quark matter (SQM) in β equilibrium is briefly discussed. Thermodynamical relations in external fields as well as the form of the stress tensor in a quantum relativistic medium are obtained. The ferromagnetic symmetry breaking is briefly discussed for very dense matter. It is concluded that stable matter cannot exist for fields greater than B = 10¹⁸ G.

Three static models with two interacting phantom and ghost scalar fields are considered: a model of a traversable wormhole, a branelike model and a spherically symmetric problem. It is shown numerically that regular solutions exist for all three cases.

Self-similar solutions of a collapsing perfect fluid and a massless scalar field with kinematic self-similarity of the first kind in 2+1 dimensions are obtained. The local and global properties of the solutions are studied. It is found that some of them represent gravitational collapse, in which black holes are always formed, and some may be interpreted as representing cosmological models.

A Gauss–Bonnet dark energy model is considered. It is inspired by string/M-theory and also takes into account quantum contributions, which are introduced from a conformal quantum anomaly. The corresponding solutions for the Hubble rate, H, are studied starting from the Friedmann–Robertson–Walker equation. It is seen that, as a pure effect of the quantum contributions, a new solution for H exists in some region, which does not appear in the classical case. The behavior of all encountered solutions is studied with care, in particular the role played by the quantum correction term — which depends on the number of matter fields — in the stability of the solutions around its asymptotic value. It is argued that, contrary to what happens in the classical case, quantum effects remarkably lead to the realization of a de Sitter stage which corresponds to the inflation/dark energy stages, even for positive values of the f₀ constant (coupling of the field with the Gauss–Bonnet invariant).

The cosmic neutrinos background (CNB) comprises primordial neutrinos that were decoupled when the Universe was very young. Its detection is complicated, especially if we take into account neutrino mass and a possible breaking of Lorentz invariance at high energy, but has a fundamental relevance to studying the Big Bang. In this paper, we will see that a Lorentz violation does not produce important modification, but the mass does. We will show how the neutrino current velocity, with respect to the comobile system of the universe expansion, is of the order of 1065 km/s, much less than the velocity of light. Besides, we will see that the neutrino distribution is complex due to planetary motion. This prediction differs totally from the usual massless case, where we would get a correction similar to the dipolar moment of the CMB.