Early Cosmological Models with Bulk Viscosity in Brans-Dicke Theory

The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously as the Universe expands, are obtained by using the power-law relation φ = bR ⁿ , which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation dominated era of the universe. The physical properties of cosmological solutions are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Homogeneous Universes in Extended Inflation I

We have found that for the Bianchi types I–II–III–V in the Brans-Dicke theory, the scalar field of the theory φ has the same form in the isotropic case. It is shown that the isotropization of the Universe occurs in a very short time when the Universe is dominated by vacuum energy, proving that an isotropic Robertson-Walker model is a good approximation to use in the extended inflation scenario. This revised version was published online in July 2006 with corrections to the Cover Date.     

Brans-Dicke Cosmology with non-vanishing Cosmological Constant and Non-Zero Curvature

A model of the universe based on Brans-Dicke theory with non-vanishing cosmological constant and non-zero curvature is studied. Equations (13) and (16) have been obtained by the assumption f(t) = φ(t)a ³ (t),which give the values of the scale factor, a(t) and scalar field, φ(t) in terms of the observable parameters. Also, for a particular case of matter dominated universe, Equation (20) is obtained which gives the relation between various parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

LRS Bianchi-I anisotropic cosmological model with dominance of dark energy

The present study deals with spatially homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi type I cosmological model with dominance of dark energy. To get the deterministic model of Universe, we assume that the shear scalar (σ) in the model is proportional to expansion scalar (θ). This condition leads to A=B ⁿ , where A, B are metric potential and n is positive constant. It has been found that the anisotropic distribution of dark energy leads to the present accelerated expansion of Universe. The physical behavior of the Universe has been discussed in detail.     

Bianchi type-V cosmological model with perfect fluid using negative constant deceleration parameter in a scalar tensor theory based on Lyra Manifold

An exact Bianchi type-V perfect fluid cosmological model is obtained in a scalar tensor theory proposed by Sen (Z. Phys. 149:311, 1957) based on Lyra Manifold in case of β is a constant and it is shown that this cosmological model exists only in the case of Radiation Universe (ρ=3p) if β is a function of ‘t’ using negative constant deceleration parameter. Some physical and geometrical properties of these models are discussed.     

Kantowski-Sachs String Cosmological Model with Bulk Viscosity In General Relativity

String cosmological models with bulk viscosity are investigated in Kantowski-Sachs space-time. To obtain a determinate solution, it is assumed that the coefficient of bulk viscosity is a power function of the scalar of expansion ζ = kθ^m and the scalar of expansion is proportional to the shear scalar θ ∝ σ, which leads to a relation between metric potentials R = AS ⁿ . The physical and geometrical aspects of the model are also discussed. It is shown that the bulk viscosity has significant influence on the evolution of the universe. There is a ‘big bang’ start in the model when m ≤ 1 but there is no ‘big bang’ start when m > 1.     

Star Formation in NGC 4038/4039 as seen in the NIR

We obtained various sets of near infrared observations of the prototypical merger, NGC 4038/4039 (‘the Antennae’). Integral field spectroscopy and broad- and narrow band imaging aimed at obtaining age and extinction estimates of the young star clusters seen in large numbers distributed throughout the disks of the interacting galaxies. High resolution spectroscopy led to estimates of the dynamical masses of the clusters. The clusters have ages ranging from 3.7 to ≈ 20 Myrs. Those in the ‘overlap region’ are very young (below 8 Myrs), while in the nothwestern loop ages are above that limit, and the nuclear starbursts are much older (∼ 100 Myrs). Some photometric cluster masses lie above 10⁶ M_⊙. The stellar velocity dispersions determined from the medium- to high resolution spectra yielded virial cluster masses again up to a few 10⁶ M_⊙. Large differences in the estimated photometric and virial masses suggest a variation of the IMF between the clusters. At least some of the clusters have masses, concentrations and IMFs that could allow them to evolve into globular clusters. This revised version was published online in September 2006 with corrections to the Cover Date.     

Some magnetized bulk viscous string cosmological models in?General Relativity

Some Bianchi type-I viscous fluid string cosmological models with magnetic field are investigated. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density ξ(t)=ξ ₀ ρ ^m , where ξ ₀ and m are constants. To get a determinate model, we assume conditions ρ=(1+ω)λ, where ρ is rest energy density, ω a positive constant and λ the string tension density and expansion θ is proportional to eigen value σ ₁¹ of the shear tensor σ _j ⁱ . The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is discussed.      

Scalar field as dark energy accelerating expansion of the Universe

The features of a homogeneous scalar field ϕ with classical Lagrangian L = ϕ_;i ϕ^;i /2 − V(ϕ) and tachyon field Lagrangian L = −V(ϕ)√1 − ϕ_;i ϕ^;i causing the observable accelerated expansion of the Universe are analyzed. The models with constant equation-of-state parameter w _de = p _de/ρ_de < −1/3 are studied. For both cases the fields ϕ(a) and potentials V(a) are reconstructed for the parameters of cosmological model of the Universe derived from the observations. The effect of rolling down of the potential V(ϕ) to minimum is shown. Published in Ukrainian in Kinematika i Fizika Nebesnykh Tel, 2008, Vol. 24, No. 5, pp. 345–359. The article was translated by the authors.     

Fundamentals of collisionless shocks for astrophysical application, 2. Relativistic shocks

In this concise review of the recent developments in relativistic shock theory in the Universe we restrict ourselves to shocks that do not exhibit quantum effects. On the other hand, emphasis is given to the formation of shocks under both non-magnetised and magnetised conditions. We only briefly discuss particle acceleration in relativistic shocks where much of the results are still preliminary. Analytical theory is rather limited in predicting the real shock structure. Kinetic instability theory is briefed including its predictions and limitations. A recent self-similar relativistic shock theory is described which predicts the average long-term shock behaviour to be magnetised and to cause reasonable power-law distributions for energetic particles. The main focus in this review is on numerical experiments on highly relativistic shocks in (i) pair and (ii) electron-nucleon plasmas and their limitations. These simulations do not validate all predictions of analytic and self-similar theory and so far they do not solve the injection problem and the self-modification by self-generated cosmic rays. The main results of the numerical experiments discussed in this review are: (i) a confirmation of shock evolution in non-magnetised relativistic plasma in 3D due to either the lepton-Weibel instability (in pair plasmas) or to the ion-Weibel instability; (ii) the sensitive dependence of shock formation on upstream magnetisation which causes suppression of Weibel modes for large upstream magnetisation ratios σ>10⁻³; (iii) the sensitive dependence of particle dynamics on the upstream magnetic inclination angle θ _Bn , where particles of θ _Bn >34° cannot escape upstream, leading to the distinction between ‘subluminal’ and ‘superluminal’ shocks; (iv) particles in ultra-relativistic shocks can hardly overturn the shock and escape to upstream; they may oscillate around the shock ramp for a long time, so to speak ‘surfing it’ and thereby becoming accelerated by a kind of SDA; (v) these particles form a power-law tail on the downstream distribution; their limitations are pointed out; (vi) recently developed methods permit the calculation of the radiation spectra emitted by the downstream high-energy particles; (vii) the Weibel-generated downstream magnetic fields form large-amplitude vortices which could be advected by the downstream flow to large distances from the shock and possibly contribute to an extended strong field region; (viii) if cosmic rays are included, Bell-like modes can generate upstream magnetic turbulence at short and, by diffusive re-coupling, also long wavelengths in nearly parallel magnetic field shocks; (ix) advection of such large-amplitude waves should cause periodic reformation of the quasi-parallel shock and eject large-amplitude magnetic field vortices downstream where they contribute to turbulence and to maintaining an extended region of large magnetic fields.     

The Galactic Thick Disk – Status

The status of the Galactic thick disk is reviewed. Consideration of the recent literature suggests that its vertical scale height and normalisation with respect to the thin disk remain uncertain to within a factor two, with values reported in the ranges 750–1500 pc, and 0.02–0.13, respectively. The bulk of the thick disk has kinematics (σ_U, σ_V, σ_W) = (65, 54, 38 km s^-1), and lags the thin disk by some 40 km s^-1; differences of opinion exists as to whether kinematics change with distance from the Galactic plane. The bulk of the thick disk has [Fe/H] ∼ −0.6, with little or no evidence for a vertical gradient. The question of gradients is critical for an understanding of thick disk cosmogony and needs closer attention. The reality of the so-called metal-weak thick disk (material having disklike kinematics and [Fe/H] ≤ −1.0) is also considered. The case for such material seems to be steadily growing: in the range −1.6 ≤ [Fe/H] ≤ −1.0, recent estimates suggest ρ_MWTD/ρ_Halo ∼ 0.1-0.3. While many workers regard the thick disk as a discrete entity, the caveat is made that this is a sufficient condition, but not one necessarily required by the observations. Best practice requires that both the discrete model and the alternative extended configuration be compared with observational data to examine the relative likelihood of their relevance. Recent theoretical advances are also discussed, together with the need for in situ measurements of the thick disk away from the Galactic plane. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Evolution of Star-Forming Galaxies: The SFR Density of the Universe

The evolution of the Star Formation Rate (SFR) density of the Universe as a function of look-back time is a fundamental parameter in order to understand the formation and evolution of galaxies. The current picture, only outlined in the last years, is that the global SFR density has dropped by about an order of magnitude from a redshift of z∼1.5 to the current value at z=0. Because these SFR density studies are now extended to the whole range in redshift, it becomes mandatory to combine data from different SFR tracers. At low redshifts, optical emission lines are the most widely used. Using Hα as current-SFR tracer, the Universidad Complutense de Madrid (UCM) Survey provided the first estimation of the global SFR density in the Local Universe. The Hα flux in emission is directly related to the number of ionizing photons and, modulo IMF, to the total mass of stars formed. Metallic lines like [OII]λ3727 and [OIII]λ5007 are affected by metallicity and excitation. Beyond redshifts z∼0.4, Hα is not observable in the optical and [OII]λ3727 or UV luminosities have to be used. The UCM galaxy sample has been used to obtain a calibration between [OII]λ3727 luminosity and SFR specially suitable for the different types of star-forming galaxies found by deep spectroscopic surveys in redshifts up to z∼1.5. These calibrations, when applied to recent deep redshift surveys confirm the drop of the SFR density of the Universe since z∼1 previously infered in the UV. However, the fundamental parameter that determines galactic evolution is mass, not luminosity. The mass function for local star-forming galaxies is critical for any future comparison with other galaxy populations of different evolutionary status. Hα velocity-widths for UCM galaxies indicate that besides a small fraction of 10¹⁰-10¹¹ M_⊙ starburst nuclei spirals, the majority have dynamical masses in the ∼10⁹ M_⊙ range. A comparison with published data for faint blue galaxies suggests that star-forming galaxies at z∼1 would have SFR per unit mass and burst strengths similar to those at z=0, but being intrinsically more massive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Plane symmetric cosmological models with perfect fluid and dark energy

We consider a self-consistent system of Plane symmetric cosmology and binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be one obeying the usual equation of state p=γρ with γ∈[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding Einstein’s field equations are obtained as a quadrature. The cases of Zeldovich Universe, Dust Universe and Radiation Universe and models with power-law and exponential expansion have discussed in detail. For large t, the models tend to be isotropic.     

Born–Infeld type phantom model in the ω–ω′ plane

In this paper, we investigate the dynamics of Born–Infeld (B–I) phantom model in the ω–ω′ plane, which is defined by the equation of state parameter for the dark energy and its derivative with respect to N (the logarithm of the scale factor a). We find the scalar field equation of motion in ω–ω′ plane, and show mathematically the property of attractor solutions which correspond to ω _φ ∼−1, Ω _φ =1, which avoid the “Big rip” problem and meets the current observations well.      

Allan Sandage and the cosmic expansion

This is an account of Allan Sandage’s work on (1) The character of the expansion field. For many years he has been the strongest defender of an expanding Universe. He later explained the CMB dipole by a local velocity of 220±50 km s⁻¹ toward the Virgo cluster and by a bulk motion of the Local supercluster (extending out to ∼3500 km s⁻¹) of 450–500 km s⁻¹ toward an apex at l=275, b=12. Allowing for these streaming velocities he found linear expansion to hold down to local scales (∼300 km s⁻¹). (2) The calibration of the Hubble constant. Probing different methods he finally adopted—from Cepheid-calibrated SNe Ia and from independent RR Lyr-calibrated TRGBs—H ₀=62.3±1.3±5.0 km s⁻¹ Mpc⁻¹.     

THE ATMOSPHERES OF SATURN AND TITAN IN THE NEAR-INFRARED: FIRST RESULTS OF CASSINI/VIMS

The wide spectral coverage and extensive spatial, temporal, and phase-angle mapping capabilities of the Visual Infrared Mapping Spectrometer (VIMS) onboard the Cassini-Huygens Orbiter are producing fundamental new insights into the nature of the atmospheres of Saturn and Titan. For both bodies, VIMS maps over time and solar phase angles provide information for a multitude of atmospheric constituents and aerosol layers, providing new insights into atmospheric structure and dynamical and chemical processes. For Saturn, salient early results include evidence for phosphine depletion in relatively dark and less cloudy belts at temperate and mid-latitudes compared to the relatively bright and cloudier Equatorial Region, consistent with traditional theories of belts being regions of relative downwelling. Additional Saturn results include (1) the mapping of enhanced trace gas absorptions at the south pole, and (2) the first high phase-angle, high-spatial-resolution imagery of CH₄ fluorescence. An additional fundamental new result is the first nighttime near-infrared mapping of Saturn, clearly showing discrete meteorological features relatively deep in the atmosphere beneath the planet’s sunlit haze and cloud layers, thus revealing a new dynamical regime at depth where vertical dynamics is relatively more important than zonal dynamics in determining cloud morphology. Zonal wind measurements at deeper levels than previously available are achieved by tracking these features over multiple days, thereby providing measurements of zonal wind shears within Saturn’s troposphere when compared to cloudtop movements measured in reflected sunlight. For Titan, initial results include (1) the first detection and mapping of thermal emission spectra of CO, CO₂, and CH₃D on Titan’s nightside limb, (2) the mapping of CH₄ fluorescence over the dayside bright limb, extending to ∼ ∼750 km altitude, (3) wind measurements of ∼ ∼0.5 ms⁻¹, favoring prograde, from the movement of a persistent (multiple months) south polar cloud near 88° S latitude, and (4) the imaging of two transient mid-southern-latitude cloud features.     

Ionized gas towards galactic centre — Constraints from low-frequency recombination lines

Observations of the H272α recombination line towards the galactic centre show features near V_LSR= 0, −50 and + 36 kms⁻¹. We have combined the parameters of these features with the available H166α measurements to obtain the properties of the ionized gas present along the line of sight and also in the ‘3 kpc arm’. For the line-of-sight ionized gas we get an electron density around 7 cm⁻³ and a pathlength through it ∼ 10–60 pc. The emission measure and the electron temperature are in the range 500–2900 pc cm⁻⁶ and 2000–6000 K. respectively. The ionized gas in the 3 kpc arm has an electron density of 30 cm⁻³ and extends over 9 pc along the line of sight if we assume an electron temperature of 10⁴ K. Using the available upper limit to the intensity of the H351α recombination line, we show that the distributed ionized gas responsible for the dispersion of pulsar signals should have a temperature >4500 K. and a minimum filling factor of 20 per cent. We also show that recombination lines from the ‘warm ionized’ gas proposed by McKee & Ostriker (1977) should be detectable in the frequency range 100–150 MHz towards the galactic centre with the sensitivity available at present.     

Accelerating universe with time variation of G and Λ

We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant. Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum density as ρ∼a ⁻⁴. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing exotic type matter like cosmic strings.     

L.R.S. Bianchi Type I string dust magnetized cosmological models

L.R.S. Bianchi Type I string dust cosmological models with and without magnetic field following the techniques used by Letelier and Stachel, is investigated. To get a determinate solution, we assume a conditionσ is proportional to scalar of expansion θ where σ is shear and θ is scalar of expansion and which leads to A=ℓ B ⁿwhere n is a constant and ℓ is proportionality constant. Some special models are also investigated by introducing the transformation, , which leads to Riccati type differential equation. The physical and geometrical aspects of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.