Showing papers on "Big Rip published in 2008"

Future evolution and finite-time singularities in F(R) gravity unifying inflation and cosmic acceleration

Abstract: We study the future evolution of quintessence/phantom-dominated epoch in modified F(R) gravity which unifies the early-time inflation with late-time acceleration and which is consistent with observational tests. Using the reconstruction technique it is demonstrated that there are models where any known (big rip, II, III, or IV type) singularity may classically occur. From another side, in Einstein frame (scalar-tensor description) only IV type singularity occurs. Near the singularity the classical description breaks up, and it is demonstrated that quantum effects act against the singularity and may prevent its appearance. The realistic F(R) gravity which is future singularity free is proposed. We point out that additional modification of any F(R) gravity by the terms relevant at the early universe is possible, in such a way that future singularity does not occur even classically.

Lemaitre–Tolman–Bondi model and accelerating expansion

Abstract: I discuss the spherically symmetric but inhomogeneous Lemaitre–Tolman–Bondi (LTB) metric, which provides an exact toy model for an inhomogeneous universe. Since we observe light rays from the past light cone, not the expansion of the universe, spatial variation in matter density and Hubble rate can have the same effect on redshift as acceleration in a perfectly homogeneous universe. As a consequence, a simple spatial variation in the Hubble rate can account for the distant supernova data in a dust universe without any dark energy. I also review various attempts towards a semirealistic description of the universe based on the LTB model.

Emergent universe and the phantom tachyon model

Abstract: In this work, I have considered that the universe is filled with normal matter and a phantom field (or tachyonic field). If the universe is filled with a scalar field, Ellis et al have shown that an emergent scenario is possible only for k = +1, i.e. for a closed universe. Here I have shown that the emergent scenario is possible for a closed universe if the universe contains the normal tachyonic field. But for a phantom field (or tachyonic field), the negative kinetic term can generate the emergent scenario for all values of k(=0, ± 1). From recently developed statefinder parameters, the behaviour of different stages of the evolution of the emergent universe has been studied. The static Einstein universe and the stability analysis have been briefly discussed for both phantom and tachyon models.

On the generalized chaplygin gas: worse than a big rip or quieter than a sudden singularity?

Abstract: Although it has been believed that models with generalized Chaplygin gas (GCG) do not contain singularities, in previous work we have studied how a big freeze could take place in some kinds of phantom-generalized Chaplygin gas. In the present work, we study some types of generalized Chaplygin gas in order to show how different sorts of singularities could appear in such models, either in the future or in the past. We point out that (i) singularities may not originate from the phantom nature of the fluid, and (ii) if initially the tension of the brane in a brane-world Chaplygin model is large enough, then an infrared cutoff appears in the past.

Description of our cosmological spacetime as a perturbed conformal Newtonian metric and implications for the backreaction proposal for the accelerating universe

Abstract: It has been argued that the spacetime of our universe can be accurately described by a perturbed conformal Newtonian metric, and hence even large density inhomogeneities in a dust universe cannot change the observables predicted by the homogeneous dust model. In this paper we study a spherically symmetric dust model and illustrate conditions under which large spatial variations in the expansion rate can invalidate the argument.

Cosmic inhomogeneities and averaged cosmological dynamics.

Abstract: If general relativity (GR) describes the expansion of the Universe, the observed cosmic acceleration implies the existence of a "dark energy." However, while the Universe is on average homogeneous on large scales, it is inhomogeneous on smaller scales. While GR governs the dynamics of the inhomogeneous Universe, the averaged homogeneous Universe obeys modified Einstein equations. Can such modifications alone explain the acceleration? For a simple generic model with realistic initial conditions, we show the answer to be "no." Averaging effects negligibly influence the cosmological dynamics.

How strong is the evidence for accelerated expansion

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

Astronomical bounds on a future Big Freeze singularity

Abstract: It was recently found that dark energy in the form of phantom generalized Chaplygin gas may lead to a new form of a cosmic doomsday, the Big Freeze singularity. Like the Big Rip singularity, the Big Freeze singularity would also take place at finite future cosmic time, but, unlike the Big Rip, it happens for a finite scale factor. Our goal is to test if a universe filled with phantom generalized Chaplygin gas can conform to the data of astronomical observations. We shall see that if the universe is only filled with generalized phantom Chaplygin gas with the equation of state p = −c 2 s 2/ρ α with α < −1, then such a model cannot be matched to the observational data; generally speaking, such a universe has an infinite age. To construct more realistic models, one actually need to add dark matter. This procedure results in cosmological scenarios which do not contradict the values of universe age and expansion rate and allow one to estimate how long we are now from the future Big Freeze doomsday.

Singularities in loop quantum cosmology.

Abstract: We show that simple scalar field models can give rise to curvature singularities in the effective Friedmann dynamics of loop quantum cosmology (LQC). We find singular solutions for spatially flat Friedmann-Robertson-Walker cosmologies with a canonical scalar field and a negative exponential potential, or with a phantom scalar field and a positive potential. While LQC avoids big bang or big rip type singularities, we find sudden singularities where the Hubble rate is bounded, but the Ricci curvature scalar diverges. We conclude that the effective equations of LQC are not in themselves sufficient to avoid the occurrence of curvature singularities.

Non-minimally coupled scalar field cosmology on the phase plane

Abstract: In this publication we investigate dynamics of a flat FRW cosmological model with a non-minimally coupled scalar field with the coupling term $\xi R \psi^{2}$ in the scalar field action. The quadratic potential function $V(\psi)\propto \psi^{2}$ is assumed. All the evolutional paths are visualized and classified in the phase plane, at which the parameter of non-minimal coupling $\xi$ plays the role of a control parameter. The fragility of global dynamics with respect to changes of the coupling constant is studied in details. We find that the future big rip singularity appearing in the phantom scalar field cosmological models can be avoided due to non-minimal coupling constant effects. We have shown the existence of a finite scale factor singular point (future or past) where the Hubble function as well as its first cosmological time derivative diverges.

Dynamics of holographic vacuum energy in the DGP model

Abstract: We consider the evolution of the vacuum energy in the Dvali-Gabadadze-Porrati (DGP) model according to the holographic principle under the assumption that the relation linking the IR and UV cutoffs still holds in this scenario. The model is studied when the IR cutoff is chosen to be the Hubble scale H(-1), the particle horizon R(ph), and the future event horizon R(eh), respectively. The two branches of the DGP model are also taken into account. Through numerical analysis, we find that in the cases of H(-1) in the (+) branch and R(eh) in both branches, the vacuum energy can play the role of dark energy. Moreover, when considering the combination of the vacuum energy and the 5D gravity effect in both branches, the equation of state of the effective dark energy may cross -1, which may lead to the big rip singularity. Besides, we constrain the model with the Type Ia supernovae and baryon oscillation data and find that our model is consistent with current data within 1 sigma, and that the observations prefer either a pure holographic dark energy or a pure DGP model.

Duality extended Chaplygin cosmologies with a big rip

Abstract: We consider modifications to the Friedmann equation motivated by recent proposals along these lines pursuing an explanation to the observed late time acceleration. Here we show those modifications can be framed within a theory with self-interacting gravity, where the term self-interaction refers here to the presence of functions of $\rho$ and $p$ in the right hand side of the Einstein equations. We then discuss the construction of the duals of the cosmologies generated within that framework. After that we investigate the modifications required to generate generalized and modified Chaplygin cosmologies and show that their duals belong to a larger family of cosmologies we call extended Chaplygin cosmologies. Finally, by letting the parameters of those models take values not earlier considered in the literature we show some representatives of that family of cosmologies display sudden future singularities, which indicates their behavior is rather different from generalized or modified Chaplygin gas cosmologies. This reinforces the idea that modifications of gravity can be responsible for unexpected evolutionary features in the universe.

Cosmology: patchy solutions.

Abstract: susceptible to other classes of anthelmintics, providing further credence to the idea that AADs have a new biological target. The authors then looked to see whether AADs attack the same target in parasitic nematodes. They selected for AAD-resistant H. contortus larvae in vitro, and for adult worms in vivo, by giving sheep doses of the new anthelmintic that were too low to get rid of the parasite completely. They then used nAChR-specific DNA probes on both AAD-resistant and AAD-susceptible parasites to find nAChR genes likely to be responsible for the resistance. They found that part of a nematode-specific nAChR gene (named Hcdes-2H by the authors) was missing in the AAD-resistant worms, and that this gene was closely related to acr-23. This suggests that the mode of action of AADs is similar in both C. elegans and H. contortus. It also suggests that the presence of evolutionarily related nAChRs in other species of parasitic nematode can be used to predict the effectiveness of AADs against those species, which could be useful Figure 2 | Determining the biological target of antiparasitic drugs. Kaminsky et al. report a class of compounds known as amino-acetonitrile derivatives (AADs) that kill parasitic worms such as H. contortus in livestock. R and R represent general chemical groups, and may be attached to the aromatic rings at any vacant position. AADs also kill the non-parasitic worm C. elegans, the genome of which has been sequenced. The authors were therefore able to determine the biological target of AADs by studying AADresistant C. elegans, as shown in the flow chart. The gene responsible for AAD resistance must produce the protein targeted by the compounds. O

First Light in the Universe

Abstract: First Light.- Cosmological Feedbacks from the First Stars.- Observations of the High Redshift Universe.

Crossing the phantom divide in an interacting generalized Chaplygin gas

Abstract: Unified generalized Chaplygin gas models assuming an interaction between dark energy and dark matter fluids have been previously proposed. Following these ideas, we consider a particular relation between dark densities, which allows the possibility of a time varying equation of state for dark energy that crosses the phantom divide at a recent epoch. Moreover, these densities decay throughout the evolution of the Universe, avoiding a big rip. We find also a scaling solution, i.e. these densities are asymptotically proportional in the future, which contributes to the solution of the coincidence problem.

The cosmological slingshot scenario: a stringy early times universe

Abstract: A cosmological model for the early time universe is proposed. In this model, the universe is a wandering brane moving in a warped throat of a Calabi–Yau space. A nonzero angular momentum induces a turning point in the brane trajectory, and leads to a bouncing cosmology as experienced by an observer living on the brane. The universe undergoes a decelerated contraction followed by an accelerating expansion and no big-bang singularity. Although the number of e-folds of accelerated motion is low (less than 2), standard cosmological problems are not present in our model; thanks to the absence of an initial singularity and the violation of energy conditions of mirage matter at high energies. Density perturbations are also calculated in our model and we find a slightly red spectral index with negligible tensorial perturbations in compatibility with WMAP data.

Dynamical Attractor of Modified Chaplygin Gas

Abstract: The dynamical attractor of the modified Chaplygin gas (MCG) model is studied. The dynamical analysis indicates that the phase ωMCG = -1 is a dynamical attractor and the equation of state of the MCG approaches it from either ωMCG > −1 or ωMCG < −1, independent of the choice of its initial density parameter and the ratio of pressure to critical energy density. Therefore our universe will not end up with Big Rip in the future. Moreover, the evolutions of the density parameters Ωγ and ΩMCG are quite different. For different initial values of x and Ωγ, decreases and ΩMCG increases as time increases, and they will eventually approach Ωγ = 0 and ΩMCG = 1, i.e., de Sitter phase. This implies that when there is not the interaction (i.e., the energy transfer) between the barotropic background fluid and modified Chaplygin gas (MCG), the behaviour of the MCG will be similar to ΛCDM in the future.

The effect of structure formation on the expansion of the universe

Abstract: Observations of the expansion rate of the universe at late times disagree by a factor of 15-2 with the prediction of homogeneous and isotropic models based on ordinary matter and gravity We discuss how the departure from linearly perturbed homogeneity and isotropy due to structure formation could explain this discrepancy We evaluate the expansion rate in a dust universe which contains non-linear structures with a statistically homogeneous and isotropic distribution The expansion rate is found to increase relative to the exactly homogeneous and isotropic case by a factor of 11-13 at some tens of billion of years The timescale follows from the cold dark matter transfer function and the amplitude of primordial perturbations without additional free parameters

Particle production in an expanding universe dominated by dark energy fluid

Abstract: We investigate particle production in an expanding universe dominated by a perfect fluid with the equation of state p = αρ. The rate of particle production, using the Bogoliubov coefficients, is determined exactly for any value of α in the case of a flat universe. When the strong energy condition is satisfied, the particle production rate decreases with time; the opposite occurs when the strong energy condition is violated. In the phantom case, the particle production rate diverges in a finite time for each mode represented by a wavenumber k. Nevertheless, the energy density associated with the produced particles tends to zero as the big rip is approached.

The thermodynamic universe : exploring the limits of physics

Abstract: The Limits of Physics Law without Law The Universe of Fluctuations The Thermodynamic Universe Spacetime Models and Tests The Origin of Mass, Spin and Interaction Enigma of Gravitation An Adventurer's Miscellany

Viscous modified gravity on a RS brane embedded in AdS5

Abstract: We consider a modified gravity fluid on a Randall–Sundrum II brane situated at y=0, the action containing a power α of the scalar curvature. As is known from 4D spatially flat modified gravity, the presence of bulk viscosity may drive the cosmic fluid into the phantom region (w −1). The condition for this to occur is that the bulk viscosity contains the power (2α−1) of the scalar expansion. We combine this with the 5D RS II model, and we find that the Big Rip, occurring for α>1/2, carries over to the metric for the bulk metric, |y|>0. Actually, the scale factors on the brane and in the bulk become simply proportional to each other.

Weighing the spatial and temporal fluctuations of the dark universe

Abstract: A generic prediction of the standard cosmology, based on general relativity (GR), dark matter and the cosmological constant (and more generally, smooth dark energy), is that, the two gravitational potentials describing the spatial and temporal scalar perturbations of the universe are equivalent. Modifications in GR or dark energy clustering in general violate this relation. Thus this ratio serves as a smoking gun of the dark universe. We propose a method to extract this ratio at various cosmological scales and redshifts from a set of measurements, in a model independent way. The ratio measured by future surveys has strong discriminating power for a variety of dark universe scenarios.