Cosmic Time Variation of the Gravitational Constant

A pre-relativistic cosmological approach to electromagnetism and gravitation is explored that leads to a cosmic time variation of the fundamental constants. Space itself is supposed to have physical substance, which manifests by its permeability. The scale factors of the permeability tensor induce a time variation of the fundamental constants. Atomic radii, periods, and energy levels scale in cosmic time, which results in dispersionless redshifts without invoking a space expansion. Hubble constant and deceleration parameter are reviewed in this context. The time variation of the gravitational constant at the present epoch can be expressed in terms of these quantities. This provides a completely new way to restrain the deceleration parameter from laboratory bounds on the time variation of the gravitational constant. This variation also affects the redshift dependence of angular diameters and the surface brightness, and we study in some detail the redshift scaling of the linear sizes of radio sources. The effect of the varying constants on source counts is discussed, and an estimate on the curvature radius of the hyperbolic3-space is inferred from the peak in the quasar distribution. The background radiation in this dispersionless, permeable space-time stays perfectly Planckian. Cosmic time is discussed in terms of atomic and gravitational clocks, as well as cosmological age dating, in particular how the age of the Universe relates to the age of the Galaxy in a permeable space-time. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solutions to the field equations and the deceleration parameter

We utilise a form for the Hubble parameter to generate a number of solutions to the Einstein field equations with variable cosmological constant and variable gravitational constant. The Hubble law utilised yields a constant value for the deceleration parameter. A variety of solutions is presented in the Robertson-Walker spacetimes. A generalisation of the cosmic scale factor is utilised in the anisotropic Bianchi I spacetime to illustrate that new solutions may also be found in spacetimes with less symmetry than Robertson-Walker. We also show that the constant deceleration parameter used is consistent with alternate theories of gravity by considering the scalar-tensor theory of Lau and Prokhovnik with ak = 0 Robertson-Walker background.     

Stability of viscous fluid in Bianchi type-VI model with cosmological constant

In this paper, we investigate Bianchi type-VI cosmological model for the universe filled with dark energy and viscous fluid in the presence of cosmological constant. Also, we show accelerating expansion of the universe by drawing volume scale, pressure and energy density versus cosmic time. In order to solve the Einstein’s field equations, we assume the expansion scalar is proportional to a component of the shear tensor. Therefore, we obtain the directional scale factors and show the EOS parameter crosses over phantom divided-line.     

Redshift evolution of angular diameters and surface brightness: how rigid are galactic measuring rods?

The effect of a cosmic time variation of Newton’s constant on galactic angular diameters, linear size, apparent magnitude, and surface brightness is investigated. The redshift scaling of the gravitational constant is proportional to the Hubble parameter, derived from the constancy of a moderate dimensionless ratio of fundamental constants, and manifested in galactic linear-size evolution. The latter is demonstrated by fitting the angular size–redshift relation to spectroscopically and photometrically selected samples of high-redshift rotators. The intrinsic luminosity evolution of the rotators and their magnitude–redshift and surface brightness–redshift relations are studied. The galactic luminosity scales with a power of the Hubble parameter, and the scaling exponent is inferred from a moderate dimensionless ratio involving the gravitational constant, the Galactic luminosity, and the velocity of the Galaxy in the microwave background. The fits are performed with a cosmic expansion factor derived from paleoplanetary surface temperatures. This expansion factor is tested by comparing the corresponding redshift evolution of the angular-size distance to the distance estimates of two samples of galaxy clusters.     

Cosmic microwave background constraints on the epoch of reionization

We use a compilation of cosmic microwave anisotropy data to constrain the epoch of reionization in the Universe, as a function of cosmological parameters. We consider spatially flat cosmologies, varying the matter density Ω₀ (the flatness being restored by a cosmological constant), the Hubble parameter h and the spectral index n of the primordial power spectrum. Our results are quoted both in terms of the maximum permitted optical depth to the last-scattering surface, and in terms of the highest allowed reionization redshift assuming instantaneous reionization. For critical-density models, significantly tilted power spectra are excluded as they cannot fit the current data for any amount of reionization, and even scale-invariant models must have an optical depth to last scattering of below 0.3. For the currently favoured low-density model with Ω₀=0.3 and a cosmological constant, the earliest reionization permitted to occur is at around redshift 35, which roughly coincides with the highest estimate in the literature. We provide general fitting functions for the maximum permitted optical depth, as a function of cosmological parameters. We do not consider the inclusion of tensor perturbations, but if present they would strengthen the upper limits that we quote.     

The probable extragalactic nature of some low surface brightness clouds at high galactic latitude

Low surface brightness clouds observed at optical, infrared, and radio wavelengths are discussed. We present evidence that some clouds at high galactic latitudes are associated with the Local Group, M81, and possibly even with higher redshift extragalactic objects.Low temperature clouds at high latitude must affect at some level the short wave length side of the cosmic background radiation. If some of these clouds are extragalactic there should be a further effect on the interpretation of CBR measures.     

The effect of reionization on the cosmic microwave background–density correlation

In this paper, we show how the rescattering of cosmic microwave background photons after cosmic reionization can give a significant linear contribution to the temperature–matter cross-correlation measurements. These anisotropies, which arise via a late-time Doppler effect, are on scales much larger than the typical scale of non-linear effects at reionization; they can contribute to degree scale cross-correlations and could affect the interpretation of similar correlations resulting from the integrated Sachs–Wolfe effect. While expected to be small at low redshifts, these correlations can be large given a probe of the density at high redshift, and so could be a useful probe of the cosmic reionization history.     

Superluminal Kinematics in the Milne Universe: Causality in the Cosmic Time Order

The causality of superluminal signal transfer in the galaxy background is scrutinized. The cosmic time of the comoving galaxy frame determines a distinguished time order for events connected by superluminal signals. Every observer can relate his rest frame to the galaxy frame, and compare so the time order of events in his proper time to the cosmic time order. In this way all observers arrive at identical conclusions on the causality of events connected by superluminal signals. The energy of tachyons (superluminal particles) is defined in the comoving galaxy frame analogous to the energy of subluminal particles. It is positive in the galaxy frame and bounded from below in the rest frames of geodesically moving observers, so that particle-tachyon interactions can be based on energy-momentum conservation. We study tachyons in a Robertson-Walker cosmology with linear expansion factor and open, negatively curved 3-space (Milne universe). This cosmology admits globally geodesic rest frames for uniformly moving observers, synchronized by Lorentz boosts. In this context we show that no signals can be sent into the past of observers. If an observer emits a tachyonic signal, then the response of a second observer can never reach him prior to the emission, i.e., no predetermination can occur. The proof is based on the positivity of tachyonic energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

A class of cosmological solutions of Brans-Dicke theory with cosmological constant

We present a class of exact cosmological solutions of Brans-Dicke (B-D) equations with cosmological constant in flat Robertson-Walker metric. These solutions are based on the relation øR ⁿ= constant between the B-D field and the scale factor of the universe. This relation turns out to be consistent with the equation of statep =m for the cosmic matter, provided thatn andm are suitably related to each other. Several special cases and asymptotic solutions are derived and discussed.     

The dependence of galaxy formation on cosmological parameters: can we distinguish between the WMAP1 and WMAP3 parameter sets?

We combine N -body simulations of structure growth with physical modelling of galaxy evolution to investigate whether the shift in cosmological parameters between the first- and third-year results from the Wilkinson Microwave Anisotropy Probe ( WMAP ) affects predictions for the galaxy population. Structure formation is significantly delayed in the WMAP3 cosmology, because the initial matter fluctuation amplitude is lower on the relevant scales. The decrease in dark matter clustering strength is, however, almost entirely offset by an increase in halo bias, so predictions for galaxy clustering are barely altered. In both cosmologies, several combinations of physical parameters can reproduce observed, low-redshift galaxy properties; the star formation, supernova feedback and active galactic nucleus feedback efficiencies can be played off against each other to give similar results. Models which fit observed luminosity functions predict projected two-point correlation functions which scatter by about 10–20 per cent on large scale and by larger factors on small scale, depending both on cosmology and on details of galaxy formation. Measurements of the pairwise velocity distribution prefer the WMAP1 cosmology, but careful treatment of the systematics is needed. Given present modelling uncertainties, it is not easy to distinguish between the WMAP1 and WMAP3 cosmologies on the basis of low-redshift galaxy properties. Model predictions diverge more dramatically at high redshift. Better observational data at   z > 2  will better constrain galaxy formation and perhaps also cosmological parameters.     

The Ionization Rate During the Decoupling Period in a Universe with Nonzero Cosmological Constant

Nonzero cosmological constant favours cosmological models with larger Hubble constant. The evolution of ionization during decoupling period in a universe with nonzero cosmological constant is computed by using a corrected recombination coefficient. Also presented in this paper is the redshift distribution of the last scattering surfaces of the cosmic background photons while the cosmological constant is nonvanishing. Finally, we give a brief estimation about the influence of H_e on the last scattering surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preliminary results of the multisite time-series campaign on the sdB pulsator PG 1325+101

I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new 3D chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z = 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. The feedback of stars leads to turbulent motions and large-scale flows in the ISM. As one result the galactic disk is significantly enriched by chemical elements synthesized in bulge stars.     

The influence of gravitational binding energy on cosmic expansion dynamics: new perspectives for cosmology

Confronted with microwave background observations by WMAP and with consternating supernova locations in the magnitude–redshift diagram modern cosmology feels enforced to call for cosmic vacuum energy as a necessary cosmological ingredient. Most often this vacuum energy is associated with Einstein’s cosmological constant Λ or with so-called “dark energy”. A positive value of Λ describes an inflationary action on cosmic dynamics which in view of recent cosmological data appears as an absolute need. In this article, however, we question the hypothesis of a constant vacuum energy density since not justifiable on physical grounds. Instead we show that gravitational binding energy of cosmic matter, connected with ongoing structure formation during cosmic expansion, acts similar to vacuum energy, since it reduces the effective gravitating proper mass density. Thus one may be encouraged to believe that actions of cosmic vacuum energy and gravitational binding energy concerning their cosmological effects are closely related to each other, perhaps in some respects even have identical phenomenologies.     

Robertson-Walker cosmological models with perfect fluid in general relativity

Einstein's field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for a Robertson-Walker universe by assuming the cosmological term to be proportional to R-m(R is a scale factor and m is a constant).A variety of solutions is presented.The physical significance of the cosmological models has also been discussed.     

Cosmological tensor perturbations in brane world models

We discuss the problems faced in trying to deduce the evolution of cosmological perturbations in brane-world models. There are two natural ways to formulate the problem: one which makes the equations of motion simple and the other which makes the boundary condition simple. Unfortunately the problem is difficult to solve, even numerically, in either formalism. We present a more phenomenological approach which, while it does not solve the problem for any given model, illustrates some generic features one might expect to see in the tensor part of the cosmic microwave background power spectrum. We find that the observed scale invariance of the cosmic microwave background provides bounds on brane world models. This revised version was published online in July 2006 with corrections to the Cover Date.     

The redshift revisited

We analyse the history of modern cosmology based on the redshift phenomenon and on the cosmic background radiation (CBR). We show the models of different authors for the interpretation of the redshift and how the tired light models predicted the correct value of 2.7 K temperature previous to Gamow and collaborators.     

Testing Cosmological Models using the Kinematics of High Redshift Galaxies

The scaling of the apparent angular diameter of galaxies with redshift θ(z) is a powerful discriminator of cosmological models. In this paper we argue that the rotational velocity of distant galaxies, when interpreted as size indicator, may be used as an interesting tool to select high redshift standard rods. Upcoming deep redshift surveys will allow an implementation of this classical geometrical test to measure directly the amplitude of the cosmological constant Λ, or to constrain the cosmic equation of state parameter for a smooth dark energy component (w = p/ρ, —1 ≤ w < 0).     

The cross-correlation of the CMB polarization and the 21-cm line fluctuations from cosmic reionization

The cosmic microwave background (CMB) polarization and the 21-cm line fluctuations are powerful probes of cosmological reionization. We study how the cross-correlation between the CMB polarization ( E modes) and the 21-cm line fluctuations can be used to gain further understanding of the reionization history, within the framework of inhomogeneous reionization. Since the E -mode polarization reflects the amplitude of the quadrupole component of the CMB temperature fluctuations, the angular power spectrum of the cross-correlation exhibits oscillations at all multipoles. The first peak of the power spectrum appears at the scale corresponding to the quadrupole at the redshift, which is probed by the 21-cm line fluctuations. The peak reaches its maximum value in redshift when the average ionization fraction of the universe is about half. On the other hand, on small scales, there is a damping that depends on the duration of reionization. Thus, the cross-correlation between the CMB polarization and the 21-cm line fluctuations has the potential to accurately constrain the epoch and the duration of reionization.     

Some string cosmological models in Bianchi Type I space-time

We obtain some cosmological models that are exact solutions of Einstein's field equations. The metric utilized is Marder's metric which is Bianchi Type I and the curvature source is a cloud of strings which are one dimensional objects. Bianchi type cosmological models play an important role in the study of the universe on a scale which anisotropy is not ignored. In this paper we have investigated the effect of cosmic strings on the cosmic microwave background anisotropy. Various physical and geometrical properties of the model are also discussed. The solutions have reported that the cosmic microwave background anisotropy may due to the cosmic strings.     

Looking Down the Light Cone: Can Deep Redshift Surveys Alone Measure the Power Spectrum?

We analyse the window functions for the spherical harmonic mode estimators of all-sky, volume-limited surveys, considering evolutionary effects along the past light-cone which include the deviation of the distance scale from a linear relationship with redshift, linear peculiar velocity corrections, and linear evolution of the density perturbations. The spherical harmonic basis functions are considered, because they correspond most closely to the symmetries of typical survey geometries and of the light-cone effects we consider. Our results show substantial broadening of the windows over that expected by ignoring light-cone effects, indicating the difficulty of measuring the power spectrum independently from cosmology. We suggest that because of light-cone effects, deep redshift surveys should be analysed either in conjunction with CMBR data which determines the cosmological parameters, or by using a Bayesian likelihood scheme in which varying cosmological parameters and a simple parametrization of the primordial power spectrum are assumed as the priors, so that observed data can be mapped from redshift to real space. The derived power spectrum can then be compared with underlying models of fluctuation generation and growth in structure formation to evaluate both these models and the cosmological priors.