Showing papers on "Big Rip published in 2009"

On initial conditions for the Hot Big Bang

Abstract: We analyse the process of reheating the Universe in the electroweak theory where the Higgs field plays a role of the inflaton. We estimate the maximal temperature of the Universe and fix the initial conditions for radiation-dominated phase of the Universe expansion in the framework of the Standard Model (SM) and of the νMSM - the minimal extension of the SM by three right-handed singlet fermions. We show that the inflationary epoch is followed by a matter dominated stage related to the Higgs field oscillations. We investigate the energy transfer from Higgs-inflaton to the SM particles and show that the radiation dominated phase of the Universe expansion starts at temperature Tr (3-15) × 1013GeV, where the upper bound depends on the Higgs boson mass. We estimate the production rate of singlet fermions at preheating and find that their concentrations at T r are negligibly small. This suggests that the sterile neutrino Dark Matter (DM) production and baryogenesis in the νMSM with Higgs-driven inflation are low energy phenomena, having nothing to do with inflation. We study then a modification of the νMSM, adding to its Lagrangian higher dimensional operators suppressed by the Planck scale. The role of these operators in Higgs-driven inflation is clarified. We find that these operators do not contribute to the production of Warm Dark Matter (WDM) and to baryogenesis. We also demonstrate that the sterile neutrino with mass exceeding 100keV (a Cold Dark Matter (CDM) candidate) can be created during the reheating stage of the Universe in necessary amounts. We argue that the mass of DM sterile neutrino should not exceed few MeV in order not to overclose the Universe. © 2009 IOP Publishing Ltd and SISSA.

Approaches to Understanding Cosmic Acceleration

Abstract: Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard general relativity coupled to conventional sources of energy–momentum. We then address three broad methods of addressing an accelerating universe: the introduction of a cosmological constant, its problems and origins; the possibility of dark energy and the associated challenges for fundamental physics and the option that an infrared modification of general relativity may be responsible for the large-scale behavior of the universe.

Crossing of the phantom divide in modified gravity

Abstract: We reconstruct an explicit model of modified gravity in which a crossing of the phantom divide can be realized It is shown that the (finite-time) Big Rip singularity appears in the model of modified gravity (ie, in the so-called Jordan-frame), whereas that in the corresponding scalar field theory obtained through the conformal transformation (ie, in the so-called Einstein frame) the singularity becomes the infinite-time one Furthermore, we investigate the relations between the scalar field theories with realizing a crossing of the phantom divide and the corresponding modified gravitational theories by using the inverse conformal transformation It is demonstrated that the scalar field theories describing the nonphantom phase (phantom one with the Big Rip) can be represented as the theories of real (complex) F(R) gravity through the inverse (complex) conformal transformation We also study a viable model of modified gravity in which the transition from the de Sitter universe to the phantom phase can occur In addition, we explore the stability for the obtained solutions of the crossing of the phantom divide under a quantum correction coming from conformal anomaly

Thermodynamics and classification of cosmological models in the Horava-Lifshitz theory of gravity

Abstract: We study thermodynamics of cosmological models in the Horava-Lifshitz theory of gravity, and systematically investigate the evolution of the universe filled with a perfect fluid that has the equation of state p = wρ, where p and ρ denote, respectively, the pressure and energy density of the fluid, and w is an arbitrary real constant. Depending on specific values of the free parameters involved in the models, we classify all of them into various cases. In each case the main properties of the evolution are studied in detail, including the periods of deceleration and/or acceleration, and the existence of big bang, big crunch, and big rip singularities. We pay particular attention on models that may give rise to a bouncing universe.

Thermodynamics and classification of cosmological models in the Horava-Lifshitz theory of gravity

Abstract: We study thermodynamics of cosmological models in the Horava-Lifshitz theory of gravity, and systematically investigate the evolution of the universe filled with a perfect fluid that has the equation of state $p=w\rho$, where $p$ and $\rho$ denote, respectively, the pressure and energy density of the fluid, and $w$ is an arbitrary real constant. Depending on specific values of the free parameters involved in the models, we classify all of them into various cases. In each case the main properties of the evolution are studied in detail, including the periods of deceleration and/or acceleration, and the existence of big bang, big crunch, and big rip singularities. We pay particular attention on models that may give rise to a bouncing universe.

Archipelagian Cosmology: Dynamics and Observables in a Universe with Discretized Matter Content

Abstract: We consider a model of the Universe in which the matter content is in the form of discrete islands, rather than a continuous fluid In the appropriate limits the resulting large-scale dynamics approach those of a Friedmann-Robertson-Walker (FRW) universe The optical properties of such a space-time, however, do not This illustrates the fact that the optical and "average" dynamical properties of a relativistic universe are not equivalent, and do not specify each other uniquely We find the angular diameter distance, luminosity distance, and redshifts that would be measured by observers in these space-times, using both analytic approximations and numerical simulations While different from their counterparts in FRW, the effects found do not look like promising candidates to explain the observations usually attributed to the existence of dark energy This incongruity with standard FRW cosmology is not due to the existence of any unexpectedly large structures or voids in the Universe, but only to the fact that the matter content of the Universe is not a continuous fluid

Features of holographic dark energy under combined cosmological constraints

Abstract: We investigate the observational signatures of the holographic dark-energy models, including both the original model and a model with an interaction term between the dark energy and dark matter. We first delineate the dynamical behavior of such models, especially considering whether they would have a “big rip” for different parameters; then we use several recent observations, including 182 high-quality type Ia supernovae data observed with the Hubble Space Telescope, the SNLS and ESSENCE surveys, 42 latest Chandra X-ray cluster gas mass fraction, 27 high-redshift gamma-ray burst samples, the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey, and the CMB shift parameter from the WMAP three-year result to give more reliable and tighter constraints on the holographic dark-energy models. The results of our constraints for the holographic dark-energy model without interaction is c=0.748 −0.009 +0.108 , Ω m0=0.276 −0.016 +0.017 , and for the model with interaction (c=0.692 −0.107 +0.135 , Ω m0=0.281 −0.017 +0.017 , α=−0.006 −0.024 +0.021 , where α is an interacting parameter). As these models have more parameters than the ΛCDM model, we use the Bayesian Evidence as a model-selection criterion to make comparisons. We found that the holographic dark-energy models are mildly favored by the observations as compared to the ΛCDM model.

Internal space structure generalization of the quintom cosmological scenario

Abstract: We introduce the Lagrangian for a multiscalar-field configuration in an N-dimensional internal space endowed with a constant metric Q{sub ik}, and generalize the quintom cosmological scenario. We find the energy momentum tensor of the model and show that the set of dual transformations, which preserve the form of the Einstein equations in the Friedmann-Robertson-Walker cosmology, is enlarged. We show that the stability of the power-law solutions leads to an exponential potential which is invariant under linear transformations in the internal space. Moreover, we obtain the general exact solution of the Einstein-Klein-Gordon equations for that potential. There exist solutions that cross the phantom divide and solutions that blow up at a finite time, exhibiting a superaccelerated behavior and ending in a big rip. We show that the quintom model with a separable potential can be identified with a mixture of several fluids. This framework includes the {lambda}CDM model, a bouncing model, and a setting sourced by a cosmic string network plus a cosmological constant. Then we concentrate on the case where the dimension of the internal quintessence sector N{sub q} exceeds the dimension of the internal phantom sector N{sub ph}. For (N{sub q},N{sub ph})=(2,1) the dark energy density derived from themore » 3-scalar field crosses the phantom divide, and its negative component plays the role of the negative part of a classical Dirac field.« less

Can a matter-dominated model with constant bulk viscosity drive the accelerated expansion of the universe?

Abstract: We test a cosmological model which the only component is a pressureless fluid with a constant bulk viscosity as an explanation for the present accelerated expansion of the universe. We classify all the possible scenarios for the universe predicted by the model according to their past, present and future evolution and we test its viability performing a Bayesian statistical analysis using the SCP ``Union'' data set (307 SNe Ia), imposing the second law of thermodynamics on the dimensionless constant bulk viscous coefficient and comparing the predicted age of the universe by the model with the constraints coming from the oldest globular clusters. The best estimated values found for and the Hubble constant H0 are: = 1.922±0.089 and H0 = 69.62±0.59 (km/s)Mpc−1 with a χ2min = 314 (χ2d.o.f = 1.031). The age of the universe is found to be 14.95±0.42 Gyr. We see that the estimated value of H0 as well as of χ2d.o.f are very similar to those obtained from ΛCDM model using the same SNe Ia data set. The estimated age of the universe is in agreement with the constraints coming from the oldest globular clusters. Moreover, the estimated value of is positive in agreement with the second law of thermodynamics (SLT). On the other hand, we perform different forms of marginalization over the parameter H0 in order to study the sensibility of the results to the way how H0 is marginalized. We found that it is almost negligible the dependence between the best estimated values of the free parameters of this model and the way how H0 is marginalized in the present work. Therefore, this simple model might be a viable candidate to explain the present acceleration in the expansion of the universe.

Discovering the Expanding Universe

Abstract: Acknowledgments Foreword 1 Introduction 2 Cosmological concepts at the end of the Middle Ages 3 Nebulae as a new astronomical phenomenon 4 On the construction of the Heavens 5 Island universes turn into astronomical facts: a universe of galaxies 6 The early cosmology of Einstein and de Sitter 7 The dynamical universe of Friedmann 8 Redshifts: how to reconcile Slipher and de Sitter? 9 Lemaitre discovers the expanding universe 10 Hubble's contribution of 1929 11 The breakthrough for the expanding universe 12 Hubble's anger about de Sitter 13 Robertson and Tolman join the game 14 The Einstein-de Sitter universe 15 Are Sun and Earth older than the universe? 16 In search of alternative tracks 17 The seed for the Big Bang 18 Summary and postscript Appendix References Index

F(R) cosmology in the presence of a phantom fluid and its scalar-tensor counterpart: Towards a unified precision model of the evolution of the Universe

Abstract: The behavior of cosmological evolution is studied in the case when a phantom fluid that contributes to the accelerated expansion of the Universe is introduced in an $F(R)$ model. At the early stages of the history of the Universe, the dark fluid is seen to give rise to a deceleration of its expansion. For $t$ close to present time it works as an additional contribution to the effective cosmological constant and, later, it produces the transition to a phantom era, which could actually be taking place right now in some regions of the cosmos, and might have observable consequences. For $t$ close to the rip time, the Universe becomes completely dominated by the dark fluid, whose equation of state is phantomlike at that time. Our model, which is able to reproduce the dark energy period quite precisely, may still be modified in such a way that the epoch dominated by an effective cosmological constant---produced by the $F(R)$ term and by the dark fluid contribution---becomes significantly shorter, which is what happens when a matter term is included. The dark fluid with phantom behavior gives rise to a superaccelerated phase, as compared with the case where just the viable $F(R)$ term contributes. It is also seen explicitly that an $F(R)$ theory can be constructed from a phantom model in a scalar-tensor theory, in which the scalar field does not behave as phantom (in the latter case the action for $F(R)$ would be complex). Promising $F(R)$ models that are able to cross the phantom divide in a convenient way are constructed explicitly.

k-essence model of inflation, dark matter, and dark energy

Abstract: We investigate the possibility for k-essence dynamics to reproduce the primary features of inflation in the early universe, generate dark matter subsequently, and finally account for the presently observed acceleration. We first show that for a purely kinetic k-essence model the late-time energy density of the universe when expressed simply as a sum of a cosmological constant and a dark matter term leads to a static universe. We then study another k-essence model in which the Lagrangian contains a potential for the scalar field as well as a noncanonical kinetic term. We show that such a model generates the basic features of inflation in the early universe, and also gives rise to dark matter and dark energy at appropriate subsequent stages. Observational constraints on the parameters of this model are obtained.

Classical stability of sudden and big rip singularities

Abstract: We introduce a general characterization of sudden cosmological singularities and investigate the classical stability of homogeneous and isotropic cosmological solutions of all curvatures containing these singularities to small scalar, vector, and tensor perturbations using gauge-invariant perturbation theory. We establish that sudden singularities at which the scale factor, expansion rate, and density are finite are stable except for a set of special parameter values. We also apply our analysis to the stability of Big Rip singularities and find the conditions for their stability against small scalar, vector, and tensor perturbations.

How flat can you get? A model comparison perspective on the curvature of the Universe

Abstract: The question of determining the spatial geometry of the Universe is of greater relevance than ever, as precision cosmology promises to verify inflationary predictions about the curvature of the Universe. We revisit the question of what can be learnt about the spatial geometry of the Universe from the perspective of a three-way Bayesian model comparison. By considering two classes of phenomenological priors for the curvature parameter, we show that, given the current data, the probability that the Universe is spatially infinite lies between 67 and 98 per cent, depending on the choice of priors. For the strongest prior choice, we find odds of the order of 50:1 (200:1 ) in favour of a flat Universe when compared with a closed (open) model. We also report a robust, prior-independent lower limit to the number of Hubble spheres in the Universe, N U ≥ 5 (at 99 per cent confidence). We forecast the accuracy with which future cosmic microwave background (CMB) and baryonic acoustic oscillation (BAO) observations will be able to constrain curvature, finding that a cosmic variance-limited CMB experiment together with an Square Kilometer Array (SKA)-like BAO observation will constrain curvature independently of the equation of state of dark energy with a precision of about σ ∼ 4.5 x 10 ―4 . We demonstrate that the risk of 'model confusion' (i.e. wrongly favouring a flat Universe in the presence of curvature) is much larger than might be assumed from parameter error forecasts for future probes. We argue that a 5σ detection threshold guarantees a confusion- and ambiguity-free model selection. Together with inflationary arguments, this implies that the geometry of the Universe is not knowable if the value of the curvature parameter is below |Ω κ | ∼ 10- 4 . This bound is one order of magnitude larger than what one would naively expect from the size of curvature perturbations, ∼10 ―5 .

Reconstruction and deceleration-acceleration transitions in modified gravity

Abstract: We discuss the cosmological reconstruction in modified Gauss-Bonnet and F(R) gravities Two alternative representations of the action (with and without auxiliary scalar) are considered The approximate description of deceleration-acceleration transition cosmologies is reconstructed It is shown that cosmological solution containing Big Bang and Big Rip singularities may be reconstructed only using the representation with the auxiliary field The analytical description of the deceleration-acceleration transition cosmology in modified Gauss-Bonnet gravity is demonstrated to be impossible at sufficiently general conditions

Constraining the dark energy equation of state with cosmic voids

Abstract: Our universe is observed to be accelerating due to the dominant dark energy with negative pressure. The dark energy equation of state (w) holds a key to understanding the ultimate fate of the universe. The cosmic voids behave like bubbles in the universe so that its shapes must be quite sensitive to the background cosmology. Assuming a flat universe and using the priors on the matter density parameter (Ω m ) and the dimensionless Hubble parameter (h), we demonstrate analytically that the ellipticity evolution of cosmic voids may be a sensitive probe of the dark energy equation of state. We also discuss the parameter degeneracy between w and Ω m .

Dark energy with dark spinors

Abstract: Ever since the first observations that we are living in an accelerating universe, it has been asked what dark energy is There are various explanations all of which with have various draw backs or inconsistencies Here we show that using a dark spinor field it is possible to have an equation of state that crosses the phantom divide, becoming a dark phantom spinor which evolves into dark energy This type of equation of state has been mildly favored by experimental data, however, in the past there were hardly any candidate theories that satisfied this crossing without creating ghosts or causing a singularity which results in the universe essentially ripping The dark spinor model converges to dark energy in a reasonable time frame avoiding the big rip and without attaining negative kinetic energy as it crosses the phantom divide

Model independent constraints on the cosmological expansion rate

Abstract: We investigate what current cosmological data tells us about the cosmological expansion rate in a model independent way. Specifically, we study if the expansion was decelerating at high redshifts and is accelerating now, without referring to any model for the energy content of the universe, nor to any specific theory of gravity. This differs from most studies of the expansion rate which, e.g., assumes some underlying parameterised model for the dark energy component of the universe. To accomplish this, we have devised a new method to probe the expansion rate without relying on such assumptions. Using only supernova data, we conclude that there is little doubt that the universe has been accelerating at late times. However, contrary to some previous claims, we can not determine if the universe was previously decelerating. For a variety of methods used for constraining the expansion history of the universe, acceleration is detected from supernovae alone at >5σ, regardless of the curvature of the universe. Specifically, using a Taylor expansion of the scale factor, acceleration today is detected at >12σ. If we also include the ratio of the scale of the baryon acoustic oscillations as imprinted in the cosmic microwave background and in the large scale distribution of galaxies, it is evident from the data that the expansion decelerated at high redshifts, but only with the assumption of a flat or negatively curved universe.

Dark energy and the return of the phoenix universe

Abstract: In cyclic universe models based on a single scalar field (e.g., the radion determining the distance between branes in M theory), virtually the entire Universe makes it through the ekpyrotic smoothing and flattening phase, bounces, and enters a new epoch of expansion and cooling. This stable evolution cannot occur, however, if scale-invariant curvature perturbations are produced by the entropic mechanism because it requires two scalar fields (e.g., the radion and the Calabi-Yau dilaton) evolving along an unstable classical trajectory. In fact, we show here that an overwhelming fraction of the Universe fails to make it through the ekpyrotic phase; nevertheless, a sufficient volume survives and cycling continues forever provided the dark energy phase of the cycle lasts long enough, of order a trillion years. Two consequences are a new role for dark energy and a global structure of the Universe radically different from that of eternal inflation.

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 ξRψ2 in the scalar field action. The quadratic potential function V(ψ) ∝ ψ2 is assumed. All the evolutional paths are visualized and classified in the phase plane, at which the parameter of non-minimal coupling ξ 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 diverge.

Relation between the isotropy of the CMB and the geometry of the universe

Abstract: The near isotropy of the cosmic microwave background (CMB) is considered to be the strongest indication for the homogeneity and isotropy of the Universe, a cornerstone of most cosmological analysis. We derive new theorems which extend the Ehlers-Geren-Sachs result that an isotropic CMB implies that the Universe is either stationary or homogeneous and isotropic, and its generalization to the almost isotropic case. We discuss why the theorems do not apply to the real Universe, and why the CMB observations do not imply that the Universe would be nearly homogeneous and isotropic.

Large-Scale Structure of the Universe as a Cosmic Standard Ruler

Abstract: We propose to use the large-scale structure of the universe as a cosmic standard ruler, based on the fact that the pattern of galaxy distribution should be maintained in the course of time on large scales. By examining the scale-dependence of the pattern in different redshift intervals it is possible to reconstruct the expansion history of the universe, and thus to measure the cosmological parameters governing the expansion of the universe. The features in the galaxy distribution that can be used as standard rulers include the topology of large-scale structure and the overall shapes of galaxy power spectrum and correlation function. The genus, being an intrinsic topology measure, is resistant against the non-linear gravitational evolution, galaxy biasing, and redshift-space distortion effects, and thus is ideal for quantifying the primordial topology of the large-scale structure. The expansion history of the universe can be constrained by comparing among the genus measured at different redshifts. In the case of initially Gaussian fluctuations the genus accurately recovers the slope of the primordial power spectrum near the smoothing scale, and the expansion history can be constrained by comparing between the predicted and measured genus.

Tachyon cosmology, supernovae data and the Big Brake singularity

Abstract: We compare the existing observational data on type Ia supernovae with the evolutions of the Universe predicted by a one-parameter family of tachyon models which we have introduced recently [Phys. Rev. D 69, 123512 (2004)]. Among the set of the trajectories of the model which are compatible with the data there is a consistent subset for which the Universe ends up in a new type of soft cosmological singularity dubbed big brake. This opens up yet another scenario for the future history of the Universe besides the one predicted by the standard {lambda}CDM model.

Model- and calibration-independent test of cosmic acceleration

Abstract: We present a calibration-independent test of the accelerated expansion of the universe using supernova type Ia data. The test is also model-independent in the sense that no assumptions about the content of the universe or about the parameterization of the deceleration parameter are made and that it does not assume any dynamical equations of motion. Yet, the test assumes the universe and the distribution of supernovae to be statistically homogeneous and isotropic. A significant reduction of systematic effects, as compared to our previous, calibration-dependent test, is achieved. Accelerated expansion is detected at significant level (4.3σ in the 2007 Gold sample, 7.2σ in the 2008 Union sample) if the universe is spatially flat. This result depends, however, crucially on supernovae with a redshift smaller than 0.1, for which the assumption of statistical isotropy and homogeneity is less well established.