There is mounting observational evidence that the expansion of our universe is undergoing anacceleration. A dark energy component has usually been invoked as the most feasible mechanism for the acceleration. However, it is desirable to explore alternative possibilitiesmotivated by particle physics before adopting such an untested entity. In this work, we focus our attention on an acceleration mechanism arising from gravitational leakage into extra dimensions. We test this scenario with high-z Type Ia supernovae compiled by Tonry and coworkers and recent measurements of the X-ray gas mass fractions in clusters of galaxies published by Allen and coworkers. A combination of the two databases gives, at a 99% confidence level, m

Recent measurements of Type Ia supernovae, as well as other concordant observations, suggest that the expansion of our universe is accelerating. A dark energy component has usually been invoked as the most feasible mechanism for the acceleration. However, effects arising from possible extra dimensions can mimic dark energy through a modified Friedmann equation. In this work, we investigate some observational constraints on ascenario in which this modification is given by H2

Cosmic acceleration is one of the most remarkable cosmological findings of recent years. Although a dark energy component has usually been invoked as the mechanism for the acceleration, a modification of the Friedmann equation from various higher dimensional models provides a feasible alternative. Cardassian expansion is one of these scenarios, in which the universe is flat, matter (and radiation) dominated, and accelerating but contains no dark energy component. This scenario is fully characterized by n, the power index of the socalled Cardassian term in the modified Friedmann equation, and m, the matter density parameter of the universe. In this work, we first consider the constraints on the parameter space from the turnaround redshift, zq

Although various cosmological observations congruously suggest that our universe is dominated by two dark components, the cold dark matter without pressure and the dark energy with negative pressure, the nature and origin of these components is yet unknow. The generalized Chaplygin gas (gCg), parametrized by an equation of state, p=−A/ρα gCg, was recently proposed to be a candidate of the unified dark matter/energy (UDME) scenarios. In this work, we investigate some observational constraints on it. We mainly focus our attention on the constraints from recent measurements of the X-ray gas mass fractions in clusters of galaxies published by Allen et al. (2002,MNRAS, 334, L11; 2003, 342, 257) and the dimensionless coordinate distances to type Ia supernovae and Fanaro Riley type IIb radio galaxies compiled by Daly & Djorgovski (2003, ApJ, 597, 9). We obtain the confidence region on the two parameters fully characterizing gCg, As ≡ A/ρ(1+α) gCg0 and α, from a combined analysis of these databases, where ρgCg0 is the energy density of gCg at present. It is found that As=0.70+0.16−0.17 and −0.33, at a 95% confidence level, which is consistent within the errors with the standard dark matter+ dark energy model, i.e., the case of α=0. Particularly, the standard Chaplygin gas (α=1) is ruled out as a feasible UDME by the data at a 99% confidence level.

Using recent measurements of angular size of high-z milliarcsecond compact radio sources compiled by Gurvitset al. (1999) and X-ray gas mass fraction of galaxy clusters published by Allen et al. (2002, 2003), we explore their bounds on the equation of state, ωx=px/ρx, of the dark energy, whose existence has been congruously suggested by various cosmological observations. We relax the usual constraint ωx ≥ −1, and find that combining the two databases yields a nontrivial lower bound on ωx. Under the assumption of a flat universe, we obtain a bound −2.22<ωx<−0.62 at 95.4% confidence level. The 95.4% confidence bound goes to −1 ≤ωx<−0.60 when the constraint ωx ≥ −1 is imposed.

The distant Type Ia supernovae data compiled by Perlmutter et al. are used to analyze the Cardassian expansion scenario, which was recently proposed by Freese & Lewis as an alternative to a cosmological constant (or more generally a dark energy component) in explaining the currently accelerating universe. We show that the allowed intervals for n and zeq, the two parameters of the Cardassian model, will give rise to a universe with a very low matter density, which can hardly be reconciled with the current value derived from the measurements of the cosmic microwave background anisotropy and galaxy clusters (cluster baryon fraction). As a result, this Cardassian expansion proposal does not seem to survive the magnitude-redshift test for the present Type Ia supernovae data, unless the universe contains primarily baryonic matter.

The "Cardassian Expansion Scenario" was recently proposed by Freese & Lewis as an alternative to a cosmological constant in explaining the current accelerating universe. In this paper we investigate observational constraints on this scenario from recent measurements of the angular size of high-z compact radio sources compiled by Gurvits and coworkers. We show that the allowed intervals for n and zeq, the two parameters of the Cardassian model, are heavily dependent on the value of the mean projected linear size l. However, the best fit to the current angular size data prefers the conventional flat cold dark matter model to this Cardassian expansion proposal, though the latter is cosmologically credible and compatible with the-z diagramfor some values of l.

The expected chirp mass distribution of observed events for future gravitational wave detectors is extensively investigated in the presence of an exotic fluid component with an arbitrary equation of state, −1 !xpx=x<0, i. e., the so-called dark energy component. The results for a flat model dominated by dark energy are compared to those for the standard flat model dominated by cold dark matter. It is found that for a flat universe the chirp mass distribution shows a sensitive dependence on !x, which may provide an independent and robust constraint on the cosmic equation of state.

a wide variety of cosmological models characterized by the cosmic mass density M and the normalized cosmological constant, we derive an analytic expression for the estimate of magnification cross-sections by an ensemble of isothermal spheres as models of galactic mass distributions. This provides a simple approach to demonstrate how the lensing probability by galaxies depends on the cosmological parameters. An immediate consequence is that, while a non-zero cosmological constant indeed leads to a significant increase of the lensing probability as it has been shown in the literature, only a small fraction of sky to 23 can be moderately (1:3) lensed by galaxies even in a-dominated flat universe. Therefore, whether or not there is a nonzero cosmological constant, it is unlikely that the overall quasar counts have been seriously contaminated by the presence of galactic lenses.

The theoretically expected amplitude of the associations of background quasars with foreground galaxies as a result of gravitational lensing has been updated in this paper. Since the galactic matter alone yields an amplitude of quasar overdensity smaller than that observed, special attention has been paid to the examination or reexamination of the uncertainties in the estimate of the quasar enhancement factor arising from the cosmic evolution of galaxies, the core radius and velocity bias of galactic matter distributions, the clusters of galaxies, the obstruction etect by galactic disks, the nonzero cosmological constant, etc. Unfortunately, none of these factors has been shown to be able to improve the situation signi