Showing papers on "Deceleration parameter published in 2001"

An alternative to quintessence

Abstract: We consider a FRW cosmological model with an exotic fluid known as Chaplygin gas. We show that the resulting evolution of the universe is not in disagreement with the current observation of cosmic acceleration. The model predict an increasing value for the effective cosmological constant.

Consequences on variable Λ-models from distant type Ia supernovae and compact radio sources

Abstract: We study the magnitude-redshift relation for the type?Ia supernovae data and the angular size-redshift relation for the updated compact radio sources data (from Gurvits et?al) by considering four variable ?-models: ?~S-2, ?~H2, ?~? and ?~t-2. It is found that all the variable ?-models, as well as the constant ?-Friedmann model, fit the supernovae data equally well with ?2/dof?1 and require non-zero, positive values of ? and an accelerating expansion of the universe. The estimates of the density parameter for the variable ?-models are found to be higher than those for the constant ?-Friedmann model. From the compact radio sources data, it is found, by assuming the no-evolution hypothesis, that the Gurvits et al model (Friedmann model with ? = 0) is not the best-fitting model for the constant ? case. The best-fitting Friedmann model (with constant ?) is found to be a low-density, vacuum-dominated accelerating universe. The fits of this data set to the (variable, as well as, constant ?-) models are found to be very good with ?2/dof?0.5 and require non-zero, positive values of ? with either sign of the deceleration parameter. However, for realistic values of the matter density parameter, the only interesting solutions are (a) estimated from the supernovae data: the best-fitting solutions for the flat models (including the constant ? case); (b) estimated from the radio sources data: the global best-fitting solutions for the models ?~H2 and ?~?, the best-fitting solution for the flat model with ? = {}constant and the Gurvits et al model. It is noted that, as in the case of recent cosmic microwave background analyses, the data sets seem to favour a spherical universe (k>0).

An introduction to mathematical cosmology

Abstract: This book provides a concise introduction to the mathematical aspects of the origin, structure and evolution of the universe. The book begins with a brief overview of observational and theoretical cosmology, along with a short introduction to general relativity. It then goes on to discuss Friedmann models, the Hubble constant and deceleration parameter, singularities, the early universe, inflation, quantum cosmology and the distant future of the universe. This edition contains a rigorous derivation of the Robertson–Walker metric. It also discusses the limits to the parameter space through various theoretical and observational constraints, and presents a new inflationary solution for a sixth degree potential. This book is suitable as a textbook for advanced undergraduates and beginning graduate students. It will also be of interest to cosmologists, astrophysicists, applied mathematicians and mathematical physicists.

Study of the Magnitude-Redshift Relation for Type Ia Supernovae in a Model Resulting from a Ricci-Symmetry

Abstract: Models with a dynamic cosmological term Λ(t) are becoming popular as they solve the cosmological constant problem in a natural way Instead of considering any ad-hoc assumption for the variation of Λ, we consider a particular symmetry, the contracted Ricci-collineation along the fluid flow, in Einstein's theory We show that apart from having interesting properties, this symmetry does demand Λ to be a function of the scale factor of the Robertson-Walker metric In order to test the consistency of the resulting model with observations, we study the magnitude-redshift relation for the type Ia supernovae data from Perlmutter et al The data fit the model very well and require a positive non-zero Λ and a negative deceleration parameter The best-fitting flat model is obtained as Ω0 ≈ 05 with q0 ≈ −02

Bulk Viscous FRW Cosmology in Lyra Geometry

Abstract: We have studied an isotropic homogeneous FRW universe in the presence of a bulk viscous fluid within the framework of Lyra's geometry. We have obtained exact solutions of the Sen equations assuming the deceleration parameter to be constant. The coefficient of bulk viscosity has been assumed to be a power function of the mass density. With this assumption, we have considered the behavior of the displacement field and the energy density for both power-law and exponential expansions of the universe. We show that our models are generalised and we obtain the results of previous works by considering k=0 and k=-1.

Some Cosmological Models in Scalar-Tensor Theory of Gravitation

Abstract: Field equations in a scalar-tensor theory of gravitation proposed by Saezand Ballester (1985) are obtained with the aid of (i) Friedmann-type metric (ii) a non static plane symmetric metric and (iii) spatially homogeneous Bianchi type – III metric. Some cosmological models corresponding to perfect fluid and bulk viscous fluid are presented. Physical and kinematical properties of the models are also discussed.

Exact anisotropic brane cosmologies

Abstract: We present exact solutions of the gravitational field equations in the generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I and V geometry, with perfect fluid and scalar fields as matter sources. Under the assumption of a conformally flat bulk (with a vanishing Weyl tensor) for a cosmological fluid obeying a linear barotropic equation of state the general solution of the field equations can be expressed in an exact parametric form for both Bianchi type I and V space-times. In the limiting case of a stiff cosmological fluid with pressure equal to the energy density, for a Bianchi type I universe the solution of the field equations is obtained in an exact analytic form. Several classes of scalar field models of evolution on the brane are also considered, corresponding to different choices of the scalar field potential. For all models the behavior of the observationally important parameters such as shear, anisotropy, and the deceleration parameter is considered in detail.

Non-homogeneity-driven universe acceleration

Abstract: A class of spherically symmetric Stephani cosmological models is examined in the context of evolution type. It is assumed that the equation of state at the symmetry centre of the models is barotropic (p(t) = αρ(t)) and the function k(t) playing the role of spatial curvature is proportional to the Stephani version of the Friedmann-Robertson-Lemaitre-Walker scale factor R(t) (k(t) = βR(t)). A classification of the cosmological models is performed depending on different values and signs of parameters α and β. It is shown that for β<0 (hyperbolic geometry) a dust-like (α = 0) cosmological model exhibits accelerated expansion at later stages of evolution. The Hubble and deceleration parameters are defined in the model and it is shown that the deceleration parameter decreases with the distance becoming negative for sufficiently distant galaxies. The redshift-magnitude relation m(z) is calculated and discussed in the context of SnIa observational data. It is noted that the most distant supernovae of type Ia fit quite well to the relation m(z) calculated in the considered model (H0 = 65 km s-1 Mpc-1, Ω0≤0.3) without introducing the cosmological constant. It is also shown that the age of the universe in the model is longer than in the Friedmann model corresponding to the same H0 and Ω0 parameters.

Cosmology with Self-Adjusting Vacuum Energy Density from a Renormalization Group Fixed Point

Abstract: Cosmologies with a time dependent Newton constant and cosmological constant are investigated. The scale dependence of $G$ and $\Lambda$ is governed by a set of renormalization group equations which is coupled to Einstein's equation in a consistent way. The existence of an infrared attractive renormalization group fixed point is postulated, and the cosmological implications of this assumption are explored. It turns out that in the late Universe the vacuum energy density is automatically adjusted so as to equal precisely the matter energy density, and that the deceleration parameter approaches $q = -1/4$. This scenario might explain the data from recent observations of high redshift type Ia Supernovae and the cosmic microwave background radiation without introducing a quintessence field.

Higher Dimensional Cosmological Model With Gravitational and Cosmological "Constants"

Abstract: In this paper we have considered a cosmological model representing a flat viscous universe with variable G and Λ in the context of higher dimensional spacetime. It has been observed that in this model the particle horizon exists and the cosmological term varies as inverse square of time. The deceleration parameter and temperature are well within the observational limits. The model indicates matter and entropy generation in the early stages of the universe. Further, it is shown that our model generates all models obtained by Arbab and Singh et al. in four-dimensional space-time.

The cosmological constants

Abstract: The evolution of the large scale of the Universe is but one chapter of cosmology, yet it is fundamental to all of its aspects. The understanding of the large-scale evolution has once been described as the “Search for two numbers” (***Sandage 1970). The first of these numbers is the Hubble constant H 0 which measures the present expansion rate. The second number was thought to account for the gravitational deceleration of the Universe. But since evidence has been found for an additional acceleration, the original deceleration parameter has been split into a decelerating term Ωm and an accelerating term ΩΛ. Ωm is a measure of the gravitating mean matter density of the Universe, while ΩΛ corresponds to the energy density of Einstein’s “cosmological constant” Λ.

Apocalypse soon

Abstract: Based upon a simple vacuum Lagrangian, comprising cosmological and quadratic scalar field terms, a cosmological model is presented whose history is indistinguishable from that of an innocuous low-density cold dark matter (CDM) universe, but whose future is very much shorter. For sensible values of the deceleration parameter (0

Distance measurements as a probe of cosmic acceleration

Abstract: A major recent development in observational cosmology has been an accurate measurement of the luminosity distance–redshift relation out to redshifts z=0.8 from Type Ia supernova standard candles. The results have been argued as evidence for cosmic acceleration. It is well known that this assertion depends on the assumption that we know the equation of state for all mass–energy other than normal pressureless matter; popular models are based either on the cosmological constant or on the more general quintessence formulation. However, this assertion also depends on a number of other assumptions, implicit in the derivation of the standard cosmological field equations: large-scale isotropy and homogeneity, the flatness of the Universe, and the validity of general relativity on cosmological scales (where it has not been tested). A detailed examination of the effects of these assumptions on the interplay between the luminosity distance–redshift relation and the acceleration of the Universe is not possible unless one can define the precise nature of the failure of any particular assumption. However a simple quantitative investigation is possible and reveals a number of considerations about the relative importance of the different assumptions. In this paper we present such an investigation. We find that the relationship between the distant-redshift relation and the sign of the deceleration parameter is fairly robust and is unaffected if only one of the assumptions that we investigate is invalid so long as the deceleration parameter is not close to zero (it would not be close to zero in the currently favoured ΩΛ=1−Ωmatter=0.7 or 0.8 Universe, for example). Failures of two or more assumptions in concordance may have stronger effects.

Motor vehicle has data connection for transmitting deceleration parameter from slip regulator to controller for control of deceleration warning light

Abstract: The device has a slip regulator (100) for preventing a wheel from slipping when starting off in accordance with a slip parameter and a controller (200) for controlling at least one deceleration warning light (300) depending on current deceleration. The slip regulator computes a deceleration parameter taking into account at least some slip parameters. The vehicle has a data connection (180) for sending the deceleration parameter to the controller.

Application of the 5-Dimensional Projective Unified Field Theory to Cosmology and Astrophysics

Abstract: Projective Unified Field Theory of the author leads to a cosmology without big bang. Since details have been published in a series of papers, the aim of this article is to inform about the theoretical and numerical results with respect to cosmology and astrophysics: age of the universe, Hubble factor, deceleration parameter, equation of motion of a testparticle, motion of a body around a gravitative central body, Einstein effects, time dependence of the "effective gravitational constant" etc. Further, this theory predicts heat production by the cosmological expansion in a moving body. Numerical results are presented for orbiting stars, planets, moons, satellites and terrestrial bodies.

Study of the Magnitude-Redshift Relation for type Ia Supernovae in a Model resulting from a Ricci-Symmetry

Abstract: Models with a dynamic cosmological term \Lambda (t) are becoming popular as they solve the cosmological constant problem in a natural way. Instead of considering any ad-hoc assumption for the variation of \Lambda, we consider a particular symmetry, the contracted Ricci-collineation along the fluid flow, in Einstein's theory. We show that apart from having interesting properties, this symmetry does require \Lambda to be a function of the scale factor of the Robertson-Walker metric. In order to test the consistency of the resulting model with observations, we study the magnitude-redshift relation for the type Ia supernovae data from Perlmutter et al. The data fit the model very well and require a positive non-zero \Lambda and a negative deceleration parameter. The best-fitting flat model is obtained as \Omega_0 \approx 0.5 with q_0 \approx -0.2.

Neutrinos in a vacuum dominated cosmology

Abstract: We explore the dynamics of neutrinos in a vacuum dominated cosmology. First we show that such a geometry will induce a phase change in the eigenstates of a massive neutrino and we calculate the phase change. We also calculate the delay in the neutrino flight times in this geometry. Applying our results to the presently observed background vacuum energy density, we find that for neutrino sources further than $1.5 Gpc$ away both effects become non-trivial, being of the order of the standard relativistic corrections. Such sources are within the obsevable Hubble Deep Field. The results which are theoretically interesting are also potentially useful, in the future, as detection techniques improve. For example such effects on neutrinos from distant sources like supernovae could be used, in an independent method alternative to standard candles, to constrain the dark energy density and the deceleration parameter. The discussion is extended to investigate Caianiello's inertial or maximal acceleration (MA) effects of such a vacuum dominated spacetime on neutrino oscillations. Assuming that the MA phenomenon exists, we find that its form as generated by the presently observed vacuum energy density would still have little or no measurable effect on neutrino phase evolution.

Possible cosmological implications of a time varying fine structure constant.

Abstract: Webb et al. [2] experimental results on the fine strucutre “constant ” variation with the age of the Universe, is here analized. By using the experimental data on the fine-structure “constant ”’s variation with the age of the Universe, and Dirac’s LNH (Large Number Hypothesis), we find how should vary the total number of nucleons in the Universe, the speed of light, Newton’s gravitational “constant ” and the energy density, and we make an estimate on the deceleration parameter, finding that the Universe would be accelerating, just as Supernovae observations have concluded. PACS 98.80 Hw

The Hubble diagram of metal-rich QSOs and deceleration parameter q(0)

Abstract: For 163 metal-rich Quasars, the m V -log Z diagram shows a very close correlation. Using multiple regression analysis for these sources ( N =163), we obtained q 0 =1.142 and correlation coefficient γ =0.69. These results suggested that the Universe is closed and all metal-rich quasars are of a single category. On the other hand, the evolution is very small at Z ≤2 for metal-rich quasars.

Properties of the quantum universe in quasistationary states and cosmological puzzles

Abstract: Old and new puzzles of cosmology are reexamined from the point of view of quantum theory of the universe developed here. It is shown that in proposed approach the difficulties of the standard cosmology do not arise. The theory predicts the observed dimensions of nonhomogeneities of matter density and the amplitude of fluctuations of the cosmic background radiation temperature in the Universe and points to a new quantum mechanism of their origin. The large scale structure in the Universe is explained by the growth of nonhomogeneities which arise from primordial quantum fluctuations due to finite width of quasistationary states. The theory allows to obtain the value of the deceleration parameter which is in good agreement with the recent SNe Ia measurements. It explains the large value of entropy of the Universe and describes other parameters.

On a time varying fine structure 'constant'

Abstract: By employing Dirac LNH, and a further generalization by Berman (GLNH), we estimate how should vary the total number of nucleons, the energy density, Newton Gravitational constant, the cosmological constant, the magnetic permeability and electric permitivity, of the Universe,in order to account for the experimentally observed time variation of the fine structure constant. As a bonus,we find an acceptable value for the deceleration parameter of the present Universe, compatible with the Supernovae observations.