Showing papers on "Deceleration parameter published in 2008"

Two new diagnostics of dark energy

Abstract: We introduce two new diagnostics of dark energy (DE). The first, $Om$, is a combination of the Hubble parameter and the cosmological redshift and provides a null test of dark energy being a cosmological constant $\ensuremath{\Lambda}$. Namely, if the value of $Om(z)$ is the same at different redshifts, then $\mathrm{DE}\ensuremath{\equiv}\ensuremath{\Lambda}$, exactly. The slope of $Om(z)$ can differentiate between different models of dark energy even if the value of the matter density is not accurately known. For DE with an unevolving equation of state, a positive slope of $Om(z)$ is suggestive of phantom ($wl\ensuremath{-}1$) while a negative slope indicates quintessence ($wg\ensuremath{-}1$). The second diagnostic---acceleration probe $\overline{q}$---is the mean value of the deceleration parameter over a small redshift range. It can be used to determine the cosmological redshift at which the universe began to accelerate, again without reference to the current value of the matter density. We apply the $Om$ and $\overline{q}$ diagnostics to the Union data set of type Ia supernovae combined with recent data from the cosmic microwave background (Wilkinson Microwave Anisotropy Probe 5) and baryon acoustic oscillations.

Improved Constraints on the Acceleration History of the Universe and the Properties of the Dark Energy

Abstract: We extend and apply a model-independent analysis method developed earlier by Daly & Djorgovski to new supernova, radio galaxy, and galaxy cluster samples to study the acceleration history of the universe and the properties of the dark energy. There is good agreement between results obtained with radio galaxies and supernovae, suggesting that both distance indicators are reliable. The deceleration parameter q(z) is obtained assuming only the validity of the FRW metric, allowing for a range of values of space curvature, and independent of a gravity theory and the physical nature of the contents of the universe. We show that q_0 is independent of space curvature, and obtain q_0 = − 0.48 ± 0.11. The transition redshift when q0 = 0 is z_T = 0.78(^+0.08)_(−0.27) for zero space curvature, and has a weak dependence on space curvature. We find good agreement between model-independent quantities and those predicted by general relativity, indicating that GR provides a good description of the data over look-back times of ten billion years.

Transition redshift: new kinematic constraints from supernovae

Abstract: The transition redshift (deceleration/acceleration) is discussed by expanding the deceleration parameter to first order around its present value. A detailed study is carried out by considering two different parametrizations, q = q0 + q and q = q0 + q1 z(1 + z) -1 , and the associated free parameters (q0, q1 ) are constrained by three different supernovae (SNe) samples. A previous analysis by Riess et al. using the first expansion is slightly improved and confirmed in light of their recent data (Gold07 sample). However, by fitting the model with the Supernova Legacy Survey (SNLS) type Ia sample, we find that the best fit to the redshift transition is Zt = 0.61, instead of z t = 0.46 as derived by the High-z Supernovae Search (HZSNS) team. This result based in the SNLS sample is also in good agreement with the sample of Davis et al., z t = 0.60 +0.28 -0.11 (1σ). Such results are in line with some independent analyses and accommodate more easily the concordance flat model (ACDM). For both parametrizations, the three SNe Ia samples considered favour recent acceleration and past deceleration with a high degree of statistical confidence level. All the kinematic results presented here depend neither on the validity of general relativity nor on the matter-energy contents of the Universe.

Cosmographic Hubble fits to the supernova data

Abstract: The Hubble relation between distance and redshift is a purely cosmographic relation that depends only on the symmetries of a Friedmann-Lemaitre-Robertson-Walker spacetime, but does not intrinsically make any dynamical assumptions. This suggests that it should be possible to estimate the parameters defining the Hubble relation without making any dynamical assumptions. To test this idea, we perform a number of interrelated cosmographic fits to the $\mathtt{l}\mathtt{e}\mathtt{g}\mathtt{a}\mathtt{c}\mathtt{y}\mathtt{05}$ and $\mathtt{g}\mathtt{o}\mathtt{l}\mathtt{d}\mathtt{06}$ supernova data sets. Based on this supernova data, the ``preponderance of evidence'' certainly suggests an accelerating universe. However, we would argue that (unless one uses additional dynamical and observational information) this conclusion is not currently supported ``beyond reasonable doubt.'' As part of the analysis we develop two particularly transparent graphical representations of the redshift-distance relation---representations in which acceleration versus deceleration reduces to the question of whether the relevant graph slopes up or down. Turning to the details of the cosmographic fits, three issues in particular concern us: First, the fitted value for the deceleration parameter changes significantly depending on whether one performs a ${\ensuremath{\chi}}^{2}$ fit to the luminosity distance, proper motion distance, angular diameter distance, or other suitable distance surrogate. Second, the fitted value for the deceleration parameter changes significantly depending on whether one uses the traditional redshift variable $z$ or what we shall argue is, on theoretical grounds, an improved parametrization $y=z/(1+z)$. Third, the published estimates for systematic uncertainties are sufficiently large that they certainly impact on, and to a large extent undermine, the usual purely statistical tests of significance. We conclude that the supernova data should be treated with some caution.

Infrared propagator corrections for constant deceleration

Abstract: We derive the propagator for a massless, minimally coupled scalar on a D-dimensional, spatially flat, homogeneous and isotropic background with arbitrary constant deceleration parameter. Our construction uses the operator formalism by integrating the Fourier mode sum. We give special attention to infrared corrections from the nonzero lower limit associated with working on finite spatial sections. These corrections eliminate infrared divergences that would otherwise be incorrectly treated by dimensional regularization, resulting in off-coincidence divergences for those special values of the deceleration parameter at which the infrared divergence is logarithmic. As an application we compute the expectation value of the scalar stress–energy tensor.

Cosmological model with non-minimally coupled fermionic field

Abstract: A model for the Universe is proposed whose constituents are: a) a dark energy field modeled by a fermionic field non-minimally coupled with the gravitational field, b) a matter field which consists of pressureless baryonic and dark matter fields and c) a field which represents the radiation and the neutrinos. The coupled system of Dirac's equations and Einstein field equations is solved numerically by considering a spatially flat homogeneous and isotropic Universe. It is shown that the proposed model can reproduce the expected red-shift behaviors of the deceleration parameter, of the density parameters of each constituent and of the luminosity distance. Furthermore, for small values of the red-shift the constant which couples the fermionic and gravitational fields has a remarkable influence on the density and deceleration parameters.

Apparent and average accelerations of the Universe

Abstract: In this paper we consider the relation between the volume deceleration parameter obtained within the Buchert averaging scheme and the deceleration parameter derived from supernova observation. This work was motivated by recent findings that showed that there are models which despite having Λ = 0 have volume deceleration parameter qvol 0, while those models which we have been able to find which exhibit qvol<0 turn out to be unrealistic. This indicates that care must be exercised in relating the deceleration parameter to observations.

Bianchi type-V perfect fluid space-time models in general relativity

Abstract: The law of variation for the generalized mean Hubble’s parameter in the case of a spatially homogeneous and anisotropic Bianchi type-V space-time metric that yields a constant value of deceleration parameter, is presented The variation for Hubble’s parameter generates two types of solutions for the average scale factor one is of power-law type and second is of the exponential form Using these two forms, Einstein’s field equations for perfect fluid Bianchi type-V models are solved separately that correspond to singular and non-singular models respectively We find that the constant value of deceleration parameter is reasonable for description of the present day universe We also find that the universe decelerates for positive value of deceleration parameter where as it accelerates for negative one The behaviors of observationally important parameters such as expansion scalar, mean anisotropic parameter and shear scalar are discussed Exact expressions for look-back time, luminosity distance and event horizon versus redshift are derived and their significances are discussed in detail It has been observed that the solutions are compatible with the results of recent observations

Bianchi type-V cosmological model with perfect fluid using negative constant deceleration parameter in a scalar tensor theory based on Lyra Manifold

Abstract: An exact Bianchi type-V perfect fluid cosmological model is obtained in a scalar tensor theory proposed by Sen (Z. Phys. 149:311, 1957) based on Lyra Manifold in case of β is a constant and it is shown that this cosmological model exists only in the case of Radiation Universe (ρ=3p) if β is a function of ‘t’ using negative constant deceleration parameter. Some physical and geometrical properties of these models are discussed.

Viscous dissipative Chaplygin gas dominated homogenous and isotropic cosmological models

Abstract: The generalized Chaplygin gas, which interpolates between a high density relativistic era and a nonrelativistic matter phase, is a popular dark energy candidate. We consider a generalization of the Chaplygin gas model, by assuming the presence of a bulk viscous type dissipative term in the effective thermodynamic pressure of the gas. The dissipative effects are described by using the truncated Israel-Stewart model, with the bulk viscosity coefficient and the relaxation time functions of the energy density only. The corresponding cosmological dynamics of the bulk viscous Chaplygin gas dominated universe is considered in detail for a flat homogeneous isotropic Friedmann-Robertson-Walker geometry. For different values of the model parameters we consider the evolution of the cosmological parameters (scale factor, energy density, Hubble function, deceleration parameter, and luminosity distance, respectively), by using both analytical and numerical methods. In the large time limit the model describes an accelerating universe, with the effective negative pressure induced by the Chaplygin gas and the bulk viscous pressure driving the acceleration. The theoretical predictions of the luminosity distance of our model are compared with the observations of the type Ia supernovae. The model fits well the recent supernova data. From the fitting we determine both the equation ofmore » state of the Chaplygin gas, and the parameters characterizing the bulk viscosity. The evolution of the scalar field associated to the viscous Chaplygin fluid is also considered, and the corresponding potential is obtained. Hence the viscous Chaplygin gas model offers an effective dynamical possibility for replacing the cosmological constant, and for explaining the recent acceleration of the universe.« less

How strong is the evidence for accelerated expansion

Abstract: We test the present expansion of the universe using supernova type Ia data without making any assumptions about the matter and energy content of the universe or about the parametrization of the deceleration parameter. We assume the cosmological principle to apply in a strict sense. The result strongly depends on the data set, the light curve fitting method and the calibration of the absolute magnitude used for the test, indicating strong systematic errors. Nevertheless, in a spatially flat universe there is at least 5σ evidence for acceleration which drops to 1.8σ in an open universe.

3-loop Yang-Mills Condensate Dark Energy Model And Its Cosmological Constraints

Abstract: This work is a comprehensive investigation of the Yang-Mills condensate (YMC) dark energy (DE) model, which is extended to include the 3-loop quantum corrections. We study its cosmic evolution and the possibility of crossing phantom divide $w=-1$, examine in details the Hubble parameter $H$, the deceleration parameter $q$, the statefinder diagnosis $(r,s)$, and the $w-w^\prime$ diagnosis of the model without and with interaction, and compare our results with other DE models. Besides, by using the observational data of type Ia supernovae (SNIa), the shift parameter from cosmic microwave background (CMB), and the baryon acoustic oscillation (BAO) peak from large scale structures (LSS), we give the cosmological constraints on 3-loop YMC model. It is found that the model can naturally solve the coincidence problem, and its prediction of the afore-mentioned parameter is much closer to the $\Lambda$CDM model than other dynamics DE models; the introduction of the matter-DE interaction will make the YMC model deviating from the $\Lambda$CDM model, and will give an equation of state (EOF) crossing -1. Moreover, it is also found that, to fit the latest SNIa data alone, the $\Lambda$CDM model is slightly better than the 3-loop YMC model; but in fitting of the combination of SNIa, CMB and LSS data, the 3-loop YMC model performs better than the $\Lambda$CDM model.

Apparent and average acceleration of the Universe

Abstract: In this paper we consider the relation between the volume deceleration parameter obtained within the Buchert averaging scheme and the deceleration parameter derived from the supernova observation. This work was motivated by recent findings that showed that there are models which despite $\Lambda=0$ have volume deceleration parameter $q^{vol} < 0$. This opens the possibility that backreaction and averaging effects may be used as an interesting alternative explanation to the dark energy phenomenon. We have calculated $q^{vol}$ in some Lema\^itre--Tolman models. For those models which are chosen to be realistic and which fit the supernova data, we find that $q^{vol} > 0$, while those models which we have been able to find which exhibit $q^{vol} < 0$ turn out to be unrealistic. This indicates that care must be exercised in relating the deceleration parameter to observations.

The three-loop Yang–Mills condensate dark energy model and its cosmological constraints

Abstract: This work is a comprehensive investigation of the Yang–Mills condensate (YMC) dark energy (DE) model, which is extended to include the three-loop quantum corrections. We study its cosmic evolution and the possibility of crossing the phantom divide w = −1, examine in detail the Hubble parameter H, the deceleration parameter q, the statefinder (r,s) diagnostic and the w−w' diagnostic for the model without and with interaction, and compare our results with other DE models. Also, using the observational data for type Ia supernovae (SNIa), the shift parameter from the cosmic microwave background (CMB), and the baryon acoustic oscillation peak from large scale structures (LSS), we give the cosmological constraints on the three-loop YMC model. It is found that the model can solve the coincidence problem naturally, and its prediction of the aforementioned parameter is much closer to the ΛCDM (CDM: cold dark matter) model one than those from other dynamical DE models; the introduction of the matter–DE interaction will make the YMC model deviate from the ΛCDM model, and will give an equation of state crossing −1. Moreover, it is also found that, for fitting the latest SNIa data alone, the ΛCDM model is slightly better than the three-loop YMC model; but in fitting the combination of SNIa, CMB and LSS data, the three-loop YMC model performs better than the ΛCDM model.

Bianchi Type-I Universe with wet dark fluid

Abstract: The Bianchi Type-I Universe filled with dark energy from a wet dark fluid has been considered. A new equation of state for the dark energy component of the Universe has been used. It is modeled on the equation of state p = γ(ρ − ρ*) which can describe a liquid, for example water. The exact solutions to the corresponding field equations are obtained in quadrature form. The solution for constant deceleration parameter have been studied in detail for both power-law and exponential forms. The cases γ = 1 and γ = 0 have also been analysed.

$Λ$-CDM Universe: A Phenomenological Approach With Many Possibilities

Abstract: A time-dependent phenomenological model of Λ, viz. $\dot{\Lambda} \sim H^{3}$, is selected to investigate the Λ-CDM cosmology. The time-dependent form of the equation-of-state parameter ω is derived and it has been possible to obtain the sought-for flip of sign of the deceleration parameter q. The present age of the Universe, calculated for some specific values of the parameters, agrees very well with the observational data.

Bulk viscous cosmology in early Universe

Abstract: The effect of bulk viscosity on the early evolution of Universe for a spatially homogeneous and isotropic Robertson-Walker model is considered. Einstein’s field equations are solved by using ‘gamma-law’ equation of state p = (γ − 1)ρ, where the adiabatic parameter gamma (γ) depends on the scale factor of the model. The ‘gamma’ function is defined in such a way that it describes a unified solution of early evolution of the Universe for inflationary and radiation-dominated phases. The fluid has only bulk viscous term and the coefficient of bulk viscosity is taken to be proportional to some power function of the energy density. The complete general solutions have been given through three cases. For flat space, power-law as well as exponential solutions are found. The problem of how the introduction of viscosity affects the appearance of singularity, is briefly discussed in particular solutions. The deceleration parameter has a freedom to vary with the scale factor of the model, which describes the accelerating expansion of the Universe.

Exact Bianchi Type-I Cosmological Models in a Scalar-Tensor Theory

Abstract: In this paper, a spatially homogeneous and anisotropic Bianchi type-I space-time filled with perfect fluid is investigated within the framework of a scalar-tensor theory proposed by Saez and Ballester. Two different physically viable models of the universe are obtained by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration parameter. One of the models is found to generalize a model recently investigated by Reddy et al. (Astrophys. Space Sci. 306:171, 2006). The Einstein’s field equations are solved exactly and the solutions are found to be consistent with the recent observations of type Ia supernovae. A detailed study of physical and kinematical properties of the models is carried out.

Transition Redshift: New Kinematic Constraints from Supernovae

Abstract: The transition redshift (deceleration/acceleration) is discussed by expanding the deceleration parameter to first order around its present value. A detailed study is carried out by considering two different parameterizations: $q=q_0 + q_1z$ and $q=q_0 + q_1 z(1+z)^{-1}$, and the associated free parameters ($q_o, q_1$) are constrained by 3 different supernova samples. The previous analysis by Riess {\it{et al.}} [ApJ 607, 665, 2004] using the first expansion is slightly improved and confirmed in light of their recent data ({\emph{Gold}}07 sample). However, by fitting the model with the Supernova Legacy Survey (SNLS) type Ia sample we find that the best fit to the redshift transition is $z_t = 0.61$ instead of $z_t = 0.46$ as derived by the High-z Supernovae Search (HZSNS) team. This result based in the SNLS sample is also in good agreement with the Davis {\it{et al.}} sample, $z_t=0.60^{+0.28}_{-0.11}$ ($1\sigma$). Such results are in line with some independent analyzes and accommodates more easily the concordance flat model ($\Lambda$CDM). For both parameterizations, the three SNe type Ia samples considered favor recent acceleration and past deceleration with a high degree of statistical confidence level. All the kinematic results presented here depend neither on the validity of general relativity nor the matter-energy contents of the Universe.

An Exact Bianchi Type-I Cosmological Model in Lyra's Manifold

Abstract: A spatially homogeneous and anisotropic Bianchi type-I space–time has been studied within the framework of Lyra's geometry. Exact solutions of the Einstein's field equations have been obtained with a time dependent gauge function by using a special law of variation for Hubble's parameter that yields a constant value of deceleration parameter. It has been found that the solutions generalize the solutions obtained by Rahaman et al. [Astrophys. Space Sci.299, 211 (2005)] and are consistent with the recent observations of type Ia supernovae. A detailed study of physical and kinematical properties of the model has been carried out.

Constraints on accelerating universe using ESSENCE and Gold supernovae data combined with other cosmological probes

Abstract: We use recent data: the 192 ESSENCE type Ia supernovae (SNe Ia), the 182 Gold SNe Ia, the three-year WMAP, the SDSS baryon acoustic peak, the X-ray gas mass fraction in clusters and the observational H(z) data, to constrain models of the accelerating universe. Combining the 192 ESSENCE data with the observational H(z) data to constrain the parameterized deceleration parameter, we obtain the best-fit values of the transition redshift and current deceleration parameter z T=0.632 −0.127 +0.256 and q 0=−0.788 −0.182 +0.182 . Furthermore, using the ΛCDM model and two model-independent equations of state of the dark energy, we find that the combined constraint from the 192 ESSENCE data and four other cosmological observations gives smaller values for Ω 0m and q 0, but a larger value for z T than the combined constraint from the 182 Gold data with four other observations. Finally, according to the Akaike information criterion it is shown that the recently observed data equally support three dark energy models: ΛCDM, w de(z)=w 0 and w de(z)=w 0+w 1ln (1+z).

Non-homogeneity-driven Universe acceleration

Abstract: 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 center of the models is barotropic. Classification of cosmological models is performed depending on different values and signs of two free parameters. It is shown that for (hyperbolic geometry) dust-like 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. Redshift-magnitude relation is calculated and discussed in the context of SnIa observational data. It is noticed that the most distant supernovae of type Ia fit quite well to the redshift-magnitude relation calculated in the considered model 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 Hubble and energy density parameters.

Cosmological backreaction and spatially averaged spatial curvature

Abstract: It has been suggested that the accelerated expansion of the Universe is due to backreaction of small scale density perturbations on the large scale spacetime geometry. While evidence against this suggestion has accumulated, it has not yet been definitively ruled out. Many investigations of this issue have focused on the Buchert formalism, which computes spatial averages of quantities in synchronous comoving gauge. We argue that, for the deceleration parameter of this formalism to agree with observations, the spatial average of the three dimensional Ricci scalar (spatial curvature) must be large today, with an $\Omega_k$ in the range of $1 \le \Omega_k \le 1.3$. We argue that this constraint is difficult to reconcile with observations of the location of the first Doppler peak of the CMBR. We illustrate the argument with a simple toy model for the effect of backreaction, which we show is generically incompatible with observations.

Bianchi type v cosmological models with perfect fluid and heat conduction in lyra's geometry

Abstract: The field equations within the framework of Lyra's geometry with a time-dependent displacement vector field for a Bianchi type-V space–time filled with a perfect fluid and heat flow are presented. Two different classes of physically viable solutions are obtained by using a special law of variation for the generalized mean Hubble's parameter which correspond to singular and nonsingular models with constant deceleration parameter. These models are found to be consistent with the observations on the present day universe. Some thermodynamical relations are studied. The physical and kinematical behaviors of the models are also discussed.

Some Exact Bianchi Type V Perfect Fluid Solutions with Heat Flow

Abstract: The variation law for generalized mean Hubble’s parameter is discussed in a spatially homogeneous and anisotropic Bianchi type V space-time with perfect fluid along with heat-conduction. The variation law for Hubble’s parameter, that yields a constant value of deceleration parameter, generates two types of solutions for the average scale factor, one is of power-law type and other one of exponential form. Using these two forms of the average scale factor, exact solutions of Einstein field equations with a perfect fluid and heat conduction are presented for a Bianchi type V space-time, which represent expanding singular and non-singular cosmological models. We find that the constant value of deceleration parameter is reasonable for the present day universe. The physical and geometrical properties of the models are also discussed in detail.

Bianchi Type-I Space-Time with Variable Cosmological Constant

Abstract: A spatially homogeneous and anisotropic Bianchi type-I perfect fluid model is considered with variable cosmological constant. Einstein’s field equations are solved by using a law of variation for mean Hubble’s parameter, which is related to average scale factor and that yields a constant value of deceleration parameter. An exact and singular Bianchi-I model is presented, where the cosmological constant remains positive and decreases with the cosmic time. It is found that the solutions are consistent with the recent observations of type Ia supernovae. A detailed study of physical and kinematical properties of the model is carried out.

Probing the cosmic acceleration history and the properties of dark energy from the ESSENCE supernova data with a model independent method

Abstract: With a model independent method the expansion history H(z), the deceleration parameter q(z) of the universe and the equation of state w(z) for the dark energy are reconstructed directly from the 192 Sne Ia (type Ia supernovae) data points which are contained in the new ESSENCE (Equation of State: Supernovae Trace Cosmic Expansion) Sne Ia data and the high redshift Sne Ia data. We find that the evolving properties of q(z) and w(z) reconstructed from the 192 Sne Ia data seem to be weaker than those obtained from the Gold set, but stronger than those from the SNLS (Supernova Legacy Survey) set. With a combination of the 192 Sne Ia and BAO (Baryonic Acoustic Oscillation) data, a tight constraint on Ωm0 is obtained. At the 1σ confidence level Ωm0 = 0.278−0.023+0.024, which is highly consistent with the values from the Gold + BAO and SNLS + BAO data.

Constraints on Deceleration Parameter of a 5D Bounce Cosmological Model from Recent Cosmic Observations

Abstract: We study the constraint on deceleration parameter q from the recent SNeIa Gold dataset and observational Hubble data by using a model-independent deceleration parameter q(z) = ½ – a/(1 + z)b under the five-dimensional bounce cosmological model. For the cases of SNeIa Gold dataset, Hubble data, and their combination, the present results show that the constraints on transition redshift zT are 0.35+0.14−0.07, 0.68+1.47 0.58, and 0.55+0.18−0.09 with 1σ errors, respectively.