Showing papers on "Deceleration parameter published in 2004"
TL;DR: For a flat universe with a cosmological constant, the transition between the two epochs is constrained to be at z = 0.46 ± 0.13 as mentioned in this paper, and w = -1.02 ± (and w < -0.76 at the 95% confidence level) for an assumed static equation of state of dark energy.
Abstract: We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest redshift SNe Ia known, all at z > 1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these objects and to 170 previously reported SNe Ia have been determined using empirical relations between light-curve shape and luminosity. A purely kinematic interpretation of the SN Ia sample provides evidence at the greater than 99% confidence level for a transition from deceleration to acceleration or, similarly, strong evidence for a cosmic jerk. Using a simple model of the expansion history, the transition between the two epochs is constrained to be at z = 0.46 ± 0.13. The data are consistent with the cosmic concordance model of ΩM ≈ 0.3, ΩΛ ≈ 0.7 (χ = 1.06) and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat universe with a cosmological constant, we measure ΩM = 0.29 ± (equivalently, ΩΛ = 0.71). When combined with external flat-universe constraints, including the cosmic microwave background and large-scale structure, we find w = -1.02 ± (and w < -0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc2. Joint constraints on both the recent equation of state of dark energy, w0, and its time evolution, dw/dz, are a factor of ~8 more precise than the first estimates and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w0 = -1.0, dw/dz = 0) and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the universe.
4,236 citations
TL;DR: In this article, the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration was provided by the discovery of 16 Type Ia supernovae with the Hubble Space Telescope (HST).
Abstract: We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest-redshift SNe Ia known, all at z>1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these and 170 previous SNe Ia are provided. A purely kinematic interpretation of the SN Ia sample provides evidence at the > 99% confidence level for a transition from deceleration to acceleration or similarly, strong evidence for a cosmic jerk. Using a simple model of the expansion history, the transition between the two epochs is constrained to be at z=0.46 +/- 0.13. The data are consistent with the cosmic concordance model of Omega_M ~ 0.3, Omega_Lambda~0.7 (chi^2_dof=1.06), and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat Universe with a cosmological constant. When combined with external flat-Universe constraints we find w=-1.02 + 0.13 - 0.19 (and $<-0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = w\rho c^2. Joint constraints on both the recent equation of state of dark energy, $w_0$, and its time evolution, dw/dz, are a factor of ~8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w_0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the Universe.
3,528 citations
TL;DR: In this article, a model for holographic dark energy is proposed, following the idea that the short distance cutoff is related to the infrared cut-off, and the model predicts the equation of state of the dark energy at the present time to be characterized by w = -0.90.
1,834 citations
TL;DR: In this article, the authors proposed a method to determine the expansion rate E(z) and the deceleration parameter q(z), in a largely model-independent way, directly from the data on coordinate distances y(z).
Abstract: Understanding the nature of dark energy, which appears to drive the expansion of the universe, is one of the central problems of physical cosmology today. In an earlier paper we proposed a novel method to determine the expansion rate E(z) and the deceleration parameter q(z) in a largely model-independent way, directly from the data on coordinate distances y(z). Here we expand this methodology to include measurements of the pressure of dark energy p(z), its normalized energy density fraction f(z), and the equation-of-state parameter w(z). We then apply this methodology to a new, combined data set of distances to supernovae and radio galaxies. In evaluating E(z) and q(z), we make only the assumptions that the FRW metric applies and that the universe is spatially flat (an assumption strongly supported by modern cosmic microwave background radiation measurements). The determinations of E(z) and q(z) are independent of any theory of gravity. For evaluations of p(z), f(z), and w(z), a theory of gravity must be adopted, and general relativity is assumed here. No a priori assumptions regarding the properties or redshift evolution of the dark energy are needed. We obtain trends for y(z) and E(z) that are fully consistent with the standard Friedmann-Lemaitre concordance cosmology with Ω_0 = 0.3 and Λ_0 = 0.7. The measured trend for q(z) deviates systematically from the predictions of this model on a ~1-2 σ level but may be consistent for smaller values of Λ_0. We confirm our previous result that the universe transitions from acceleration to deceleration at a redshift zT ≈ 0.4. The trends for p(z), f(z), and w(z) are consistent with being constant at least out to z ~ 0.3-0.5 and broadly consistent with being constant out to higher redshifts, but with large uncertainties. For the present values of these parameters we obtain E_0 = 0.97 ± 0.03, q_0 = -0.35 ± 0.15, p_0 = -0.6 ± 0.15, f_0 = -0.62 - (Ω_0 - 0.3) ± 0.05, and w_0 = -0.9 - e(Ω_0 - 0.3) ± 0.1, where Ω_0 is the density parameter for nonrelativistic matter and e ≈ 1.5 ± 0.1. We note that in the standard Friedmann-Lemaitre models p_0 = -Λ_0, and thus we can measure the value of the cosmological constant directly and obtain results in agreement with other contemporary results.
145 citations
TL;DR: In this paper, a model-independent analysis of Type Ia supernovae (SNe Ia) data without reference to the energy densities is presented, based on the observationally justifiable assumptions of homogeneity and isotropy.
Abstract: The apparent magnitude-redshift data of Type Ia supernovae (SNe Ia) call for modifications in the standard model energy densities. Under the circumstance that this modification cannot be limited to the addition of a mere cosmological constant, a serious situation has emerged in cosmology in which the energy densities in the universe have become largely speculative. In this situation, an equation of state of the form p = wρ itself is not well motivated. In this paper, we argue that the reasonable remaining option is to make a model-independent analysis of SNe data without reference to the energy densities. In this basically kinematic approach, we limit ourselves to the observationally justifiable assumptions of homogeneity and isotropy, i.e., to the assumption that the universe has a Robertson-Walker metric. This cosmographic approach is historically the original one in cosmology. We perform the analysis by expanding the scale factor into a fifth-order polynomial, an assumption that can be further generalized to any order. The present expansion rates h, q0, r0, etc., are evaluated by computing the marginal likelihoods for these parameters. These values are relevant since any cosmological solution would ultimately need to explain them.
94 citations
TL;DR: In this article, the authors used the supernova data to explore the property of dark energy by some model independent methods and found that the transition redshift from deceleration phase to acceleration phase is 0.3.
Abstract: The nature of dark energy is a mystery to us. This paper uses the supernova data to explore the property of dark energy by some model independent methods. We first Talyor expanded the scale factor $a(t)$ to find out the deceleration parameter $q_0<0$. This result just invokes the Robertson-Walker metric. Then we discuss several different parameterizations used in the literature. We find that $\Omega_{\rm DE0}$ is almost less than -1 at $1\sigma$ level. We also find that the transition redshift from deceleration phase to acceleration phase is $z_{\rm T}\sim 0.3$.
91 citations
TL;DR: In this paper, the authors make a model-independent analysis of SNe data, without reference to the energy densities, by expanding the scale factor into a polynomial of order 5, which assumption can be further generalized to any order.
Abstract: The apparent magnitude-redshift data of SNe Ia call for modifications in the standard model energy densities. Under the circumstance that this modification cannot be limited to the addition of a mere cosmological constant, a serious situation has emerged in cosmology, in which the energy densities in the universe have become largely speculative. In this situation, an equation of state of the form p=w \rho itself is not well-motivated. In this paper, we argue that the reasonable option left is to make a model-independent analysis of SNe data, without reference to the energy densities. In this basically kinematic approach, we limit ourselves to the observationally justifiable assumptions of homogeneity and isotropy; i.e., to the assumption that the universe has a RW metric. This cosmographic approach is historically the original one to cosmology. We perform the analysis by expanding the scale factor into a polynomial of order 5, which assumption can be further generalised to any order. The present expansion rates h, q_0, r_0 etc. are evaluated by computing the marginal likelihoods for these parameters. These values are relevant, since any cosmological solution would ultimately need to explain them.
87 citations
TL;DR: In this article, the viability of f(R) theories in the framework of the Palatini approach as solutions to the problem of the observed accelerated expansion of the universe is investigated.
Abstract: We investigate the viability of f(R) theories in the framework of the Palatini approach as solutions to the problem of the observed accelerated expansion of the universe. Two physically motivated popular choices for f(R) are considered: power law, f(R) = \beta R^n, and logarithmic, f(R) = \alpha \ln{R}. Under the Palatini approach, both Lagrangians give rise to cosmological models comprising only standard matter and undergoing a present phase of accelerated expansion. We use the Hubble diagram of type Ia Supernovae and the data on the gas mass fraction in relaxed galaxy clusters to see whether these models are able to reproduce what is observed and to constrain their parameters. It turns out that they are indeed able to fit the data with values of the Hubble constant and of the matter density parameter in agreement with some model independent estimates, but the today deceleration parameter is higher than what is measured in the concordance LambdaCDM model.
78 citations
TL;DR: In this article, a new class of exact solutions has been obtained by considering a time dependent displacement field for constant deceleration parameter models of the universe, and the physical behaviour of the models is examined in vacuum and in the presence of perfect fluids.
54 citations
TL;DR: In this article, the authors presented the general analytic solution for the evolution of radiative supernova remnants in a uniform interstellar medium, under thin-shell approximation, for a given set of parameters, showing a transient in which the deceleration parameter reaches a maximum value of 0.33.
Abstract: We present the general analytic solution for the evolution of radiative supernova remnants in a uniform interstellar medium, under thin-shell approximation. This approximation is shown to be very accurate approach to this task. For a given set of parameters, our solution closely matches the results of numerical models, showing a transient in which the deceleration parameter reaches a maximum value of 0.33, followed by a slow convergence to the asymptotic value 2/7. Oort (1951) and McKee & Ostriker (1977) analytic solutions are discussed, as special cases of the general solution we have found.
39 citations
TL;DR: In this article, the general solution of the gravitational field equations for a full causal bulk viscous stiff cosmological fluid, with bulk viscosity coefficient proportional to the energy density to the power 1/4, is obtained in the flat Friedmann-Robertson-Walker geometry.
Abstract: The general solution of the gravitational field equations for a full causal bulk viscous stiff cosmological fluid, with bulk viscosity coefficient proportional to the energy density to the power 1/4, is obtained in the flat Friedmann–Robertson–Walker geometry. The solution describes a non-inflationary Universe, which starts its evolution from a singular state. The time variation of the scale factor, deceleration parameter, viscous pressure, viscous pressure-thermodynamic pressure ratio, co-moving entropy and Ricci and Kretschmann invariants is considered in detail.
TL;DR: In this paper, the authors focus on models where the vacuum energy decreases as an inverse power law of the scale factor and show that the transition from decelerated to accelerated cosmic expansion occurs at z approx 0.48 +/- 0.20, which is compatible with SNe data.
Abstract: Braneworld theory provides a natural setting to treat, at a classical level, the cosmological effects of vacuum energy. Non-static extra dimensions can generally lead to a variable vacuum energy, which in turn may explain the present accelerated cosmic expansion. We concentrate our attention in models where the vacuum energy decreases as an inverse power law of the scale factor. These models agree with the observed accelerating universe, while fitting simultaneously the observational data for the density and deceleration parameter. The redshift at which the vacuum energy can start to dominate depends on the mass density of ordinary matter. For Omega = 0.3, the transition from decelerated to accelerated cosmic expansion occurs at z approx 0.48 +/- 0.20, which is compatible with SNe data. We set a lower bound on the deceleration parameter today, namely q > - 1 + 3 Omega/2, i.e., q > - 0.55 for Omega = 0.3. The future evolution of the universe crucially depends on the time when vacuum starts to dominate over ordinary matter. If it dominates only recently, at an epoch z 0.64, then the deceleration comes back and the universe recollapses at some point in the distant future. In the first case, quintessence and Cardassian expansion can be formally interpreted as the low energy limit of our model, although they are entirely different in philosophy. In the second case there is no correspondence between these models and ours.
TL;DR: Anisotropic Bianchi Type-I cosmological models have been studied on the basis of Lyra's geometry as mentioned in this paper, and two types of models, one with constant deceleration parameter and the other with variable decelerance parameter have been derived by considering a time-dependent displacement field.
Abstract: Anisotropic Bianchi Type-I cosmological models have been studied on the basis of Lyra’s geometry. Two types of models, one with constant deceleration parameter and the other with variable deceleration parameter have been derived by considering a time-dependent displacement field.
TL;DR: In this paper, a geometrical interpretation for dark energy as warp in the universe given by the extrinsic curvature is proposed, which is consistent with the presently known observational data on inflation for the deSitter bulk, but not for the anti-deSitter case.
Abstract: Experimental evidence suggests that we live in a spatially flat, accelerating universe composed of roughly one-third of matter (baryonic + dark) and two-thirds of a negative-pressure dark component, generically called dark energy. The presence of such energy not only explains the observed accelerating expansion of the Universe but also provides the remaining piece of information connecting the inflationary flatness prediction with astronomical observations. However, despite of its good observational indications, the nature of the dark energy still remains an open question. In this paper we explore a geometrical explanation for such a component within the context of brane-world theory without mirror symmetry, leading to a geometrical interpretation for dark energy as warp in the universe given by the extrinsic curvature. In particular, we study the phenomenological implications of the extrinsic curvature of a Friedman-Robertson-Walker universe in a five-dimensional constant curvature bulk, with signatures (4,1) or (3,2), as compared with the X-matter (XCDM) model. From the analysis of the geometrically modified Friedman's equations, the deceleration parameter and the Weak Energy Condition, we find a consistent agreement with the presently known observational data on inflation for the deSitter bulk, but not for the anti-deSitter case.
Posted Content•
TL;DR: In this article, the authors describe a method to derive the expansion and acceleration rates directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy.
Abstract: We describe a method to derive the expansion and acceleration rates directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy. If one also specifies a theory of gravity we can also determine the pressure, energy density, and equation of state of the dark energy as functions of redshift. We then apply this methodology on a modern data set of distances to Supernovae and to radio galaxies. We find that the universe transitions from deceleration to acceleration at a redshift of about 0.4, and the present value of deceleration parameter is -0.35 +/- 0.15. The standard ``concordance model'' provides a reasonably good fit to the dimensionless expansion rate as a function of redshift, though it fits the dimensionless acceleration rate as a function of redshift less well. Adopting General Relativity as the theory of gravity, we obtain the redshift trends for the pressure, energy density, and equation of state of the dark energy out to a redshift of about one. They are generally consistent with the concordance model, at least out to a redshift of about 0.5, but the existing data preclude any stronger conclusions at this point.
Posted Content•
TL;DR: In this article, the authors obtain the metric of a charge-tempered cosmological model, a slightly modified Standard Cosmological Model by a small excess of charge density, distributed uniformly in accordance with the Cosmology Principle, the global Coulomb interaction incorporated in this metric.
Abstract: The main purpose of this work is to obtain the metric of a Charge Tempered Cosmological Model, a slightly modified Standard Cosmological Model by a small excess of charge density, distributed uniformly in accordance with the Cosmological Principle, the global Coulomb interaction incorporated in this metric. The particularity of this model is that the commoving observer referential where the metric belongs is non inertial, which consequence is that clocks at different position can not be synchronized. The new metric is constrained to k goint to 0, with dependence on a charge parameter, and related to a modified Friedmann equation, but it is constrained to a positive deceleration parameter and the hyperbolic solution . Nevertheless, there are corrections to do, valid just for a long range distances. For example, the red shift has, now, dependences on the gravitational potential together the recessional motion. In any way, this model accepts as well the cosmological constant and its physical counterpart, the dark energy.
TL;DR: In this paper, the authors proposed a method to determine the expansion rate and the deceleration parameter in a largely model-independent way, directly from the data on coordinate distances, including measurements of the pressure of dark energy, its normalized energy density, and the equation of state parameter as functions of redshift.
Abstract: Understanding of the nature of dark energy, which appears to drive the expansion of the universe, is one of the central problems of physical cosmology today. In an earlier paper [Daly & Djorgovski (2003)] we proposed a novel method to determine the expansion rate and the deceleration parameter in a largely model-independent way, directly from the data on coordinate distances. Here we expand this methodology to include measurements of the pressure of dark energy, its normalized energy density, and the equation of state parameter as functions of redshift. We then apply this methodology to a new, combined data set of distances to supernovae and radio galaxies.
TL;DR: In this article, the authors focus on spatially-flat, homogeneous and isotropic, brane-universes where the matter density decreases as an inverse power of the scale factor, similar (but at different rate) to the power law in FRW-UNiverses of general relativity.
Abstract: In braneworld models a variable vacuum energy may appear if the size of the extra dimension changes during the evolution of the universe. In this scenario the acceleration of the universe is related not only to the variation of the cosmological term, but also to the time evolution of $G$ and, possibly, to the variation of other fundamental "constants" as well. This is because the expansion rate of the extra dimension appears in different contexts, notably in expressions concerning the variation of rest mass and electric charge. We concentrate our attention on spatially-flat, homogeneous and isotropic, brane-universes where the matter density decreases as an inverse power of the scale factor, similar (but at different rate) to the power law in FRW-universes of general relativity.
We show that these braneworld cosmologies are consistent with the observed accelerating universe and other observational requirements. In particular, $G$ becomes constant and $\Lambda_{(4)} \approx const \times H^2$ asymptotically in time. Another important feature is that the models contain no "adjustable" parameters. All the quantities, even the five-dimensional ones, can be evaluated by means of measurements in 4D. We provide precise constrains on the cosmological parameters and demonstrate that the "effective" equation of state of the universe can, in principle, be determined by measurements of the deceleration parameter alone. We give an explicit expression relating the density parameters $\Omega_{\rho}$, $\Omega_{\Lambda}$ and the deceleration parameter $q$. These results constitute concrete predictions that may help in observations for an experimental/observational test of the model.
TL;DR: In this article, the authors used the type Ia supernovae (SNe Ia) observational data to estimate the parameters of a cosmological model with cold dark matter and the generalized Chaplygin gas model (GCGM).
Abstract: The type Ia supernovae (SNe Ia) observational data are used to estimate the parameters of a cosmological model with cold dark matter and the generalized Chaplygin gas model (GCGM). The GCGM depends essentially on five parameters: the Hubble constant, the parameter $\bar{A}$ related to the velocity of the sound, the equation of state parameter $\alpha$, the curvature of the Universe and the fraction density of the generalized Chaplygin gas (or the cold dark matter). The parameter $\alpha$ is allowed to take negative values and to be greater than 1. The Bayesian parameter estimation yields $\alpha = - 0.86^{+6.01}_{-0.15}$, $H_0 = 62.0^{+1.32}_{-1.42} km/Mpc.s$, $\Omega _{k0}=-1.26_{-1.42}^{+1.32}$, $\Omega_{m0} = 0.00^{+0.86}_{-0.00}$, $\Omega_{c0} = 1.39^{+1.21}_{-1.25}$, $\bar A =1.00^{+0.00}_{-0.39}$, $t_0 = 15.3^{+4.2}_{-3.2}$ and $q_0 = -0.80^{+0.86}_{-0.62}$, where $t_0$ is the age of the Universe and $q_0$ is the value of the deceleration parameter today. Our results indicate that a Universe completely dominated by the generalized Chaplygin gas is favoured, what reinforces the idea that the this gas may unify the description for dark matter and dark energy, at least as the SNe Ia data is concerned. A closed and accelerating Universe is also favoured. The traditional Chaplygin gas model (CGM), $\alpha = 1$ is not ruled out, even if it does not give the best-fitting. Particular cases with four or three independent free parameters are also analysed.