Showing papers on "Big Rip published in 2005"

Properties of singularities in the (phantom) dark energy universe

Abstract: The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an equation of state $wl\ensuremath{-}1$ and numerically confirm that this is actually a late-time attractor in the phantom-dominated universe. We also construct a phantom dark energy scenario coupled to dark matter that reproduces singular behaviors of the Big Rip type for the energy density and the curvature of the universe. The effect of quantum corrections coming from conformal anomaly can be important when the curvature grows large, which typically moderates the finite-time singularities.

Inhomogeneous equation of state of the universe: Phantom era, future singularity and crossing the phantom barrier

Abstract: The dark energy universe equation of state (EOS) with inhomogeneous, Hubble parameter dependent term is considered. The motivation to introduce such a term comes from time-dependent viscosity considerations and modifications of general relativity. For several explicit examples of such EOS it is demonstrated how the type of future singularity changes, how the phantom epoch emerges and how the crossing of a phantom barrier occurs. Similar cosmological regimes are considered for the universe with two interacting fluids and for the universe with implicit EOS. For instance, the crossing of the phantom barrier is realized in an easier way, thanks to the presence of inhomogeneous term. The thermodynamical dark energy model is presented where the universe entropy may be positive even at the phantom era as a result of the crossing of the $w=\ensuremath{-}1$ barrier.

Gauss-Bonnet dark energy

Abstract: We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with a Gauss-Bonnet invariant. It is demonstrated that the effective phantom (or quintessence) phase of the late universe may occur in the presence of such a term when the scalar is phantom or for nonzero potential (for canonical scalar). However, with the increase of the curvature, the Gauss-Bonnet term may become dominant so that the phantom phase is transient and the $w=\ensuremath{-}1$ barrier may be passed. Hence, the current acceleration of the universe may be caused by a mixture of scalar phantom and/or potential or stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the big rip occurrence also in phantom cosmology.

Dark energy: Vacuum fluctuations, the effective phantom phase, and holography

Abstract: We aim at the construction of dark energy models without exotic matter but with a phantomlike equation of state (an effective phantom phase) The first model we consider is decaying vacuum cosmology where the fluctuations of the vacuum are taken into account In this case, the phantom cosmology (with an effective, observational $\ensuremath{\omega}$ being less than $\ensuremath{-}1$ ) emerges even for the case of a real dark energy with a physical equation of state parameter $\ensuremath{\omega}$ larger than $\ensuremath{-}1$ The second proposal is a generalized holographic model, which is produced by the presence of an infrared cutoff It also leads to an effective phantom phase, which is not a transient one as in the first model However, we show that quantum effects are able to prevent its evolution towards a big rip singularity

Reconstructing the universe

Abstract: We provide detailed evidence for the claim that nonperturbative quantum gravity, defined through state sums of causal triangulated geometries, possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale, and at the same time provides a nontrivial consistency check of the method of causal dynamical triangulations. A closer look at the quantum geometry reveals a number of highly nonclassical aspects, including a dynamical reduction of spacetime to two dimensions on short scales and a fractal structure of slices of constant time.

Dark energy and viscous cosmology

Abstract: Singularities in the dark energy universe are discussed, assuming that there is a bulk viscosity in the cosmic fluid. In particular, it is shown how the physically natural assumption of letting the bulk viscosity be proportional to the scalar expansion in a spatially flat FRW universe can drive the fluid into the phantom region (w −1) in the non-viscous case.

An Emergent Universe from a loop

Abstract: Closed, singularity-free, inflationary cosmological models have recently been studied in the context of general relativity. Despite their appeal, these so called emergent models suffer from a number of limitations. These include the fact that they rely on an initial Einstein static state to describe the past-eternal phase of the universe. Given the instability of such a state within the context of general relativity, this amounts to a very severe fine tuning. Also in order to be able to study the dynamics of the universe within the context of general relativity, they set the initial conditions for the universe in the classical phase. Here we study the existence and stability of such models in the context of Loop Quantum Cosmology and show that both these limitations can be partially remedied, once semiclassical effects are taken into account. An important consequence of these effects is to give rise to a static solution (not present in GR), which dynamically is a center equilibrium point and located in the more natural semiclassical regime. This allows the construction of emergent models in which the universe oscillates indefinitely about such an initial static state. We construct an explicit emergent model of this type, in which amore » nonsingular past-eternal oscillating universe enters a phase where the symmetry of the oscillations is broken, leading to an emergent inflationary epoch, while satisfying all observational and semiclassical constraints. We also discuss emergent models in which the universe possesses both early- and late-time accelerating phases.« less

Effect of inhomogeneities on the expansion rate of the universe

Abstract: While the expansion rate of a homogeneous isotropic universe is simply proportional to the square-root of the energy density, the expansion rate of an inhomogeneous universe also depends on the nature of the density inhomogeneities. In this paper we calculate to second order in perturbation variables the expansion rate of an inhomogeneous universe and demonstrate corrections to the evolution of the expansion rate. While we find that the mean correction is small, the variance of the correction on the scale of the Hubble radius is sensitive to the physical significance of the unknown spectrum of density perturbations beyond the Hubble radius.

Cosmological evolution of "hessence" dark energy and avoidance of the big rip

Abstract: Recently, many dark energy models whose equation-of-state parameter can cross the phantom divide w(de)=-1 have been proposed. In a previous paper [H. Wei, R. G. Cai, and D. F. Zeng, Classical Quantum Gravity 22, 3189 (2005)], we suggest such a model named hessence, in which a noncanonical complex scalar field plays the role of dark energy. In this work, the cosmological evolution of the hessence dark energy is investigated. We consider two cases: one is the hessence field with an exponential potential, and the other is with a (inverse) power-law potential. We separately investigate the dynamical system with four different interaction forms between hessence and background perfect fluid. It is found that the big rip never appears in the hessence model, even in the most general case, beyond particular potentials and interaction forms.

Dark energy and cosmological solutions in second-order string gravity

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.

Dark energy cosmology with generalized linear equation of state

Abstract: Dark energy with the usually used equation of state p = wp, where w = const 0) and unstable (a < 0) fluids. In particular, the considered cosmological model describes the hydrodynamically stable dark (and phantom) energy. The possible types of cosmological scenarios in this model are determined and classified in terms of attractors and unstable points by using phase trajectories analysis. For the dark energy case, some distinctive types of cosmological scenarios are possible: (i) the universe with the de Sitter attractor at late times, (ii) the bouncing universe, (iii) the universe with the big rip and with the anti-big rip. In the framework of a linear equation of state the universe filled with a phantom energy, w < -1; may have either the de Sitter attractor or the big rip.

Effect of inhomogeneities on the luminosity distance-redshift relation: Is dark energy necessary in a perturbed universe?

Abstract: The luminosity distance-redshift relation is one of the fundamental tools of modern cosmology We compute the luminosity distance-redshift relation in a perturbed flat matter-dominated Universe, taking into account the presence of cosmological inhomogeneities up to second order in perturbation theory Cosmological observations implementing the luminosity distance-redshift relation tell us that the Universe is presently undergoing a phase of accelerated expansion This seems to call for a mysterious Dark Energy component with negative pressure Our findings suggest that the need of a Dark Energy fluid may be challenged once a realistic inhomogeneous Universe is considered and that an accelerated expansion may be consistent with a matter-dominated Universe

Phantom cosmology with general potentials

Abstract: We present a unified treatment of the phase space of a spatially flat homogeneous and isotropic universe dominated by a phantom field. Results on the dynamics and the late time attractors (big rip, de Sitter, etc) are derived without specifying the form of the phantom potential, using only general assumptions on its shape. Many results found in the literature are quickly recovered and predictions are made for new scenarios.

Inhomogenized sudden future singularities

Abstract: We find that sudden future singularities of pressure may also appear in spatially inhomogeneous Stephani models of the universe. They are temporal pressure singularities and may appear independently of the spatial finite density singularities already known to exist in these models. It is shown that the main advantage of the homogeneous sudden future singularities which is the fulfillment of the strong and weak energy conditions may not be the case in the inhomogeneous case.

What Is the Universe Made Of

Abstract: Independent measurements of a variety of phenomena indicate that matter makes up only about 30% of the stuff in the universe. The rest is a mysterious antigravity force known as dark energy, the nature of which is arguably the murkiest question in physics.

Escaping the big rip

Abstract: We discuss dark energy models which might describe effectively the actual acceleration of the universe. More precisely, for a four-dimensional Friedmann–Lemaitre–Robertson–Walker (FLRW) universe we consider two situations. For the first of them, we model dark energy as phantom energy described as a perfect fluid satisfying the equation of state P = (β−1)ρ (with β<0 and constant). In this case the universe reaches a 'big rip' independently of the spatial geometry of the FLRW universe. In the second situation, the dark energy is described as a phantom (generalized) Chaplygin gas which violates the dominant energy condition. Contrary to the previous case, for this material content a FLRW universe would never reach a 'big rip' singularity (indeed, the geometry is asymptotically de Sitter). We also show how this dark energy model can be described in terms of scalar fields, corresponding to a minimally coupled scalar field, a Born–Infeld scalar field and a generalized Born–Infeld scalar field. Finally, we introduce a phenomenologically viable model where dark energy is described as a phantom generalized Chaplygin gas.

Can superhorizon perturbations drive the acceleration of the Universe

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.

Brane worlds in collision.

Abstract: We obtain an exact solution of the supergravity equations of motion in which the four-dimensional observed Universe is one of a number of colliding D3 branes in a Calabi-Yau background. The collision results in the ten-dimensional spacetime splitting into disconnected regions, bounded by curvature singularities. However, near the D3 branes the metric remains static during and after the collision. We also obtain a general class of solutions representing p-brane collisions in arbitrary dimensions, including one in which the universe ends with the mutual annihilation of a positive-tension and a negative-tension 3 brane.

Cosmological Surprises from Braneworld models of Dark Energy

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).

Conservation of second-order cosmological perturbations in a scalar field dominated universe

Abstract: We discuss second-order cosmological perturbations on super-Hubble scales, in a scalar field dominated universe, such as during single field inflation. In this contest we show that the gauge-invariant curvature perturbations defined on uniform density and comoving hypersurfaces coincide and that perturbations are adiabatic in the large scale limit. Since it has been recently shown that the uniform density curvature perturbation is conserved on large scales if perturbations are adiabatic, we conclude that both the uniform density and comoving curvature perturbations at second order, in a scalar field dominated universe, are conserved. Finally, in the light of this result, we comment on the variables recently used in the literature to compute non-Gaussianities.

Understanding Our Universe : Current Status and Open Issues

Abstract: 1. Prologue: Universe as a physical system Attempts to understand the behaviour of our universe by applying the laws of physics lead to difficulties which have no parallel in the application of laws of physics to systems of more moderate scale — like atoms, solids or even galaxies. We have only one universe available for study, which itself is evolving in time; hence, different epochs in the past history of the universe are unique and have occurred only once. Standard rules of science, like repeatability, statistical stability and predictability cannot be applied to the study of the entire universe in a naive manner. The obvious procedure will be to start with the current state of the universe and use the laws of physics to study its past and future. Progress in this attempt is limited because our understanding of physical processes at energy scales above 100 GeV or so lacks direct experimental support. What is more, cosmological observations suggest that nearly 95 per cent of the matter in the universe is of types which have not been seen in the laboratory; there is also indirect, but definitive, evidence to suggest that nearly 70 per cent of the matter present in the universe exerts negative

The Running of the Cosmological and the Newton Constant controlled by the Cosmological Event Horizon

Abstract: We study the renormalisation group running of the cosmological and the Newton constant, where the renormalisation scale is given by the inverse of the radius of the cosmological event horizon. In this framework, we discuss the future evolution of the universe, where we find stable de Sitter solutions, but also "big crunch"-like and "big rip"-like events, depending on the choice of the parameters in the model.

Misconceptions about the big bang.

Abstract: The article discusses common misconceptions about the Big Bang theory and the expansion of the universe. The expansion of the universe may be the most important fact we have ever discovered about our origins. 75 years after its initial discovery, the expansion of the universe is still widely misunderstood. Because expansion is the basis of the big bang model, these misunderstandings are fundamental. Individual galaxies move around at random within clusters, but the clusters of galaxies are essentially at rest. The big bang was not an explosion in space; it was more like an explosion of space. It did not go off at a particular location and spread out from there into some imagined preexisting void. The rate at which the distance between galaxies increases follows a distinctive pattern discovered by American astronomer Edwin Hubble in 1929. According to Hubble's law, the universe does not expand at a single speed. The key to avoiding the misunderstandings is not to take the term "big bang" too literally.