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.

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.

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