Cosmological model with a conformally coupled scalar field

Modern concepts of the universe support the assumed existence of a nongravitational source, known as dark energy, for which ε + 3 P < 0 (where ε is the energy density and P is the pressure). This ensures accelerated expansion of the universe. This paper examines a tensor-scalar variant of the theory of gravitation with a conformally coupled scalar field. Various cosmological models are examined and the possible evolutionary development of the universe with accelerated expansion is discussed. Translated from Astrofizika, Vol. 51, No. 4, pp. 653–661 (November 2008).     

Cosmological implications of BL Lac objects

It has become clear in recent years that relativistic beaming is a good explanation for the BL Lac phenomenon. Of studies based on the relativistic beaming model of BL Lac objects, we note that the orientation of jet's axis to the line-of-sight is very small and, therefore, the observed flux emitted from a rapidly moving source is orders of magnitude higher than the flux in its rest-frame:F _obs = ^{3 +} F _intr, where is the bulk relativistic Doppler factor. Then the observed apparent magnitudem _v must be corrected for this effect. For our 39 samples, the corrected apparent magnitudem _v ^corr and logZ have a good correlation.     

Cosmological dynamics of EDGP model with a tachyon field on the brane

We consider the cosmological dynamics of a tachyon field localized on the extended DGP braneworld scenario. We present a detailed analysis of the critical points in the phase space of the model, their stability and late-time cosmological viability of the solutions. We study the luminosity distance behavior of this ?EDGP model and compare it with ΛCDM model. Also we show that the EDGP solutions in the presence of tachyon field can explain late time acceleration of the universe.     

Cosmological implications of the PSCz PDF and its moments

We compare the probability density function (PDF) and its low-order moments (variance and skewness) of the smoothed IRAS Point Source Catalogue Redshift Survey (PSC z ) galaxy density field and of the corresponding simulated PSC z look-alikes, generated from N -body simulations of six different dark matter models: four structure-normalized with     and     , one COBE -normalized, and the old standard cold dark matter model. The galaxy distributions are smoothed with a Gaussian window at three different smoothing scales,     , 10 and 15  h ⁻¹ Mpc. We find that the simulation PSC z look-alike PDFs are sensitive only to the normalization of the power spectrum, probably owing to the shape similarity of the simulated galaxy power spectrum on the relevant scales. We find that the only models that are consistent, at a high significance level, with the observed PSC z PDF are models with a relatively low power spectrum normalization     . From the phenomenologically derived σ ₈–moments relation, fitted from the simulation data, we find that the PSC z moments suggest     .     

Dark matter,galaxy sizes and exponential potential

Starting from the standard Newtonian theory, we derive in the propagation equation of perturbations an effectively Yukawalike potential invoked by several authors as an alternative to dark matter. This approach provides with a simple explanation of the segregation observed for dynamically ‘hot’ systems in the plane defined by their effective parameters.     

Cosmological implications and structure formation from a time varying vacuum

We study the dynamics of the Friedmann–Lemaitre–Robertson–Walker (FLRW) flat cosmological models in which the vacuum energy varies with time,  Λ( t )  . In this model, we find that the main cosmological functions such as the scale factor of the universe and the Hubble flow are defined in terms of exponential functions. Applying a joint likelihood analysis of the recent Type Ia supernovae data, the cosmic microwave background shift parameter and the baryonic acoustic oscillations traced by the Sloan Digital Sky Survey (SDSS) galaxies, we place tight constraints on the main cosmological parameters of the  Λ( t )  scenario. Also, we compare the  Λ( t )  model with the traditional Λ cosmology and we find that the former model provides a Hubble expansion which compares well with that of the Λ cosmology. However, the  Λ( t )  scenario predicts stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the usual Λ cosmology, despite the fact that they share the same equation of state parameter. In this framework, we find that galaxy clusters in the  Λ( t )  model appear to form earlier than in the Λ model.     

Cosmological viscous fluid models in the presence of electromagnetic field and zero-mass scalar field

The problem of electromagnetic field interacting with viscous fluid without and with zero-mass scalar field has been studied. It has been shown that electromagnetic field cannot interact with viscous fluid for spherically-symmetric Robertson-Walker metric. Exact solutions corresponding to the problem of electromagnetic field interactions in presence of viscous fluid and zero-mass scalar field have been obtained subject to various physical conditions. It presents a scope for the study of imperfect fluid FRW models showing the existence of the electromagnetic field due to the presence of zero-mass scalar field.     

Cosmological dynamics of a quintom field on the warped DGP brane

As a generalization of the Brans-Dicke type scalar-tensor gravity in a braneworld context, we study cosmological phase space of a braneworld model with induced gravity in the presence of a scalar field on the brane. We consider a quintom field minimally or non-minimally coupled to induced gravity on the warped DGP brane and we present a detailed analysis of the critical points, their stability and late-time cosmological viability of the solutions within a phase space approach. In particular, de Sitter solutions, different from the famous self-accelerated branch of the DGP model are found and the phase-space analysis for checking their attractor properties is performed. We analyze also the possibility of crossing of the phantom divide by the effective equation of state parameter of the model. We also focus on the classical stability of the solutions in w–w′ phase plane.     

Cosmological dynamics of a bulk scalar field in the DGP setup

We reconsider the issue of cosmological dynamics in a DGP setup with a bulk scalar field. The ghost-free, normal branch of this DGP-inspired braneworld scenario has the potential to realize a self-consistent phantom-like behavior. The roles played by the bulk canonical scalar field on this phantom-like dynamics are explored. Within a dynamical system approach, the effective phantom nature of the scenario is investigated with details. This analysis shows that there is a stable, late-time de Sitter phase.     

Geodesics around oscillatons made of exponential scalar field potential

Some of the spherically symmetric solutions to the Einstein–Klein–Gordon (EKG) equations can describe the astronomical soliton objects made of a real time-dependent scalar fields. The solutions are known as oscillatons which are non-singular satisfying the flatness conditions asymptotically with periodic (separated) time-dependency. In this paper, we investigate the geodesic motion around an oscillaton. The Spherically Symmetric Geometry allows the bound orbits in the plan \(\theta=\frac{\pi}{2}\) under a given initial conditions. The potential for the scalar field \(\varPhi=\varPhi(r,t)\), is an exponential function of the form \(V(\varPhi)=V_{0}\exp(\lambda\sqrt{k_{0}}\varPhi)\).     

Cosmological dynamics of a non-minimally coupled bulk scalar field in DGP setup

We consider cosmological dynamics of a canonical bulk scalar field, which is coupled non-minimally to 5-dimensional Ricci scalar in a DGP setup. We show that presence of this non-minimally coupled bulk scalar field affects the jump conditions of the original DGP model significantly. Within a superpotential approach, we perform some numerical analysis of the model parameter space and consider bulk-brane energy exchange in this setup. Also we show that the normal, ghost-free branch of the DGP solutions in this case has the potential to realize a self-consistent phantom-like behavior and therefore explains late time acceleration of the universe in a consistent way.     

Distances on Cosmological Scales with VLTI

We present here a new method using interferometric measurements of quasars, that allows the determination of direct geometrical distances on cosmic scales. Quasar Broad Emission Line Regions sizes provide a `meter rule' with which to measure the metric of the Universe. This method is less dependent of model assumptions, and even of variations in the fundamental constants (other than c).We discuss the spectral and spatial requirements on the VLTI observations needed to carry out these measurements. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cylindrically-symmetric matter distribution with a magnetic field in Einstein-Cartan Theory

In the first part the number of independent non-vanishing components of the 3-index torsion tensorQ _jk ⁱ is reduced from 24 (in general case) to 12 — for the case of cylindrical symmetry of the underlying manifold. In the second part of the paper we have obtained an exact solution of Einstein-Cartan-Maxwell equations representing a static cylinder of perfect fluid with an axial magnetic fieldH and non-zero spin densityK.     

The exact transformation from spherical harmonic to ellipsoidal harmonic coefficients for gravitational field modeling

The spherical and ellipsoidal harmonic series of the external gravitational potential for a given mass distribution are equivalent in their mutual region of uniform convergence. In an instructive case, the equality of the two series on the common coordinate surface of an infinitely large sphere reveals the exact correspondence between the spherical and ellipsoidal harmonic coefficients. The transformation between the two sets of coefficients can be accomplished via the numerical methods by Walter (Celest Mech 2:389–397, 1970) and Dechambre and Scheeres (Astron Astrophys 387:1114–1122, 2002), respectively. On the other hand, the harmonic coefficients are defined by the integrals of mass density moments in terms of the respective solid harmonics. This paper presents general algebraic formulas for expressing the solid ellipsoidal harmonics as a linear combination of the corresponding solid spherical harmonics. An exact transformation from spherical to ellipsoidal harmonic coefficients is found by incorporating these connecting expressions into the density integral. A computational procedure is proposed for the transformation. Numerical results based on the nearly ellipsoidal Martian moon, Phobos, are presented for validation of the method.