Showing papers on "Deceleration parameter published in 2006"

Mimicking dark energy with Lemaitre-Tolman-Bondi models: Weak central singularities and critical points

Abstract: There has been much debate over whether or not one could explain the observed acceleration of the Universe with inhomogeneous cosmological models, such as the spherically-symmetric Lema\^{\i}tre-Tolman-Bondi (LTB) models. It has been claimed that the central observer in these models can observe a local acceleration, which would contradict general theorems. We resolve the contradiction by noting that many of the models that have been explored contain a weak singularity at the location of the observer which makes them unphysical. In the absence of this singularity, we show that LTB models must have a positive central deceleration parameter ${q}_{0}$, in agreement with the general theorems. We also show that it is possible to achieve a negative apparent deceleration parameter at nonzero redshifts in LTB models that do not contain this singularity. However, we find other singularities that tend to arise in LTB models when attempting to match luminosity distance data, and these generally limit the range of redshifts for which these models can mimic observations of an accelerating Universe. Exceptional models do exist that can extend to arbitrarily large redshift without encountering these pathologies, and we show how these may be constructed. These special models exhibit regions with negative effective equation of state parameter, which may fall below negative one, but we have failed to find any singularity-free models that agree with observations. Moreover, models based on dust-filled LTB metrics probably fail to reproduce observed properties of large scale structure.

Interacting Chaplygin gas

Abstract: We investigate a kind of interacting Chaplygin gas model in which the Chaplygin gas plays the role of dark energy and interacts with cold dark matter particles. We find that there exists a stable scaling solution at late times with the Universe evolving into a phase of steady state. Furthermore, the effective equation of state of Chaplygin gas may cross the so-called phantom divide $w=\ensuremath{-}1$. The above results are derived from continuity equations, which means that they are independent of any gravity theories. Assuming standard general relativity and a spatially flat Friedamnn-Robertson-Walker (FRW) metric, we also find the deceleration parameter is well consistent with current observations.

Late-time inhomogeneity and acceleration without dark energy

Abstract: The inhomogeneous distribution of matter in the non-linear regime of galaxies, clusters of galaxies and voids is described by an exact, spherically symmetric inhomogeneous solution of Einstein's gravitational field equations, corresponding to an under-dense void. The solution becomes the homogeneous and isotropic Einstein–de Sitter solution for a red shift z > 10–20, which describes the matter dominated CMB data with small inhomogeneities δρ/ρ ~ 10−5. A spatial volume averaging of physical quantities is introduced and the averaged time evolution expansion parameter θ in the Raychaudhuri equation can give rise in the late-time universe to a volume averaged deceleration parameter that is negative for a positive matter density. This allows for a region of accelerated expansion which does not require a negative pressure dark energy or a cosmological constant. A negative deceleration parameter can be derived by this volume averaging procedure from the Lemaitre–Tolman–Bondi open void solution, which describes the late-time non-linear regime associated with galaxies and under-dense voids and solves the 'coincidence' problem.

A holographic model of dark energy and the thermodynamics of a non-flat accelerated expanding universe

Abstract: Motivated by recent results on non-vanishing spatial curvature [1] we employ a holographic model of dark energy to investigate the validity of the first and second laws of thermodynamics in a non-flat (closed) universe enclosed by the apparent horizon RA and the event horizon measured from the sphere of the horizon named L. We show that for the apparent horizon the first law is roughly respected for different epochs while the second law of thermodynamics is respected, while for L as the system's IR cut-off the first law is broken and the second law is respected for the special range of the deceleration parameter. It is also shown that for the late-time universe L is equal to RA and the thermodynamic laws hold when the universe has non-vanishing curvature. Defining the fluid temperature as being proportional to the horizon temperature the range for the coefficient of proportionality is obtained, provided that the generalized second law of thermodynamics holds.

Observational constraints on the acceleration of the Universe

Abstract: We propose a new parametrization of the deceleration parameter to study its time-variation behavior. The advantage of parameterizing the deceleration parameter is that we do not need to assume any underlying theory of gravity. By fitting the model to the 157 gold sample supernova Ia data, we find strong evidence that the Universe is currently accelerating and it accelerated in the past. By fitting the model to the 115 nearby and Supernova Legacy Survey supernova Ia data, the evidence that the Universe is currently accelerating is weak, although there is still a strong evidence that the Universe once accelerated in the past. The results obtained from the 157 gold sample supernova Ia data and those from the 115 supernova Ia data are not directly comparable because the two different data sets measure the luminosity distance up to different redshifts. We then use the Friedmann equation and a dark energy parametrization to discuss the same problem. When we fit the model to the supernova Ia data alone, we find weak evidence that the Universe is accelerating and the current matter density is higher than that measured from other experiments. After we add the Sloan Digital Sky Survey data to constrain themore » dark energy model, we find that the behavior of the deceleration parameter is almost the same as that obtained from parameterizing the deceleration parameter.« less

The Holographic Model of Dark Energy and Thermodynamics of Non-Flat Accelerated Expanding Universe

Abstract: Motivated by recent results on non-vanishing spatial curvature \cite{curve} we employ the holographic model of dark energy to investigate the validity of first and second laws of thermodynamics in non-flat (closed) universe enclosed by apparent horizon $R_A$ and the event horizon measured from the sphere of horizon named $L$. We show that for the apparent horizon the first law is roughly respected for different epochs while the second laws of thermodynamics is respected while for $L$ as the system's IR cut-off first law is broken down and second law is respected for special range of deceleration parameter. It is also shown that at late-time universe $L$ is equal to $R_A$ and the thermodynamic laws are hold, when the universe has non-vanishing curvature. Defining the fluid temperature to be proportional to horizon temperature the range for coefficient of proportionality is obtained provided that the generalized second law of thermodynamics is hold.

f( R) theories of gravity in the Palatini approach matched with observations

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) = β Rn, and logarithmic, f(R) = α 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 ΛCDM model.

Bianchi type-ii cosmological models with constant deceleration parameter

Abstract: A special law of variation for Hubble's parameter in anisotropic space–time models that yields a constant value of the deceleration parameter is presented. Also, a spatially homogeneous and anisotropic but locally rotationally symmetric (LRS) Bianchi type-II cosmological model is studied with a perfect fluid and constant deceleration parameter. Assuming the equation of state p = γρ, where 0≤γ≤1, and using a special law of variation for the Hubble parameter, we are able to construct many new solutions to Einstein's field equations of LRS Bianchi type-II for four different physical models (dust, radiation, Zel'dovich and vacuum). We discuss the solutions with power-law and exponential expansion and examine a particular class of models. A detailed study of kinematic, geometrical and observational properties is carried out.

Bayesian analysis of Friedmannless cosmologies

Abstract: Assuming a homogeneous and isotropic universe and using both the 'Gold' Supernova Type Ia sample of Riess and co-workers and the results from the Supernova Legacy Survey, we carry out a critical investigation of how much the data reveal about the kinematics of the cosmic expansion history, based on the Bayesian marginal likelihood. We consider both spatially flat and curved models. Our results show that although there is strong evidence in the data for an accelerating universe, there is as yet no model-independent evidence for a deceleration parameter that varies with redshift.

Power spectrum of the SDSS luminous red galaxies: constraints on cosmological parameters

Abstract: In this paper we determine the constraints on cosmological parameters using the CMB data from the Wmap experiment together with the recent power spectrum measurement of the SDSS Luminous Red Galaxies (LRGs). Specifically, we focus on spatially flat, low matter density models with adiabatic Gaussian initial conditions. The spatial flatness is achieved with an additional quintessence component whose equation of state parameter $w_\mathrm{eff}$ is taken to be independent of redshift. Throughout most of the paper we do not allow any massive neutrino contribution and also the influence of the gravitational waves on the CMB is taken to be negligible. The analysis is carried out separately for two cases: (i) using the acoustic scale measurements as presented in Hutsi (2006, A&A, 449, 891), (ii) using the full SDSS LRG power spectrum and its covariance matrix. We are able to obtain a very tight constraint on the Hubble constant: $H_0 = 70.8 ^{+2.1}_{-2.0}$ km s -1 Mpc -1 , which helps in breaking several degeneracies between the parameters and allows us to determine the low redshift expansion law with much higher accuracy than available from the Wmap + HST data alone. The positive deceleration parameter q 0 is found to be ruled out at $5.5\sigma$ confidence level. Finally, we extend our analysis by investigating the effects of relaxing the assumption of spatial flatness and also allow for a contribution from massive neutrinos.

Power spectrum of the SDSS luminous red galaxies: constraints on cosmological parameters

Abstract: In this paper we determine the constraints on cosmological parameters using the CMB data from the WMAP experiment together with the recent power spectrum measurement of the SDSS Luminous Red Galaxies (LRGs). Specifically, we focus on spatially flat, low matter density models with adiabatic Gaussian initial conditions. The spatial flatness is achieved with an additional quintessence component whose equation of state parameter w_eff is taken to be independent of redshift. Throughout most of the paper we do not allow any massive neutrino contribution and also the influence of the gravitational waves on the CMB is taken to be negligible. The analysis is carried out separately for two cases: (i) using the acoustic scale measurements as presented in Hutsi (2006), (ii) using the full SDSS LRG power spectrum and its covariance matrix. We are able to obtain a very tight constraint on the Hubble constant: H_0 = 70.8 ^{+2.1}_{-2.0} km/s/Mpc, which helps in breaking several degeneracies between the parameters and allows us to determine the low redshift expansion law with much higher accuracy than available from the WMAP + HST data alone. The positive deceleration parameter q_0 is found to be ruled out at 5.5 \sigma confidence level. Finally, we extend our analysis by investigating the effects of relaxing the assumption of spatial flatness and also allow for a contribution from massive neutrinos.

Cosmological evolution of a torsion-induced quintaxion

Abstract: In an affine prolongation of general relativity, the minimal coupling of Dirac fields to gravity naturally provides an axial current interaction. We demonstrate that the cancellation of the translational curvature, i.e. torsion, in the chiral anomaly induces a dynamical axion coupled with gravitational strength. Because of a geometrical identity, our torsion-induced pseudoscalar couples to the Einstein equations with an effective energy-momentum tensor which automatically satisfies the quintessence condition w<-1/3 for the equation of state parameter. In a toy model of an axion-dominated Universe, this leads to an anharmonic oscillatory evolution for which the deceleration parameter is within the range of current observations.

A Cosmological Model with Negative Constant Deceleration Parameter in a Scalar-Tensor Theory

Abstract: With the help of a special law of variation for Hubble's parameter presented by Bermann [Nuovo Cimento B (1983), 74, 182], a cosmological model with negative constant deceleration parameter is obtained in the framework of Saez-Ballester [Phys. Lett (1985), Al 13, 467] scalar -- tensor theory of gravitation. Some physical and kinematical properties of the model are, also, discussed.

The present universe in the Einstein frame, metric-affine R+1/R gravity

Abstract: We study the present, flat isotropic universe in 1/R-modified gravity. We use the Palatini (metric-affine) variational principle and the Einstein (metric-compatible connected) conformal frame. We show that the energy density scaling deviates from the usual scaling for nonrelativistic matter, and the largest deviation occurs in the present epoch. We find that the current deceleration parameter derived from the apparent matter density parameter is consistent with observations. There is also a small overlap between the predicted and observed values for the redshift derivative of the deceleration parameter. The predicted redshift of the deceleration-to-acceleration transition agrees with that in the ΛCDM model but it is larger than the value estimated from SNIa observations.

Reinventing spacetime on a dynamical hypersurface

Abstract: In braneworld models, Spacetime-Matter and other Kaluza–Klein theories, our spacetime is devised as a four-dimensional hypersurface orthogonal to the extra dimension in a five-dimensional bulk We show that the FRW line element can be "reinvented" on a dynamical four-dimensional hypersurface, which is not orthogonal to the extra dimension, without any internal contradiction This hypersurface is selected by the requirement of continuity of the metric and depends explicitly on the evolution of the extra dimension The main difference between the "conventional" FRW, on an orthogonal hypersurface, and the new one is that the latter contains higher-dimensional modifications to the regular matter density and pressure in 4D We compare the evolution of the spacetime in these two interpretations and find that a wealth of "new" physics can be derived from a five-dimensional metric if it is interpreted on a dynamical (non-orthogonal) 4D hypersurface In particular, in the context of a well-known cosmological metric in 5D, we construct a FRW model which is consistent with the late accelerated expansion of the universe, while fitting simultaneously the observational data for the deceleration parameter The model predicts an effective equation of state for the universe, which is consistent with observations

An Analytic model for the transition from decelerated to accelerated cosmic expansion

Abstract: We consider the scenario where our observable universe is devised as a dynamical four-dimensional hypersurface embedded in a five-dimensional bulk space–time, with a large extra dimension, which is the generalization of the flat FRW cosmological metric to five dimensions. This scenario generates a simple analytical model where different stages of the evolution of the universe are approximated by distinct parametrizations of the same space–time. In this model the evolution from decelerated to accelerated expansion can be interpreted as a "first-order" phase transition between two successive stages. The dominant energy condition allows different parts of the universe to evolve, from deceleration to acceleration, at different redshifts within a narrow era. This picture corresponds to the creation of bubbles of new phase, in the middle of the old one, typical of first-order phase transitions. Taking Ωm = 0.3 today, we find that the cross-over from deceleration to acceleration occurs at z ~ 1 - 1.5, regardless of the equation of state in the very early universe. In the case of primordial radiation, the model predicts that the deceleration parameter "jumps" from q ~ +1.5 to q ~ -0.4 at z ~ 1.17. At the present time q = -0.55 and the equation of state of the universe is w = p/ρ ~ -0.7, in agreement with observations and some theoretical predictions.

Universe with Time Dependent Deceleration Parameter and Λ Term in General Relativity

Abstract: A new class of exact solutions of Einstein's field equations with perfect fluid for an LRS Bianchi type-I spacetime is obtained by using a time dependent deceleration parameter. We have obtained a general solution of the field equations from which three models of the universe are derived: exponential, polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame of reference of recent supernovae Ia observations.

Universe With Time Dependent Deceleration Parameter and $\Lambda$ Term in General Relativity

Abstract: A new class of exact solutions of Einstein's field equations with perfect fluid for an LRS Bianchi type-I spacetime is obtained by using a time dependent deceleration parameter. We have obtained a general solution of the field equations from which three models of the universe are derived: exponential, polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame of reference of recent supernovae Ia observations.

Universe evolution in a 5D ricci-flat cosmology

Abstract: We use Wetterich's parametrization equation of state (EOS) of dark energy to a 5D Ricci-flat cosmological solution and we assume that the universe contains three major components: matter, radiation and dark energy. By using the relation between the scale factor and the redshift z, we show that the two arbitrary functions contained in the 5D solution could be solved out analytically in terms of the variable z. Thus the whole 5D solution could be constructed uniquely if the current values of the three density parameters Ωm0, Ωr0, Ωx0, the EOS w0, and the bending parameter b contained in the EOS are all known. Furthermore, we find that all the evolutions of the mass density Ωm, the radiation density Ωr, the dark energy density Ωx, and the deceleration parameter q depend on the bending parameter b sensitively. Therefore it is worthwhile to study observational constraints on the bending parameter b.

A cosmology with variable c

Abstract: A new varying-c cosmological model, constructed using two additional assumptions, which was introduced in our previous work, is briefly reviewed and the dynamic equation of the model is derived distinctly from a semi-Newtonian approach. The results of this model, using a term and an extra energy-momentum tensor, are considered separately. It is shown that the Universe began from a hot Big Bang and expands forever with a constant deceleration parameter regardless of its curvature. Finally, the age, the radius, and the energy content of the Universe are estimated and some discussion about the type of the geometry of the Universe is provided. PACS Nos.: 98.80.Bp, 98.80.Jk

Transition from decelerated to accelerated cosmic expansion in braneworld universes

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 $$\bar \Omega$$ m = 0.3, the transition from decelerated to accelerated cosmic expansion occurs at z T ≈ 0.48 ± 0.20, which is compatible with SNe data. We set a lower bound on the deceleration parameter today, namely $$\bar{q}$$ > − 1 + 3 $$\bar \Omega$$ m /2, i.e., $$\bar{q}$$ > − 0.55 for $$\bar \Omega $$ m = 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.

Perfect Fluid Lrs Bianchi I With Time Varying Constants

Abstract: It is investigated the behaviour of the “constants” G, c and Λ in the framework of a perfect fluid LRS Bianchi I cosmological model. It has been taken into account the effects of a c-variable into the curvature tensor. Two exact cosmological solutions are investigated, arriving t the conclusion that if q < 0 (deceleration parameter) then G, c are growing functions on time t while Λ is a negative decreasing function on time.

A kinematical approach to dark energy studies

Abstract: We present and employ a new kinematical approach to cosmological `dark energy' studies. We construct models in terms of the dimensionless second and third derivatives of the scale factor a(t) with respect to cosmic time t, namely the present-day value of the deceleration parameter q_0 and the cosmic jerk parameter, j(t). An elegant feature of this parameterization is that all LCDM models have j(t)=1 (constant), which facilitates simple tests for departures from the LCDM paradigm. Applying our model to the three best available sets of redshift-independent distance measurements, from type Ia supernovae and X-ray cluster gas mass fraction measurements, we obtain clear statistical evidence for a late time transition from a decelerating to an accelerating phase. For a flat model with constant jerk, j(t)=j, we measure q_0=-0.81+-0.14 and j=2.16+0.81-0.75, results that are consistent with LCDM at about the 1sigma confidence level. A standard `dynamical' analysis of the same data, employing the Friedmann equations and modeling the dark energy as a fluid with an equation of state parameter, w (constant), gives Omega_m=0.306+0.042-0.040 and w=-1.15+0.14-0.18, also consistent with LCDM at about the 1sigma level. In comparison to dynamical analyses, the kinematical approach uses a different model set and employs a minimum of prior information, being independent of any particular gravity theory. The results obtained with this new approach therefore provide important additional information and we argue that both kinematical and dynamical techniques should be employed in future dark energy studies, where possible. Our results provide further interesting support for the concordance LCDM paradigm.

Accelerating universe from extra spatial dimension

Abstract: We present a simple higher dimensional FRW-type of model where the acceleration is apparently caused by the presence of the extra dimensions. Assuming an ansatz in the form of the deceleration parameter, we get a class of solutions some of which shows the desirable feature of dimensional reduction as well as reasonably good physical properties of matter. Interestingly we do not have to invoke an extraneous scalar field or a cosmological constant to account for this acceleration. One argues that the terms containing the higher dimensional metric coefficients produces an extra negative pressure that apparently drives the inflation of the 4D space with an accelerating phase. It is further found that in line with the physical requirements our model admits of a decelerating phase in the early era along with an accelerating phase at present. Further the models asymptotically mimic a steady-state-type of universe although it starts from a big-bang-type of singularity. Correspondence to Wesson's induced matter theory is also briefly discussed and, in line with it, it is argued that the terms containing the higher dimensional metric coefficients apparently creates a negative pressure which drives the inflation of the 3-space with an accelerating phase.

Bayesian analysis of Friedmannless cosmologies

Abstract: Assuming only a homogeneous and isotropic universe and using both the 'Gold' Supernova Type Ia sample of Riess et al. and the results from the Supernova Legacy Survey, we calculate the Bayesian evidence of a range of different parameterizations of the deceleration parameter. We consider both spatially flat and curved models. Our results show that although there is strong evidence in the data for an accelerating universe, there is little evidence that the deceleration parameter varies with redshift.

Late-time Inhomogeneity and the Acceleration of the Universe

Abstract: The inhomogeneous distribution of matter in the non-linear regime of galaxies, clusters of galaxies and voids is described by an exact, spherically symmetric inhomogeneous solution of Einstein's gravitational field equations, corresponding to an under-dense void. The solution becomes the homogeneous and isotropic Einstein-de Sitter solution for a red shift $z > 10-20$, which describes the matter dominated CMB data with small inhomogeneities. A spatial volume averaging of physical quantities is introduced and the averaged time evolution expansion parameter $\theta$ can give rise in the late-time universe to a volume averaged deceleration parameter $ $ that is negative for a positive matter density. This allows for a region of accelerated expansion which does not require a cosmological constant. A negative deceleration parameter can be derived by this volume averaging procedure from the Lema\^{i}tre-Tolman-Bondi open void solution, which describes the late-time non-linear regime associated with galaxies and under-dense voids and solves the ``coincidence'' problem.

The evolution of generalized chaplygin gas

Abstract: We consider the generalized Chaplygin gas (GCG) proposal for the unification of dark matter and dark energy with p = pde and ρ = ρdm+ρde. The unified equation of state for GCG has been obtained: . On the basis of the function χ(z), some cosmological quantities such as the fractional contributions of different components of the universe Ωi (i respectively denotes baryons, dark matter and dark energy) to the critical density, the equation of state for dark energy ωde, the deceleration parameter q are all obtained, which are consistent with observations. In addition, the transition from deceleration to acceleration is described in our model. We find that the behavior of GCG will be like ΛCDM in the future. So, it has been ruled out in our model that our universe will end up with Big Rip in the future.

Late‐time Inhomogeneity and Acceleration of the Universe

Abstract: The inhomogeneous distribution of matter in the non‐linear regime of galaxies, clusters of galaxies and voids is described by an exact, spherically symmetric inhomogeneous solution of Einstein’s gravitational field equations, corresponding to an under‐dense void. The solution becomes the homogeneous and isotropic Einstein‐de Sitter solution for a red shift z > 10 – 20, which describes the matter dominated CMB data with small inhomogeneities. A spatial volume averaging of physical quantities is introduced and the averaged time evolution expansion parameter θ can give rise in the late‐time universe to a volume averaged deceleration parameter 〈q〉 that is negative for a positive matter density. This allows for a region of accelerated expansion which does not require a cosmological constant. A negative deceleration parameter can be derived by this volume averaging procedure from the Lemaitre‐Tolman‐Bondi open void solution, which describes the late‐time non‐linear regime associated with galaxies and under‐dense voids and solves the “coincidence” problem.

Universe Evolution in a 5D Ricci-flat Cosmology

Abstract: We use Wetterich's parameterization equation of state (EOS) of dark energy to a $5D$ Ricci-flat cosmological solution and we suppose the universe contains three major components: matter, radiation and dark energy. By using the relation between the scale factor and the redshift $z$, we show that the two arbitrary functions contained in the $5D$ solution could be solved out analytically in terms of the variable $z$. Thus the whole $5D$ solution could be constructed uniquely if the current values of the three density parameters $\Omega_{m0}$, ${\Omega_{r0}}$, $\Omega_{x0}$, the EOS $w_{0}$%, and the bending parameter $b$ contained in the EOS are all known. Furthermore, we find that all the evolutions of the mass density $\Omega_{m} $, the radiation density ${\Omega_{r}}$, the dark energy density $\Omega_{x}$, and the deceleration parameter $q$ depend on the bending parameter $b$ sensitively. Therefore it is deserved to study observational constraints on the bending parameter $b$.