Cosmological self-similarity

By use of a simple set of scaling equations it is demonstrated that atomic and stellar systems show evidence of quantitative self-similarity. Six of the basic parameters characterizing the atomic scale, when properly scaled, are nearly identical to the corresponding six basic measurements characterizing the stellar scale. Galactic scale extrapolations are mentioned and a definitive prediction with which to test the principle of self-similarity is identified.     

Cosmological hadronic fireball

Relativistic Non-Zero Pressure Cosmology describes a hadronic fireball in the primordial stage of the Universe near the singularity. It is phenomenologically matched to Hagedorn's equation of state based on thermodynamics of strong interactions.     

Cosmological considerations regarding Uranus

The cosmogony of Uranus is discussed within the context of a picture in which solid condensed materials accumulate to form a large body, which then acquires significant amounts of gas from the primitive solar nebula. Of prime cosmogonical importance is the tilt of the equatorial plane of the planet and of the plane of tilt of the planet can easily occur as a result of a major collision during the formation process; it seems most likely that the tilt of the satellite orbits requires that they were formed from a gaseous disc rotating about the planet after the tilt of the planetary rotational axis had occurred. Possible methods for tilting this gaseous disc are discussed. A strong early magnetic field may have helped in this and may have played an essential role in showing down the spin of the planet to the present observed value. These processes may have produced significant compositional differences between the satellites of Uranus and those of Jupiter and Saturn.     

Some Bianchi Type I Viscous Fluid Cosmological Models with a Variable Cosmological Constant

Some Bianchi type I viscous fluid cosmological models with a variable cosmological constant are investigated in which the expansion is considered only in two direction i.e. one of the Hubble parameter is zero. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density whereas the coefficient of shear viscosity is considered as constant in first case whereas in other case it is taken as proportional to scale of expansion in the model. The cosmological constant Λ is found to be positive and is a decreasing function of time which is supported by results from recent supernovae Ia observations. Some physical and geometric properties of the models are also discussed.     

Cosmological constant as integration constant

Einstein's field theory of elementary particles (Einstein 1919) yields black holes with a mass M ˜ G⁻¹ Λ^−1/2c² and a charge Q ˜ G^−1/2λ^−1/2c², their curvatu re radius is Λ^−1/2. Here 4Λ is an integration constant of Einstein's ‘trace-less’ gravitation equations. The choice λ = G⁻¹h⁻¹c³ for this constant defines Planckions and implies “strong gravity”. The choice λ = λ = 3H_inf²c⁻² (where H_inf means the Hubble parameter of a final de Sitter cosmos) involves “weak gravity” and describes an electro-vac spherical universe.     

Cosmological parameters and redshift periodicity

This work is the continuation of the search for such a cosmological model using which the observed redshift distribution of galaxies in the sample of Broadhurstet al. (1990) turns out to be maximally periodic in the calculated spatial distance. In a previous work, Paálet al. (1992) have demonstrated that among theflat models with non-negative cosmological constant (e.e., vacuum density) the one with a vacuum: dust ratio 2:1 provides the optimum. Now we extend that study to the case of arbitrary space curvature and find equally good periodicity in a surprisingly wide range of models. By use of the dimensionless parameters ₀= ₀/ _crit and ₀=/3H ₀ ² acceptable periodicity is obtained forall points of the parameter plane within the strip between the parallel lines 0.83₀–0.30< ₀(₀)<0.83₀+0.85(₀<1.8), whilst the best periodicities appear along the line ₀=0.83₀+0.39 fitting to the previous optimum at ₀=1/3, ₀=2/3. Any nonpositive value of ₀ gives bad periodicity unless the space curvature is strongly negative and ₀<0.4. Fairly good periodicity is observed only in the range of the deceleration parameter –1.2q ₀<0.2, corresponding to a small or even negative total gravitational attraction and an expansion time-scale longer than usually expected.     

Cosmological constraint on Brans-Dicke Model

We combine new Cosmic Microwave Background(CMB) data from Planck with Baryon Acoustic Oscillation(BAO) data to constrain the Brans-Dicke(BD) theory,in which the gravitational constant G evolves with time.Observations of type Ia supernovae(SNe Ia) provide another important set of cosmological data,as they may be regarded as standard candles after some empirical corrections.However,in theories that include modified gravity like the BD theory,there is some risk and complication when using the SNIa data because their luminosity may depend on G.In this paper,we assume a power law relation between the SNIa luminosity and G,but treat the power index as a free parameter.We then test whether the difference in distances measured with SNIa data and BAO data can be reduced in such a model.We also constrain the BD theory and cosmological parameters by making a global fit with the CMB,BAO and SNIa data set.For the CMB+BAO+SNIa data set,we find 0.08 × 10~(-2) ζ 0.33 × 10~(-2) at the 68% confidence level(CL) and-0.01 × 10~(-2) ζ 0.43 × 10~(-2) at the 95% CL,where ζ is related to the BD parameter ω by ζ = ln(1 + 1/ω).     

Cosmological parameters and cosmic topology

Geometry constrains but does not dictate the topology of the three-dimensional space. In a locally spatially homogeneous and isotropic universe, however, the topology of its spatial section dictates its geometry. We show that, besides determining the geometry, the knowledge of the spatial topology through the circles-in-the-sky offers an effective way of setting constraints on the density parameters associated with dark matter (Ω_m) and dark energy  (Ω_Λ)  . By assuming the Poincaré dodecahedral space as the circles-in-the-sky detectable topology of the spatial sections of the Universe, we re-analyse the constraints on the density parametric plane  Ω_m–Ω_Λ  from the current Type Ia supernova plus X-ray gas mass fraction data, and show that a circles-in-the sky detection of the dodecahedral space topology gives rise to strong and complementary constraints on the region of the density parameter plane currently allowed by these observational data sets.     

Radio Relics in Cosmological Simulations

Radio relics have been discovered in many galaxy clusters. They are believed to trace shock fronts induced by cluster mergers. Cosmological simulations allow us to study merger shocks in detail since the intra-cluster medium is heated by shock dissipation. Using high resolution cosmological simulations, identifying shock fronts and applying a parametric model for the radio emission allows us to simulate the formation of radio relics. We analyze a simulated shock front in detail. We find a rather broad Mach number distribution. The Mach number affects strongly the number density of relativistic electrons in the downstream area, hence, the radio luminosity varies significantly across the shock surface. The abundance of radio relics can be modeled with the help of the radio power probability distribution which aims at predicting radio relic number counts. Since the actual electron acceleration efficiency is not known, predictions for the number counts need to be normalized by the observed number of radio relics. For the characteristics of upcoming low frequency surveys we find that about thousand relics are awaiting discovery.     

Cosmological science enabled by Planck

Planck will be the first mission to map the entire cosmic microwave background (CMB) sky with mJy sensitivity and resolution better than 10′. The science enabled by such a mission spans many areas of astrophysics and cosmology. In particular it will lead to a revolution in our understanding of primary and secondary CMB anisotropies, the constraints on many key cosmological parameters will be improved by almost an order of magnitude (to sub-percent levels) and the shape and amplitude of the mass power spectrum at high redshift will be tightly constrained.     

Cosmological models of gamma-ray bursts

We review models of cosmological gamma-ray bursts (GRBs). The statistical and -ray transparency issues are summarized. Neutron-star and black-hole merger scenarios are described and estimates of merger rates are summarized. We review the simple fireball models for GRBs and the recent work on non-simple fireballs. Alternative cosmological models, including models where GRBs are analogs of active galactic nuclei and where they are produced by high-field, short period pulsars, are also mentioned. The value of neutrino astronomy to solve the GRB puzzle is briefly reviewed.     

Star Formation and Cosmological Simulations

The inclusion of a detailed modeling of the short-scale baryonic physics in a large-scale cosmological simulation is crucial for a better comparison between observations and predictions from cosmological models. From a set of 3D hydrodynamical simulations which include a chemical model to account for the complex physics of the ISM at a sub-grid scale, we have been able to obtain a statistically significant sample of galaxy-type halos with observational properties, like colors and luminosities for different cosmological scenarios. From this data base, we have studied a number of different things, like Tully-Fisher relations, luminosity functions and environmental effects. Despite the progress made during the last few years in the modeling of the physics of ISM and star formation, more work is clearly needed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cosmological information from wimp experiments

We discuss the information that can be obtained from direct and indirect detection experiments of wimp dark matter. While individual experiments yield little information about wimp cross sections or galactic abundances, we consider the possibility that data from direct and indirect experiments can be combined to give a measurement of the low-energy wimp annihilation cross section. This turns out to be possible if the wimp annihilation rate in the Earth is not balanced by the capture rate. The derived low-energy annihilation cross section can be used to constrain the cosmological density of wimps.     

Holographic View of Cosmological Perturbations

The cosmological perturbation theory is revisited from the holographic point of view. In the case of the single brane model, it turns out that the AdS/CFT correspondence plays an important role. In the case of the two-brane model, it is shown that the effective equations of motion becomes the quasi-scalar-tensor gravity. It is also demonstrated that the radion anisotropy gives the CMB fluctuations through the Sachs-Wolfe effect. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Cosmological mass model in general relativity

Relativistic cosmological field equations are obtained for a non-static stationary Bertotti-Robinson-type space-time for interacting perfect fluid and electromagnetic field. The cosmological solution to the field equations are obtained and the nature of the electromagnetic field as well the perfect fluid are studied. The electromagnetic field generated here corresponds to a special generic case and the perfect fluid distribution degenerates into a barotropic perfect fluid with equation of statep+=0, >0. It is shown here that the interacting barotropic fluid can generate gravitation only when the cosmological constant being a function ofx in a dynamic field.