Showing papers on "Deceleration parameter published in 1999"

The Case for a Positive Cosmological Lambda-term

Abstract: Recent observations of Type 1a supernovae indicating an accelerating universe have once more drawn attention to the possible existence, at the present epoch, of a small positive Lambda-term (cosmological constant). In this paper we review both observational and theoretical aspects of a small cosmological Lambda-term. We discuss the current observational situation focusing on cosmological tests of Lambda including the age of the universe, high redshift supernovae, gravitational lensing, galaxy clustering and the cosmic microwave background. We also review the theoretical debate surrounding Lambda: the generation of Lambda in models with spontaneous symmetry breaking and through quantum vacuum polarization effects -- mechanisms which are known to give rise to alarge value of Lambda hence leading to the `cosmological constant problem'. More recent attempts to generate a small cosmological constant at the present epoch using either field theoretic techniques, or by modeling a dynamical Lambda-term by scalar fields are also extensively discussed. Anthropic arguments favouring a small cosmological constant are briefly reviewed. A comprehensive bibliography of recent work on Lambda is provided.

Four testable predictions of instanton cosmology

Abstract: A new cosmological model makes the following predictions: (1) The deceleration parameter q0 is approximately zero. (2) The mass density parameter Ωm is less than 1. (3) The universe is spatially closed, but is asymptotically flat as t→∞, regardless of its matter content. (4) The age of the universe is approximately 15 Gyr if the Hubble parameter h is approximately 0.65.

Evolution of Type Ia Supernovae on Cosmological Time Scales

Abstract: Due to their high luminosity at maximum and degree of homogeneity, Type Ia supernovae have been extensively used for cosmological purpouses, in particular to estimate extragalactic distances and the Hubble constant. Recently the number of Type Ia supernovae detected at high redshift has increased, opening the possibility of determining the mass density parameter, the cosmological constant and the deceleration parameter. The observed supernovae appear to be further than expected -even for an empty Universe-, implying a low density Universe and moreover an accelerating Universe. Among the various uncertainties, we address the possibility that old supernovae are not equal to current supernovae. From first principles, an evolution of progenitors with time is expected. Additionally, some observations show a dependence of the observed properties on galaxy type and colour. Our aim in this work is to study the outcome of exploding CO white dwarfs following the evolution of the progenitor intermediate mass stars with different masses and metallicities. Once this influence of the progentitor has been determined, the observations may be corrected. At the present stage of this project we are not able to quantify this effect properly. One result is clear, that the differences at maximum are expected to be small ($\sim$0.2 mag) but this is of the same order as all the evidence for a positive cosmological constant ($\sim$0.25 mag).

A Constraint on the Standard Beaming Model for Superluminal Sources

Abstract: We have used two subsamples of superluminal quasars to test the relativistic beaming model, and to place useful constraints on the radio source orientation hypothesis and cosmology. Based on the variation of the observed ratio R of the core-to-lobe radio luminosities with proper motion mu for the subsample of lobe-selected quasars, we show that the observed R - mu data can be explained in terms of a bulk relativistic motion with Lorentz factor gamme approx 4. Also, from the observed proper motion versus redshift (mu - z) plot for this subsample, we show that gamma approx 4 implies a high density universe with deceleration parameter q_0 = 0.5. Furthermore, from the observed (mu - z) plot for the two subsamples taken separately, we show that both gamma and mu for the core-selected subsample exceed those of the lobe-selected subsample by a factor of 2 for the q_0 = 0.5 world model. This result is demonstrated to be consistent with an orientation-based unified scheme in which lobe-selected quasars lie, on the average, at an angle which is a factor of ~2-3 larger than that of their core-selected counterparts.

Four Testable Predictions of Instanton Cosmology

Abstract: A new cosmological model makes the following predictions: (1) The deceleration parameter $q_{0}$ is approximately zero. (2) The mass density parameter $\Omega_{m}$ is less than 1. (3) The universe is spatially closed, but is asymptotically flat as $t\to\infty$, regardless of its matter content. (4) The age of the universe is approximately 15 Gyr if the Hubble parameter $h$ is approximately 0.65.

Future Destiny of Quintessential Universe and Constraint on Model from Deceleration Parameter

Abstract: The evolution of the quintessence in various stages of the universe, the radiation-, matter-, and quintessence-dominated, is closely related with the tracking behavior and the deceleration parameter of the universe. We gave the explicit relation between the equation-of-state of the quintessence in the epoch of the matter-quintessence equality and the inverse power index of the quintessence potential, obtained the constraint on this potential parameter come from the present deceleration parameter, i.e., a low inverse power index. We point out that the low inverse power-law potential with a single term can not work for the tracking solution. In order to have both of the tracker and the suitable deceleration parameter it is necessary to introduce at least two terms in the quintessence potential. We give the future evolution of the quintessential universe.