Showing papers on "Dark fluid 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.

Limits of quintessence.

Abstract: We present evidence that the simplest particle-physics scalar-field models of dynamical dark energy can be separated into distinct behaviors based on the acceleration or deceleration of the field as it evolves down its potential towards a zero minimum. We show that these models occupy narrow regions in the phase-plane of w and w', the dark energy equation-of-state and its time-derivative in units of the Hubble time. Restricting an energy scale of the dark energy microphysics limits how closely a scalar field can resemble a cosmological constant. These results, indicating a desired measurement resolution of order \sigma(w')\approx (1+w), define firm targets for observational tests of the physics of dark energy.

Constraints on holographic dark energy from type Ia supernova observations

Abstract: In this paper, we use the type Ia supernovae data to constrain the holographic dark energy model proposed by Li. We also apply a cosmic age test to this analysis. We consider in this paper a spatially flat Friedmann-Robertson-Walker universe with a matter component and a holographic dark energy component. The fit result shows that the case $cl1$ ($c=0.21$) is favored, which implies that the holographic dark energy behaves as a quintom-type dark energy. Furthermore, we also perform a joint analysis of $\mathrm{SNe}+\mathrm{CMB}+\mathrm{LSS}$ to this model; the result is well improved and still upholds the quintom dark energy conclusion. The best fit results in our analysis are $c=0.81$, ${\ensuremath{\Omega}}_{m}^{0}=0.28$, and $h=0.65$, which lead to the present equation of state of dark energy ${w}_{0}=\ensuremath{-}1.03$ and the deceleration/acceleration transition redshift ${z}_{T}=0.63$. Finally, an expected supernova/acceleration probe simulation using $\ensuremath{\Lambda}\mathrm{CDM}$ as a fiducial model is performed on this model, and the result shows that the holographic dark energy model takes on $cl1$ ($c=0.92$) even though the dark energy is indeed a cosmological constant.

Cosmology with interaction between phantom dark energy and dark matter and the coincidence problem

Abstract: We study a cosmological model in which phantom dark energy is coupled to dark matter by phenomenologically introducing a coupled term to the equations of motion of dark energy and dark matter. This term is parameterized by a dimensionless coupling function δ, the Hubble parameter and the energy density of dark matter, and it describes an energy flow between the dark energy and dark matter. We discuss two cases: one is the case where the equation of state ωe of the dark energy is a constant; the other is that where the dimensionless coupling function δ is a constant. We investigate the effect of the interaction on the evolution of the universe, the total lifetime of the universe and the ratio of the period when the universe is in the coincidence state to its total lifetime. It turns out that the interaction will produce significant deviation from the case without the interaction.

Consistent modified gravity: dark energy, acceleration and the absence of cosmic doomsday

Abstract: We discuss modified gravity which includes negative and positive powers of curvature and provides gravitational dark energy. It is shown that in GR plus a term containing a negative power of curvature, cosmic speed-up may be achieved while the effective phantom phase (with w less than −1) follows when such a term contains a fractional positive power of curvature. Minimal coupling with matter makes the situation more interesting: even 1/R theory coupled with the usual ideal fluid may describe the (effective phantom) dark energy. The account of the R2 term (consistent modified gravity) may help to escape cosmic doomsday.

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.

Dark energy: The cosmological challenge of the millennium

Abstract: Recent cosmological observations suggest that nearly seventy per cent of the energy density in the universe is unclustered and has negative pressure. Several conceptual issues related to the modelling of this component ('dark energy'), which is driving an accelerated expansion of the universe, are reviewed with special emphasis on the cosmological constant as the possible choice for the dark energy.

Gamma rays from Kaluza-Klein dark matter.

Abstract: A TeV gamma-ray signal from the direction of the Galactic center (GC) has been detected by the HESS experiment. Here, we investigate whether Kaluza-Klein (KK) dark matter annihilations near the GC can be the explanation. Including the contributions from internal bremsstrahlung as well as subsequent decays of quarks and $\ensuremath{\tau}$ leptons, we find a very flat gamma-ray spectrum which drops abruptly at the dark matter particle mass. For a KK mass of about 1 TeV, this gives a good fit to the HESS data below 1 TeV. A similar model, with gauge coupling roughly 3 times as large and a particle mass of about 10 TeV, would give both the correct relic density and a photon spectrum that fits the complete range of data.

A single scalar field model of dark energy with equation of state crossing −1

Abstract: In this paper we study the possibility of building models of dark energy with equation of state across -1 and propose explicitly a model with a single scalar field which gives rise to an equation of state larger than -1 in the past and less than -1 at the present time, consistent with the current observations.

Dark matter from baryon asymmetry

Abstract: The measured densities of dark and baryonic matter are surprisingly close to each other, even though the baryon asymmetry and the dark matter are usually explained by unrelated mechanisms. We consider a scenario where the dark matter $S$ is produced nonthermally from the decay of a messenger particle $X$, which carries the baryon number and compensates for the baryon asymmetry in the Universe, thereby establishing a connection between the baryonic and dark matter densities. We propose a simple model to realize this scenario, adding only a light singlet fermion $S$ and a colored particle $X$ which could have a mass in the $\mathcal{O}$(TeV) range and a lifetime to appear long-lived in collider detector. Therefore in hadron colliders the signal is similar to that of a stable or long-lived gluino in supersymmetric models.

Anthropic principle favours the holographic dark energy

Abstract: We discuss the anthropic principle when applied to holographic dark energy. We find that if the amplitude of the density fluctuation is variable, the holographic dark energy fares better than the cosmological constant. More generally, the anthropic predictions agree better with observation for dark energy with wΛ = pΛ/ρΛ decreasing over time.

Coupled Quintessence in a Power-Law Case and the Cosmic Coincidence Problem

Abstract: The problem of the cosmic coincidence is a longstanding puzzle. This conundrum may be solved by introducing a coupling between the two dark sectors. In this letter, we study a coupled quintessence scenario in which the scalar field evolves in a power law potential and the mass of dark matter particles depends on a power law function of ϕ. It is shown that this scenario has a stable attractor solution and can thus provide a natural solution to the cosmic coincidence problem.

Haloes of k-essence

Abstract: We study gravitationally bound static and spherically symmetric configurations of k-essence fields. In particular, we investigate whether these configurations can reproduce the properties of dark matter haloes. The classes of Lagrangians we consider lead to non-isotropic fluids with barotropic and polytropic equations of state. The latter include microscopic realizations of the often-considered Chaplygin gases, which we find can cluster into dark matter halo-like objects with flat rotation curves, while exhibiting a dark energy-like negative pressure on cosmological scales. We complement our studies with a series of formal general results about the stability and initial value formulation of non-canonical scalar field theories, and we also discuss a new class of de Sitter solutions with spacelike field gradients.

Cosmological evolution of interacting phantom energy with dark matter

Abstract: We investigate the cosmological evolution of an interacting phantom energy model in which the phantom field interacts with the dark matter. We discuss the existence and stability of scaling solutions for two types of specific interactions. One is motivated by the conformal transformation in string theory and the other is motivated by analogy with dissipation. In the former case, there exist no scaling solutions. In the latter case, there exist stable scaling solutions, which may give a phenomenological solution of the coincidence problem. Furthermore, the universe either accelerates forever or ends with a singularity, which is determined by not only the model parameters but also the initial velocity of the phantom field.

Neutrino models of dark energy

Abstract: I consider a scenario proposed by Fardon, Nelson, and Weiner where dark energy and neutrinos are connected. As a result, neutrino masses are not constant but depend on the neutrino number density. By examining the full equation of state for the dark sector, I show that in this scenario the dark energy is equivalent to having a cosmological constant, but one that 'runs' as the neutrino mass changes with temperature. Two examples are examined that illustrate the principal features of the dark sector of this scenario. In particular, the cosmological constant is seen to be negligible for most of the evolution of the Universe, becoming important only when neutrinos become nonrelativistic. Some speculations on features of this scenario which might be present in a more realistic theory are also presented.

Instability of dark energy with mass-varying neutrinos

Abstract: An interesting dynamical model for dark energy which does not require extremely light scalar fields such as quintessence, and at the same time explains the (near) coincidence between the neutrino and dark energy densities is the model of dark energy coupled to mass-varying neutrinos (MaVaNs). Despite the attractions of this model, we show that, generically, this model contains a catastrophic instability which occurs when neutrinos become nonrelativistic. As a result of this instability, as neutrinos become nonrelativistic, they condense into neutrino nuggets which redshift away similar to cold dark matter, and thus cease to act as dark energy. Any stable MaVaNs dark energy model is extremely contrived and is virtually indistinguishable from a cosmological constant.

Dark energy condensation

Abstract: The two most popular candidates for dark energy, i.e. a cosmological constant and quintessence, are very difficult to distinguish observationally, mostly because the quintessence field does not have sizable fluctuations. We study a scalar field model for dark energy in which the scalar field is invariant under reflection symmetry $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{-}\ensuremath{\phi}$. Under general assumptions, there is a phase transition at late times ($z\ensuremath{\lesssim}0.5$). Before the phase transition, the field behaves as a cosmological constant. After the phase transition, a time-dependent $\ensuremath{\phi}$-condensate forms, the field couples with dark matter and develops sizable perturbations tracking those of dark matter. The background cosmological evolution is in agreement with existing observations, but might be clearly distinguished from that of a cosmological constant by future Supernovae surveys. The growth of cosmological perturbations carries the imprint of the phase transition, however a nonlinear approach has to be developed in order to study it quantitatively.

Constraints on Dark Matter interactions from structure formation: damping lengths

Abstract: Weakly Interacting Massive Particles are often said to be the best Dark Matter candidates. Studies have shown that large Dark Matter-photon or Dark Matter-baryon interactions could be allowed by cosmology. Here we address the question of the role of the Dark Matter interactions in more detail to determine at which extent Dark Matter has to be necessarily weakly interacting. To this purpose, we compute the collisional damping (and free-streaming) scales of generic interacting Dark Matter candidates and investigate the effects on structure formation. Our calculations are valid provided the Dark Matter particles have experienced a phase of statistical equilibrium at some stage during their evolution. By comparing these damping lengths to the scale of the smallest primordial structures known to exist in the Universe, we obtain necessary conditions that any candidate must satisfy. These conditions are expressed in terms of the Dark Matter particles' mass and either the total Dark Matter interaction rate or the interaction rate of Dark Matter with a specific species. The case of Dark Matter interacting with neutrinos or photons is considered in full detail. Our results are valid even for energy dependent cross-sections and for any possible initial fluctuations spectrum. We point out the existence of new Dark Matter scenarios and exhibit new damping regimes. For example, an interacting candidate may bear a similar damping than that of collisionless Warm Dark Matter particles. The main difference is due to the Dark Matter coupling to interacting (or even freely-propagating) species. Our approach yields a general classification of Dark Matter candidates which extends the definitions of the usual Cold, Warm and Hot Dark Matter scenarios when interactions, weak or strong, are considered.

Neutrino masses and the dark energy equation of state: relaxing the cosmological neutrino mass bound.

Abstract: At present, cosmology provides the nominally strongest constraint on the masses of standard model neutrinos. However, this constraint is extremely dependent on the nature of the dark energy component of the Universe. When the dark energy equation of state parameter is taken as a free (but constant) parameter, the neutrino mass bound is (95% C.L.), compared with (95% C.L.) in the standard model where the dark energy is in the form of a cosmological constant. This has important consequences for future experiments aimed at the direct measurement of neutrino masses. We also discuss prospects for future cosmological measurements of neutrino masses.

Interacting dark energy and cosmological equations of state

Abstract: Interactions within the cosmic medium modify its equation of state. We discuss implications of interacting dark energy models both for the spatially homogenous background and for the perturbation dynamics.

Time-dependent models for dark matter at the galactic center

Abstract: The prospects for indirect detection of dark matter at the galactic center with $\ensuremath{\gamma}$-ray experiments like the space telescope GLAST, and air Cherenkov telescopes like HESS, CANGAROO, MAGIC and VERITAS depend sensitively on the mass profile within the inner parsec. We calculate the distribution of dark matter on subparsec scales by integrating the time-dependent Fokker-Planck equation, including the effects of self-annihilations, scattering of dark matter particles by stars, and capture in the supermassive black hole. We consider a variety of initial dark matter distributions, including models with very high densities (``spikes'') near the black hole, and models with ``adiabatic compression'' of the baryons. The annihilation signal after ${10}^{10}\text{ }\text{ }\mathrm{yr}$ is found to be substantially reduced from its initial value, but in dark matter models with an initial spike, order-of-magnitude enhancements can persist compared with the rate in spike-free models.

Cosmological structure formation in a homogeneous dark energy background

Abstract: There is now strong evidence that the current energy density of the Universe is dominated by dark energy with an equation of state w

Gravitational theory, galaxy rotation curves and cosmology without dark matter

Abstract: Einstein gravity coupled to a massive skew symmetric field Fμνλ leads to an acceleration law that modifies the Newtonian law of attraction between particles. We use a framework of non-perturbative renormalization group equations as well as observational input to characterize special renormalization group trajectories to allow for the running of the effective gravitational coupling G and the coupling of the skew field to matter. Strong renormalization effects occur at large and small momentum scales. The latter lead to an increase of Newton's constant at large galactic and cosmological distances. For weak fields a fit to the flat rotation curves of galaxies is obtained in terms of the mass (mass-to-light ratio M/L) of galaxies. The fits assume that the galaxies are not dominated by exotic dark matter and that the effective gravitational constant G runs with the distance scale. The equations of motion for test particles yield predictions for the solar system and the binary pulsar PSR 1913+16 that agree with the observations. The gravitational lensing of clusters of galaxies can be explained without exotic dark matter. A Friedmann–Lemaitre–Robertson–Walker cosmological model with an effective G = G(t) running with time can lead to consistent fits to cosmological data without assuming the existence of exotic cold dark matter.

Growth of perturbations in dark matter coupled with quintessence

Abstract: We consider the evolution of linear perturbations in models with a nonminimal coupling between dark matter and scalar field dark energy. Growth of matter inhomogeneities in two examples of such models proposed in the literature are investigated in detail. Both of these models are based on a low-energy limit of effective string theory action, and have been previously shown to naturally lead to late acceleration of the universe. However, we find that these models can be ruled out by taking properly into account the impact of the scalar field coupling on the formation of structure in the dark matter density. In particular, when the transition to acceleration in these models begins, the interaction with dark energy enchances the small scale clustering in dark matter much too strongly. We discuss the the role of an effective small scale sound speed in such models with a coupled dark sector.