Showing papers on "Deceleration parameter published in 2005"
TL;DR: In this paper, the cosmological evolution based on a D-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (a dilaton or compactification modulus).
Abstract: We study the cosmological evolution based on a D-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (a dilaton or compactification modulus). A barotropic perfect fluid coupled to the scalar field is also allowed. Phase space analysis and the stability of asymptotic solutions are performed for a number of models which include (i) a fixed scalar field, (ii) a linear dilaton in the string frame, and (iii) a logarithmic modulus in the Einstein frame. We confront analytical solutions with observational constraints for the deceleration parameter and show that Gauss-Bonnet gravity alone (i.e., with no matter fields) may not explain the current acceleration of the universe. We also study the future evolution of the universe using the Gauss-Bonnet parametrization and find that big rip singularities can be avoided even in the presence of a phantom fluid because of the balance between the fluid and curvature corrections. A non-minimal coupling between the fluid and the modulus field also opens up the interesting possibility of avoiding a big rip regardless of the details of the fluid equation of state.
184 citations
TL;DR: In this paper, it was shown that the acceleration of the universe cannot be explained by large superhorizon fluctuations generated by inflation, and that no acceleration can be produced by this mechanism, while the discrepancy can be traced to higher-order terms that were dropped in the Kolb et al. analysis.
Abstract: We investigate the recent suggestions by Barausse et al. and Kolb et al. that the acceleration of the universe could be explained by large superhorizon fluctuations generated by inflation. We show that no acceleration can be produced by this mechanism. We begin by showing how the application of Raychaudhuri equation to inhomogeneous cosmologies results in several "no go" theorems for accelerated expansion. Next we derive an exact solution for a specific case of initial perturbations, for which application of the Kolb et al. expressions leads to an acceleration, while the exact solution reveals that no acceleration is present. We show that the discrepancy can be traced to higher-order terms that were dropped in the Kolb et al. analysis. We proceed with the analysis of initial value formulation of general relativity to argue that causality severely limits what observable effects can be derived from superhorizon perturbations. By constructing a Riemann normal coordinate system on initial slice we show that no infrared divergence terms arise in this coordinate system. Thus any divergences found previously can be eliminated by a local rescaling of coordinates and are unobservable. We perform an explicit analysis of the variance of the deceleration parameter for the case of single-field inflation using usual coordinates and show that the infrared-divergent terms found by Barausse et al. and Kolb et al. cancel against several additional terms not considered in their analysis. Finally, we argue that introducing isocurvature perturbations does not alter our conclusion that the accelerating expansion of the universe cannot be explained by superhorizon modes.
125 citations
TL;DR: In this article, the authors used supernova data to explore the property of dark energy by some model-independent methods, and Taylor expanded the scale factor a(t) and the luminosity distance dL to the fifth order.
Abstract: This paper uses supernova data to explore the property of dark energy by some model-independent methods. We first Taylor expand the scale factor a(t) and the luminosity distance dL to the fifth order to find that the deceleration parameter q0 < 0. This result just invokes the Robertson–Walker metric. So the conclusion that the universe is expanding with acceleration is more general. Then we discuss several different parametrizations used in the literature. We also propose two modified parametrizations. We find that ωDE0 is less than −1 almost at 1σ level from all the parametrizations used in this paper. We also find that the transition redshift from deceleration phase to acceleration phase is zT ~ 0.3.
107 citations
TL;DR: In this article, the authors consider a cosmology in which a spherically symmetric large scale inhomogeneous enhancement or a void are described by an inhomogenous metric and Einstein's gravitational equations.
Abstract: We consider a cosmology in which a spherically symmetric large scale inhomogeneous enhancement or a void are described by an inhomogeneous metric and Einstein's gravitational equations. For a flat matter dominated universe the inhomogeneous equations lead to luminosity distance and Hubble constant formulae that depend on the location of the observer. For a general inhomogeneous solution, it is possible for the deceleration parameter to differ significantly from the FLRW result. The deceleration parameter q0 can be interpreted as q0 > 0 in a FLRW universe (q0 = 1/2 for a flat matter dominated universe) and q0 < 0 as inferred from the inhomogeneous enhancement that is embedded in a FLRW universe. A spatial volume averaging of local regions in the backward light cone has to be performed for the inhomogeneous solution at late times to decide whether the decelerating parameter q can be negative for a positive energy condition. The CMB temperature fluctuations across the sky can be unevenly distributed in the northern and southern hemispheres in the inhomogeneous matter dominated solution, in agreement with the analysis of the WMAP power spectrum data by several authors. The model can possibly explain the anomalous alignment of the quadrupole and octopole moments observed in the WMAP data.
102 citations
TL;DR: In this paper, the authors propose higher-order energy conditions which relate time derivatives of the energy density and pressure which may be useful in general relativity, and also propose higher order energy conditions for Friedmann cosmology.
99 citations
TL;DR: In this paper, the acceleration of the universe can be explained as the backreaction effect of super-horizon perturbations using second order perturbation theory, and it has been shown that this mechanism is correct for a hypothetical, gedanken universe in which the subhorizon sub-urbation is absent, and that if q{sub 0 is negative then its magnitude is constrained to be less than or of the order of the square of the peculiar velocity on Hubble scales today.
Abstract: It has recently been suggested that the acceleration of the Universe can be explained as the backreaction effect of superhorizon perturbations using second order perturbation theory. If this mechanism is correct, it should also apply to a hypothetical, gedanken universe in which the subhorizon perturbations are absent. In such a gedanken universe it is possible to compute the deceleration parameter q{sub 0} measured by comoving observers using local covariant Taylor expansions rather than using second order perturbation theory. The result indicates that second order corrections to q{sub 0} are present, but shows that if q{sub 0} is negative then its magnitude is constrained to be less than or of the order of the square of the peculiar velocity on Hubble scales today. We argue that, since this quantity is constrained by observations to be small compared to unity, superhorizon perturbations cannot be responsible for the acceleration of the Universe.
85 citations
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TL;DR: Properties of Braneworld models of dark energy are reviewed in this article, showing that the effective equation of state can be w −1 and the deceleration parameter and all invariants of the Riemann tensor diverge to infinity within a finite interval of cosmic time.
Abstract: Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w -1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future `Big Rip' singularity which is usually encountered by Phantom models. (ii) For a class of Braneworld models the acceleration of the universe can be a transient phenomenon. In this case the current acceleration of the universe is sandwiched between two matter dominated epochs. Such a braneworld does not have a horizon in contrast to LCDM and most Quintessence models. (iii) For a specific set of parameter values the universe can either originate from, or end its existence in a Quiescent singularity, at which the density, pressure and Hubble parameter remain finite, while the deceleration parameter and all invariants of the Riemann tensor diverge to infinity within a finite interval of cosmic time. (iv) Braneworld models of dark energy can loiter at high redshifts: $6 \lleq z \lleq 40$. The Hubble parameter decreasesduring the loitering epoch relative to its value in LCDM. As a result the age of the universe at loitering dramatically increases and this is expected to boost the formation of high redshift gravitationally bound systems such as $10^9 M_\odot$ black holes at $z \sim 6$ and lower-mass black holes and/or Population III stars at $z > 10$, whose existence could be problematic within the LCDM scenario. (v) Braneworld models with a time-like extra dimension bounce at early times thereby avoiding the initial `Big Bang singularity'. (vi) Both Inflation and Dark Energy can be successfully unified within a single scheme (Quintessential Inflation).
76 citations
TL;DR: In this paper, the authors investigated the quintessence model with a minimally coupled scalar field in the context of recent supernovae observations and showed that the early matter dominated model expands with q = 1/2 as desired and enters a negative q phase quite late during the evolution.
Abstract: In this paper, we investigate the quintessence model with a minimally coupled scalar field in the context of recent supernovae observations. By choosing a particular form of the deceleration parameter q, which gives an early deceleration and late time acceleration for the dust dominated model, we show that this sign flip in q can be obtained by a simple trigonometric potential. The early matter dominated model expands with q = 1/2 as desired and enters a negative q phase quite late during the evolution.
66 citations
TL;DR: In this article, the authors show that a bias is present for models which are very far from the so-called concordance model, and a simple kinematical analysis can lead to wrong conclusions.
Abstract: Supernovae searches have shown that a simple matter-dominated and decelerating universe should be ruled out. However, a determination of the present deceleration parameter ${q}_{0}$ through a simple kinematical description is not exempt of possible drawbacks. We show that, with a time dependent equation of state for the dark energy, a bias is present for ${q}_{0}$: models which are very far from the so-called concordance model can be accommodated by the data, and a simple kinematical analysis can lead to wrong conclusions. We present a quantitative treatment of this bias and we present our conclusions when a possible dynamical dark energy is taken into account.
62 citations
TL;DR: In this paper, the generalized Chaplygin gas model (GCGM) contains five free parameters that must be constrained using the different observational data, which are: the Hubble constant H0, the parameter related to the sound velocity, the equation of state parameter?, the curvature parameter?k0 and the Chapleygin gas density parameter?c0.
Abstract: The generalized Chaplygin gas model (GCGM) contains five free parameters that must be constrained using the different observational data. These parameters are: the Hubble constant H0, the parameter related to the sound velocity, the equation of state parameter ?, the curvature parameter ?k0 and the Chaplygin gas density parameter ?c0. The pressureless matter parameter ?m0 may be obtained as a dependent quantity. Here, these parameters are constrained through the type Ia supernovae data. The 'gold sample' of 157 supernovae data is used. Negative and large positive values for ? are taken into account. The analysis is made by employing the Bayesian statistics and the prediction for each parameter is obtained by marginalizing on the remaining ones. This procedure leads to the following predictions: ? = ?0.75+4.04?0.24, H0 = 65.00+1.77?1.75, ?k0 = ?0.77+1.14?5.94, ?m0 = 0.00+1.95?0.00, ?c0 = 1.36+5.36?0.85, ? = 1.000+0.000-0.534. Through the same analysis the specific case of the ordinary Chaplygin gas model (CGM), for which ? = 1, is studied. In this case, there are now four free parameters and the predictions for them are: H0 = 65.01+1.81?1.71, ?k0 = ?2.73+1.53?0.97, ?m0 = 0.00+1.22?0.00, ?c0 = 1.34+0.94?0.70, ? = 1.000+0.000-0.270. To complete the analysis the ?CDM, with its three free parameters, is considered. For all these models, particular cases are considered where one or two parameters are fixed. The age of the universe, the deceleration parameter and the moment the universe begins to accelerate are also evaluated. The quartessence scenario, that unifies the description for dark matter and dark energy, is favoured. A closed (and in some cases a flat) and accelerating universe is also preferred. The CGM case ? = 1 is far from been ruled out, and it is even preferred in some particular cases. In most of the cases the ?CDM is disfavoured with respect to GCGM and CGM.
56 citations
TL;DR: In this article, the generalized Chaplygin gas was used as a matter source for an anisotropic brane with Bianchi type I geometry, and the generalized field equations were expressed in exact analytical form.
Abstract: We present exact solutions of the gravitational field equations in the generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I geometry, with a generalized Chaplygin gas as matter source. The generalized Chaplygin gas, which interpolates between a high density relativistic era and a nonrelativistic matter phase, is a popular dark energy candidate. For a Bianchi type I space-time brane filled with a cosmological fluid obeying the generalized Chaplygin equation of state the general solution of the gravitational field equations can be expressed in an exact parametric form, with the comoving volume taken as parameter. In the limiting cases of a stiff cosmological fluid, with pressure equal to the energy density, and for a pressureless fluid, the solution of the field equations can be expressed in an exact analytical form. The evolution of the scalar field associated to the Chaplygin fluid is also considered and the corresponding potential is obtained. The behavior of the observationally important parameters like shear, anisotropy, and deceleration parameter is considered in detail.
TL;DR: In this article, the authors studied cosmological models with negative constant deceleration parameter within the framework of Lyra geometry and showed that these models can be used to model the universe.
Abstract: Bermann [Nuovo Cimento B (1983), 74, 182] presented a law of variation of Hubble’s parameter that yields constant deceleration parameter models of the Universe. In this paper, we study some cosmological models with negative constant deceleration parameter within the framework of Lyra geometry.
TL;DR: In this article, the generalized Chaplygin gas model (GCGM) and the CGM with five free parameters are analyzed and the analysis is made by employing the Bayesian statistics and the prediction for each parameter is obtained by marginalizing on the remained ones.
Abstract: The generalized Chaplygin gas model (GCGM) contains 5 free parameters, here, they are constrained through the type Ia supernovae data, i.e., the ``gold sample'' of 157 supernovae data. Negative and large positive values for $\alpha$ are taken into account. The analysis is made by employing the Bayesian statistics and the prediction for each parameter is obtained by marginalizing on the remained ones. This procedure leads to the following predictions: $\alpha = - 0.75^{+4.04}_{-0.24}$, $H_0=65.00^{+1.77}_{-1.75}$, $\Omega_{k0} = - 0.77^{+1.14}_{-5.94}$, $\Omega_{m0} = 0.00^{+1.95}_{-0.00}$, $\Omega_{c0} = 1.36^{+5.36}_{-0.85}$, $\bar A = 1.000^{+0.000}_{-0.534}$. Through the same analysis the specific case of the ordinary Chaplygin gas model (CGM), for which $\alpha = 1$, is studied. In this case, there are now four free parameters and the predictions for them are: $H_0 = 65.01^{+1.81}_{-1.71}$, $\Omega_{k0} = - 2.73^{+1.53}_{-0.97}$, $\Omega_{m0} = 0.00^{+1.22}_{-0.00}$, $\Omega_{c0} = 1.34^{+0.94}_{-0.70}$, $\bar A = 1.000^{+0.000}_{-0.270}$. To complete the analysis the $\Lambda$CDM, with its three free parameters, is considered. For all these models, particular cases are considered where one or two parameters are fixed. The age of the Universe, the deceleration parameter and the moment the Universe begins to accelerate are also evaluated. The quartessence scenario, is favoured. A closed (and in some cases a flat) and accelerating Universe is also preferred. The CGM case $\alpha = 1$ is far from been ruled out, and it is even preferred in some particular cases. In most of the cases the $\Lambda$CDM is disfavoured with respect to GCGM and CGM.
TL;DR: In this paper, the authors used the type Ia supernovae (SNeIa) 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 (SNeIa) 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 α, the curvature of the Universe and the fraction density of the generalized Chaplygin gas (or the cold dark matter). The parameter α is allowed to take negative values and to be greater than one. The Bayesian parameter estimation yields $\alpha = - 0.86^{+6.01}_{-0.15}$, $H_0 = 62.0^{+1.32}_{-1.42} \ {\rm km/Mpc.s}$, $\Omega_{k0} = -0.74_{-1.32}^{+1.42}$, $\Omega_{m0} = 0.00^{+0.86}_{-0.00}$, $\Omega_{c0} = 1.39^{+1.21}_{-1.25}$, $\bar A =1.00^{+0.00}_{-0.39}$, $t_0 = 15.3^{+4.2}_{-3.2}$ and $q_0 = -0.80^{+0.86}_{-0.62}$, where t0 is the age of the Universe and q0 is the value of the deceleration parameter today. Our results indicate that a Universe completely dominat...
TL;DR: In this article, the FRW line element can be re-invented on a dynamical four-dimensional hypersurface, which is not orthogonal to the extra dimension, without any internal contradiction.
Abstract: In braneworld models, Space-Time-Matter and other Kaluza-Klein theories, our spacetime is devised as a four-dimensional hypersurface {\it 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 {\it 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 later contains higher-dimensional modifications to the regular matter density and pressure in 4D. We compare the evolution of the spacetime in these two interpretations. We 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.
TL;DR: In this article, exact solutions for an isotropic homogeneous universe with a bulk viscous fluid in the cosmological theory based on Lyra's geometry are obtained for the case where the viscosity coefficient of the viscous liquid is assumed to be a power function of the mass density.
Abstract: Exact solutions are obtained for an isotropic homogeneous universe with a bulk viscous fluid in the cosmological theory based on Lyra’s geometry. The viscosity coefficient of the bulk viscous fluid is assumed to be a power function of the mass density. Cosmological models with time dependent displacement field have been discussed for a constant value of the deceleration parameter. Finally some possibilities of further problems and their investigations have been pointed out.
TL;DR: In this article, the equivalence of formally different gradient expansions is demonstrated, and the deceleration parameter is always positive semi-definite, if the barotropic index vanishes.
Abstract: Motivated by recent claims stating that the acceleration of the present Universe is due to fluctuations with wavelength larger than the Hubble radius, we present a general analysis of various perturbative solutions of fully inhomogeneous Einstein equations supplemented by a perfect fluid. The equivalence of formally different gradient expansions is demonstrated. If the barotropic index vanishes, the deceleration parameter is always positive semi-definite.
TL;DR: In this article, the acceleration of the universe as a consequence of the time evolution of the vacuum energy in cosmological models based in braneworld theories in 5D was studied.
Abstract: We study the acceleration of the universe as a consequence of the time evolution of the vacuum energy in cosmological models based in braneworld theories in 5D. 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 dimensionappears 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 \({\rm \Lambda}_{(4)} \approx const \times H^2\) 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 \({\rm \Omega}_{\rho}\), \({\rm \Omega}_{\rm \Lambda}\) 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 paper, a spatial volume averaging of physical quantities is introduced and the averaged time evolution expansion parameter in the Raychoudhuri equation can give rise in the late-time universe to a volume averaged deceleration parameter $ $ that is negative for a positive matter density.
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 $\delta\rho/\rho\sim 10^{-5}$. A spatial volume averaging of physical quantities is introduced and the averaged time evolution expansion parameter $\theta$ in the Raychoudhuri 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 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.
TL;DR: In this paper, the authors consider a cosmology in which a spherically symmetric large scale inhomogeneous enhancement or a void are described by an inhomogenous metric and Einstein's gravitational equations.
Abstract: We consider a cosmology in which a spherically symmetric large scale inhomogeneous enhancement or a void are described by an inhomogeneous metric and Einstein's gravitational equations. For a flat matter dominated universe the inhomogeneous equations lead to luminosity distance and Hubble constant formulas that depend on the location of the observer. For a general inhomogeneous solution, it is possible for the deceleration parameter to differ significantly from the FLRW result. The deceleration parameter $q_0$ can be interpreted as $q_0 > 0$ ($q_0=1/2$ for a flat matter dominated universe) in a FLRW universe and be $q_0 < 0$ as inferred from the inhomogeneous enhancement that is embedded in a FLRW universe. A spatial volume averaging of local regions in the backward light cone has to be performed for the inhomogeneous solution at late times to decide whether the decelerating parameter $q$ can be negative for a positive energy condition. The CMB temperature fluctuations across the sky can be unevenly distributed in the northern and southern hemispheres in the inhomogeneous matter dominated solution, in agreement with the analysis of the WMAP power spectrum data by several authors. The model can possibly explain the anomalous alignment of the quadrupole and octopole moments observed in the WMAP data.
TL;DR: In this paper, the cosmological evolution of the universe was studied based on low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (dilaton or compactification modulus).
Abstract: We study the cosmological evolution based upon a $D$-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (dilaton or compactification modulus). A barotropic perfect fluid coupled to the scalar field is also allowed. Phase space analysis and the stability of asymptotic solutions are performed for a number of models which include ($i$) fixed scalar field, ($ii$) linear dilaton in string frame, and ($iii$) logarithmic modulus in Einstein frame. We confront analytical solutions with observational constraints for deceleration parameter and show that Gauss-Bonnet gravity (with no matter fields) may not explain the current acceleration of the universe. We also study the future evolution of the universe using the GB parametrization and find that big rip singularities can be avoided even in the presence of a phantom fluid because of the balance between the fluid and curvature corrections. A non-minimal coupling between the fluid and the modulus field also opens up the interesting possibility to avoid big rip regardless of the details of the fluid equation of state.
TL;DR: In this article, the authors present a simple higher dimensional FRW type of model where the acceleration is apparently caused by the presence of the extra dimensions and show the desirable feature of dimensional reduction as well as reasonably good physical properties of matter.
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 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.
TL;DR: In this article, the cosmological deceleration parameter Q_0 is given by the square of the string coupling, g_s^2, up to a negative sign, and it is shown that the expansion of the universe must accelerate eventually, and the observed value of q_0 coresponds to g s^2 ~ 0.6.
Abstract: Generic cosmological models in non-critical string theory have a time-dependent dilaton background at a late epoch. The cosmological deceleration parameter Q_0 is given by the square of the string coupling, g_s^2, up to a negative sign. Hence the expansion of the Universe must accelerate eventually, and the observed value of Q_0 coresponds to g_s^2 ~ 0.6. In this scenario, the string coupling is asymptotically free at large times, but its present rate of change is imperceptibly small.
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TL;DR: In this article, the authors describe the universe as a local, inhomogeneous spherical bubble embedded in a flat matter dominated FLRW universe and derive a nonperturbative expression for the deceleration parameter q that can possibly describe the acceleration of the universe without dark energy, due to the effects associated with very long wave length super-horizon inflationary perturbations.
Abstract: We describe the universe as a local, inhomogeneous spherical bubble embedded in a flat matter dominated FLRW universe. Generalized exact Friedmann equations describe the expansion of the universe and an early universe inflationary de Sitter solution is obtained. A non-perturbative expression for the deceleration parameter q is derived that can possibly describe the acceleration of the universe without dark energy, due to the effects associated with very long wave length super-horizon inflationary perturbations. The suggestion by Kolbe et al. [9] that long wave length super-horizon inflationary modes can affect a local observable through inhomogeneities is considered in the light of our exact inhomogeneous model.
TL;DR: In this paper, the evolution of a universe consisted of a scalar field, a dark matter field and non-interacting baryonic matter and radiation is investigated, which can reproduce the expected behavior of the density parameters, deceleration parameter and luminosity distance.
Abstract: In this work we investigate the evolution of a Universe consisted of a scalar field, a dark matter field and non-interacting baryonic matter and radiation. The scalar field, which plays the role of dark energy, is non-minimally coupled to space-time curvature, and drives the Universe to a present accelerated expansion. The non-relativistic dark matter field interacts directly with the dark energy and has a pressure which follows from a thermodynamic theory. We show that this model can reproduce the expected behavior of the density parameters, deceleration parameter and luminosity distance
TL;DR: In this paper, an anisotropic model of the universe with constant energy per particle was studied and a decaying cosmological constant and particle production in an adiabatic process were considered as the sources for the entropy.
Abstract: Here we study an anisotropic model of the universe with constant energy per particle. A decaying cosmological constant and particle production in an adiabatic process are considered as the sources for the entropy. The statefinder parameters {r, s} are defined and their behavior are analyzed graphically in some cases.
TL;DR: In this article, the cosmological deceleration parameter q0 is given by the square of the string coupling, up to a negative sign, and the observed value of q0 corresponds to.
Abstract: Generic cosmological models in non-critical string theory have a time-dependent dilaton background at a late epoch. The cosmological deceleration parameter q0 is given by the square of the string coupling, , up to a negative sign. Hence, the expansion of the universe must accelerate eventually, and the observed value of q0 corresponds to . In this scenario, the string coupling is asymptotically free at large times, but its present rate of change is imperceptibly small.
TL;DR: In this paper, the equivalence of formally different gradient expansions is demonstrated, and the deceleration parameter is always positive semi-definite, if the barotropic index vanishes.
Abstract: Motivated by recent claims stating that the acceleration of the present Universe is due to fluctuations with wavelength larger than the Hubble radius, we present a general analysis of various perturbative solutions of fully inhomogeneous Einstein equations supplemented by a perfect fluid. The equivalence of formally different gradient expansions is demonstrated. If the barotropic index vanishes, the deceleration parameter is always positive semi-definite.
02 Feb 2005
TL;DR: Properties of Braneworld models of dark energy are reviewed in this paper, showing that the effective equation of state can be w −1 and the deceleration parameter and all invariants of the Riemann tensor diverge to infinity within a finite interval of cosmic time.
Abstract: Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w -1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future `Big Rip' singularity which is usually encountered by Phantom models. (ii) For a class of Braneworld models the acceleration of the universe can be a transient phenomenon. In this case the current acceleration of the universe is sandwiched between two matter dominated epochs. Such a braneworld does not have a horizon in contrast to LCDM and most Quintessence models. (iii) For a specific set of parameter values the universe can either originate from, or end its existence in a Quiescent singularity, at which the density, pressure and Hubble parameter remain finite, while the deceleration parameter and all invariants of the Riemann tensor diverge to infinity within a finite interval of cosmic time. (iv) Braneworld models of dark energy can loiter at high redshifts: $6 \lleq z \lleq 40$. The Hubble parameter decreasesduring the loitering epoch relative to its value in LCDM. As a result the age of the universe at loitering dramatically increases and this is expected to boost the formation of high redshift gravitationally bound systems such as $10^9 M_\odot$ black holes at $z \sim 6$ and lower-mass black holes and/or Population III stars at $z > 10$, whose existence could be problematic within the LCDM scenario. (v) Braneworld models with a time-like extra dimension bounce at early times thereby avoiding the initial `Big Bang singularity'. (vi) Both Inflation and Dark Energy can be successfully unified within a single scheme (Quintessential Inflation).
TL;DR: In this paper, the presence of dark energy has been established by including a time-dependent $\Lambda$ term in the Einstein's field equations, which is compatible with the idea of an accelerating universe so far as the value of the deceleration parameter is concerned.
Abstract: Dark matter, the major component of the matter content of the Universe, played a significant role at early stages during structure formation. But at present the Universe is dark energy dominated as well as accelerating. Here, the presence of dark energy has been established by including a time-dependent $\Lambda$ term in the Einstein's field equations. This model is compatible with the idea of an accelerating Universe so far as the value of the deceleration parameter is concerned. Possibility of a change in sign of the deceleration parameter is also discussed. The impact of considering the speed of light as variable in the field equations has also been investigated by using a well known time-dependent $\Lambda$ model.