Showing papers on "Deceleration parameter published in 1994"

Deflationary cosmology with decaying vacuum energy density

Abstract: A nonsingular cosmological scenario based on a new phenomenological decay law for the effective $\ensuremath{\Lambda}$ term is proposed. In such a model, the cosmic history began from an instability of the de Sitter space-time rather than a singularity. The process is characterized by an arbitrary time scale ${H}_{I}^{\ensuremath{-}1}$ giving the largest value of the cosmological constant (${\ensuremath{\Lambda}}_{I}={3H}_{I}^{2}$) and the initial temperature of the Universe. Subsequently, the Universe evolved continuously toward a slightly modified Friedmann-Robertson-Walker cosmology. Among the "remnants" of the primordial inflationary period are the present values of the cosmological term ${\ensuremath{\Lambda}}_{0}\ensuremath{\sim}3\ensuremath{\beta}{H}_{0}^{2}$, the density parameter ${\ensuremath{\Omega}}_{0}\ensuremath{\sim}1\ensuremath{-}\ensuremath{\beta}$, and the deceleration parameter $\frac{{q}_{0}\ensuremath{\sim}(1\ensuremath{-}3\ensuremath{\beta})}{2}$, where $\ensuremath{\beta}$ is smaller than unity. In addition, the age parameter of the vacuum-matter-dominated phase is ${H}_{0}{t}_{0}\ensuremath{\sim}\frac{2}{3}(1\ensuremath{-}\ensuremath{\beta})$. It is also shown that, if ${H}_{I}^{\ensuremath{-}1}$ is of the order of the Planck time, the Universe started with the Planck temperature and this model is consistent with the theoretically suggested value $\frac{{\ensuremath{\Lambda}}_{1}}{{\ensuremath{\Lambda}}_{0}\ensuremath{\sim}{10}^{118}}$.

Cosmological models with constant deceleration parameter in Brans-Dicke theory

Abstract: A detailed study of cosmological models with constant deceleration parameterq is undertaken in the framework of Brans-Dicke theory. These models are divided into two categories: (i) singular models with expansion driven by big-bang impulse, (ii) non-singlar models with expansion driven by creation of matter particles. Prigogine's hypothesis of creation of matter out of gravitational energy is analysed and extended to BD cosmology. To accommodate the creation of new particles, the universe is regarded as an open thermodynamical system and the energy conservation equation is modified with the incorporation of a creation pressure termp c in the energy-momentum tensor $$\tilde T_{ab} $$ . The exact solutions of the field equations of BD theory with $$\tilde T_{ab} $$ are obtained using the power law relationΦ=KR α, which leads to models with constantq. The behaviour of the solutions is investigated for different range of values ofa. The role played by the BD scalar fieldΦ and creation of matter particles in the expansion of the universe is investigated. It is found that one particular model with constantq has exponential expansion.

Dirac's Cosmology with Varying Cosmological Constant

Abstract: The phenomenological approach to investigating the decay of the vacuum energy density is generalized in the spirit of Dirac's large number hypothesis. Different arguments are outlined to justify an universal dependence of the type λ=ΒH 2 (Β is a pure number andH is the Hubble parameter). Such time-varying A does change the predictions of the earlier Dirac's cosmology. The deceleration parameter (q 0=(2−Β)/(1+Β)) and the age parameter (H 0t0=(1+Β)/3) are now compatible with the observations. The model also allows a “power-law” inflationary phase and a rate of matter creation smaller than the one present in the steady state universe.

Qualitative analysis of diagonal Bianchi type V imperfect fluid cosmological models

Abstract: The Einstein field equations for diagonal Bianchi type V imperfect fluid cosmological models with both viscosity and heat conduction are set up as an autonomous system of differential equations using dimensionless variables and a set of dimensionless equations of state. Models with and without a cosmological constant, λ, are investigated using the techniques from dynamical systems theory. It is shown that all models that satisfy the weak energy conditions isotropize. The introduction of viscosity (in particular) allows for a variety of different qualitative behaviors (including, for example, models with a negative deceleration parameter). Exact solutions that correspond to the singular points of the dynamical system are found. It is shown that the past asymptotic states are represented by self‐similar cosmological models and, if λ=0, the future asymptotic states are also, in general, represented by self‐similar cosmological models; in the exceptional cases the late time asymptotic state is represented by ...

Cosmological models with constant deceleration parameter in Nordtvedt’s theory

Abstract: Exact solutions of the field equations of Nordtvedt’s theory for spatially flat FRW models with constant deceleration parameter have been obtained. Singular solutions with (i) power-law (ii) exponential expansion have been studied in Nordtvedt’s theory where the coupling parameterω is a function of the scalar fieldφ.

Large numbers hypothesis and Whitrow-Randall-Sciama relation in Brans-Dicke theory

Abstract: It has recently been suggested in Brans-Dicke theory that the so-called Whitrow-Randall-Sciama relation plus the assumption of a constant deceleration parameter implies the large numbers hypothesis. It is shown that this claim is not always true.

Probing the Universe with Extragalactic Radio Jets

Abstract: distances Some specially selected active radio sources can be used for measuring extragalactic the Hubble constant and the deceleration parameter of the Universe. A source is selected if available observations of flux spectra, structure and polarization of a jet emission in this source agree with a jet model, discussed herein. Using this approach on some radio observational data, the preliminary estimations of the Hubble constant are obtained as 20 km/s/Mpc ≲ Ho ≲ 200 km/s/Mpc (in a “short jet model”) and 60 km/s/Mpc ≲ 230 km/s/Mpc (in a “long jet model”).

Positioning controller for electric motor

Abstract: PURPOSE:To calculate an acceleration/deceleration parameter from the acceleration of the electric motor and automatically set the acceleration/deceleration parameter each time positioning command is instructed by performing simple, high-precision approximation by using a mathematical expression. CONSTITUTION:A motion control instruction decoding means while interpreting a positioning instruction and outputting a feed speed F and the maximum acceleration amax of the electric motor to an acceleration/deceleration parameter arithmetic means 110 inputs the feed speed and a movement distance to a position command arithmetic means 102. The position command arithmetic means 102 calculates data on an expected movement position at each specific period and inputs the data to an acceleration/deceleration arithmetic means 103. The acceleration/deceleration parameter arithmetic means 110 calculates the acceleration/deceleration parameter omega from the feed speed F, the maximum acceleration amax of the electric motor, and the proportional gain Kp of the position controller and sets it in the acceleration/deceleration arithmetic means 103. At this time, a function (equation 9) required for the calculation is approximated with (equation 10) to perform the high-precision approximation.

An alternative form for Raychaudhuri's equation

Abstract: Raychaudhuri's equation contains some features that deserve attention, such as the increase witht2 of the difference between half the density parameter and the deceleration parameter. We derive a form of the equation that shows how this happens.

An Alternative Form for Raychaudhuri's

Abstract: Raychaudhuri's equation contains some features that deserve attention, such as the increase with t 2 of the difference between half the density parameter and the deceleration parameter. We derive a form of the equation that shows how this happens.