Showing papers on "Deceleration parameter published in 1992"

Cosmological consequences of a time-dependent Λ term

Abstract: The phenomenological approach to investigate the decay of the effective cosmological constant, as recently proposed by Chen and Wu, is generalized to include a term proportional to ${H}^{2}$ on the time dependence of $\ensuremath{\Lambda}$, where $H$ is the Hubble parameter This new term can modify some features of the standard Friedmann-Robertson-Walker model and its free parameter may be adjusted in accordance with nucleosynthesis constraints The model also allows a deceleration parameter ${q}_{0}$ assuming negative values so that the density parameter ${\ensuremath{\Omega}}_{0}$ is smaller than ⅔ and the age of the Universe is always bigger than ${H}_{0}^{\ensuremath{-}1}$ In these cases, the usual matter creation rate appearing in models with a decaying vacuum energy is smaller than the one present in the steady-state model

Some remarks on luminosity distance and nucleosynthesis in higher dimensional cosmology

Abstract: Using a previous cosmological solution of the present author, the behavior of luminosity distance is analyzed in multidimensional spacetime. It is observed that the de Sitter spacetime sets an upper limit to the absolute distance of any cosmological source where the generally accepted value of the deceleration parameter q(0) greater than -1 is taken. This generalizes an earlier result of Barnes to higher dimensions. Further, an attempt is made to investigate, in brief, the phenomenon of nucleosynthesis in the early universe. While it is premature to make a definite statement in this regard, it is found that there may be significant differences, in principle at least, from the analogous situation in four-dimensional spacetime. 15 refs.

Cosmological parameters and redshift periodicity

Abstract: 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, Paalet 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 Ω0=ρ0/ρcrit andλ0=Λ/3H02 acceptable periodicity is obtained forall points of the parameter plane within the strip between the parallel lines 0.83Ω0−0.30<λ0(Ω0)<0.83Ω0+0.85(Ω0<1.8), whilst the best periodicities appear along the lineλ0=0.83Ω0+0.39 fitting to the previous optimum at Ω0=1/3,λ0=2/3. Any nonpositive value ofλ0 gives bad periodicity unless the space curvature is strongly negative and Ω0<0.4. Fairly good periodicity is observed only in the range of the deceleration parameter −1.2≤q0<0.2, corresponding to a small or even negative total gravitational attraction and an expansion time-scale longer than usually expected.

What if Ω¬=;2q?

Abstract: We discuss various possible scenarios where Ω ≠ 2q, where Ω stands for the density parameter andq for the deceleration parameter. We further estimate the corrections necessary when a variable cosmological constant is considered in the theory.

What If (1 # 2q?

Abstract: We discuss various possible scenarios where l'~ ~ 2q, where f~ stands for the density parameter and q for the deceleration parameter. We further estimate the corrections necessary when a variable cosmological constant is considered in the theory.