Showing papers on "Dark fluid published in 1996"

Bulk viscous cosmology.

Abstract: We propose a scenario in which the dark components of the Universe are manifestations of a single bulk viscous fluid. Using dynamical system methods, a qualitative study of the homogeneous, isotropic background scenario is performed in order to determine the phase space of all possible solutions. The specific model which we investigate shares similarities with a generalized Chaplygin gas in the background but is characterized by nonadiabatic pressure perturbations. This model is tested against supernova type Ia and matter power spectrum data. Different from other unified descriptions of dark matter and dark energy, the matter power spectrum is well behaved, i.e., there are no instabilities or oscillations on small perturbation scales. The model is competitive in comparison with the currently most popular proposals for the description of the cosmological dark sector.

Cold Dark Matter

Abstract: Motivated by inflation, the theory of big-bang nucleosynthesis, and the quest for a deeper understanding of fundamental forces and particles, a paradigm for the development of structure in the universe has evolved. It holds that most of the matter exists in the form of slowly moving elementary particles left over from the earliest moments—cold dark matter—and that the small density inhomogeneities that seed structure formation arose from quantum fluctuations around 10−34 seconds after the big bang. A flood of observations, from determinations of the Hubble constant to measurements of the anisotropy of cosmic background radiation, are now testing the cold dark matter paradigm.

Discovery of a hierarchical distribution of dark matter in the Fornax cluster of galaxies

Abstract: THE mass of the Universe is widely believed to be dominated by dark matter1. Dark matter is, by its very nature, extremely difficult to investigate, and the presence of dark matter is usually inferred indirectly through its gravitational influence on ordinary visible matter. Observations of X-ray-emitting gas associated with clusters of galaxies can help to constrain the large-scale distribution of dark matter; if the gas is in hydrostatic equilibrium with the gravitational potential of the cluster, it will trace the distribution of all matter present (dark and visible)2. Here we present X-ray observations of gas in the Fornax cluster of galaxies, which show that dark matter is distributed on at least two distinct length scales: the scale of the dominant central galaxy and that of the cluster as a whole. This suggests the presence of either a single form of dark matter exhibiting hierarchical clustering (analogous to the hierarchical distribution of visible matter), or two forms of dark matter which interact—and hence cluster—through different unknown mechanisms.

Particle Dark Matter

Abstract: There is plenty of evidence that most matter in the Universe is dark (non-luminous). Particle physics offers several possible explanations. In this talk I focus on cold dark matter; the most promising candidates are then axions and the lightest supersymetric particle. I briefly summarize estimates of the present relic density of these particles, and describe efforts to detect them

A Dark Matter Solution from the Supersymmetric Axion Model.

Abstract: We study the effect of the late decaying saxino (the scalar superpartner of the axion) and find out that there is a possible dark matter solution from a class of supersymmetric extensions of the invisible axion model. In this class of models, the saxino which decays into two axions acts as the late decaying particle which reconciles the cold dark matter model with high values of the Hubble constant. Recent observations of the Hubble constant are converging to H{sub 0} = 70--80 km sec{sup {minus}1} Mpc{sup {minus}1}, which would be inconsistent with the standard mixed dark matter model. This class of models provides a plausible framework for the alternative cold dark matter plus late decaying particle model, with the interesting possibility that both cold dark matter and the extra radiation consist of axion. 15 refs.

Experimental limits to the density of dark matter in the solar system

Abstract: On the scales of galaxies and beyond there is evidence for unseen dark matter. In this paper we nd the experimental limits to the density of dark matter bound in the solar system by studying its eect upon planetary motion.

Baryons, Dark Matter, and the Jeans Mass in Simulations of Cosmological Structure Formation

Abstract: We investigate the properties of hybrid gravitational/hydrodynamical simulations, examining both the numerics and the general physical properties of gravitationally driven, hierarchical collapse in a mixed baryonic/dark matter fluid We demonstrate that, under certain restrictions, such simulations converge with increasing resolution to a consistent solution The dark matter achieves convergence provided that the relevant scales dominating nonlinear collapse are resolved If the gas has a minimum temperature (as expected when intergalactic gas is heated by photoionization due to the ultraviolet background) and the corresponding Jeans mass is resolved, then the baryons also converge However, if there is no minimum baryonic collapse mass or if this scale is not resolved, then the baryon results err in a systematic fashion In such a case, as resolution is increased the baryon distribution tends toward a higher density, more tightly bound state We attribute this to the fact that under hierarchical structure formation on all scales there is always an earlier generation of smaller scale collapses, causing shocks which irreversibly alter the state of the baryon gas In a simulation with finite resolution we miss such earlier generation collapses, unless a physical scale is introduced below which structure formation is suppressed in the baryons We also find that the baryon/dark matter ratio follows a characteristic pattern, such that collapsed structures possess a baryon enriched core (enriched by factors of 2 or more over the universal average) which is embedded within a dark matter halo, even without accounting for radiative cooling of the gas The dark matter is unaffected by changing the baryon distribution (at least in the dark matter dominated case investigated here)

Gravitino Warm Dark Matter Motivated by the CDF $ee\gamma\gamma $ Event

Abstract: The $ee\gamma\gamma +\rlap/E_T$ event observed by the CDF at Fermilab is naturally explained by dynamically supersymmetry breaking models and suggests the presence of the light gravitino which can be a warm dark matter. We consider large scale structure of the universe in the worm dark matter model and find that the warm dark matter plays almost the same role in the formation of the large scale structure as a cold dark matter if its mass is about $0.5$keV. We also study the Ly~$\alpha$ absorption systems which are presumed to be galaxies at high redshifts and show that the baryon density in the damped Ly~$\alpha$ absorption systems predicted by the warm dark matter model is quite consistent with the present observation.

Effects of Dark Matter on the Stability of Supermassive Stars

Abstract: Author(s): McLaughlin, GC; Fuller, GM | Abstract: The stability of nonrotating supermassive stars (M g 105 M⊙) in the presence of a small dark matter component is examined. We find that the destabilizing effects of first-order nonlinear corrections from general relativity can be neutralized when a significant amount of nonrelativistic dark matter is present. In this case, it is possible for supermassive stars to be stabilized through the hydrogen-burning stage. Significant mass loss in this epoch might result in the ejection of the nucleosynthesis products of hot hydrogen burning. Ultimately, however, the post-Newtonian instability can only be delayed. If this instability occurs after the hydrogen-burning epoch, collapse of the star to a black hole is guaranteed. Subject headings: dark matter - gravitation - instabilities nuclear reactions, nucleosynthesis, abundances - relativity - stars: interiors © 1996. The American Astronomical Society. All rights reserved.

Void analysis as a test for dark matter composition

Abstract: We use the void probability function to compare the redshift--space galaxy distribution in the Perseus--Pisces survey with artificial samples from N-body simulations of standard cold dark matter (CDM) and broken scale invariance (BSI) models. Galaxies are identified as residing in peaks of the evolved density field in such a way as to reproduce both the observed luminosity and two--point correlation functions. Using a similar approach, it was recently shown that the VPF can discriminate between CDM and a cold+hot dark matter (CHDM) model with cold/hot/baryon fractions = 0.6/0.3/0.1. Our main result is that both CDM (as expected from a previous analysis) and BSI fit observational data. The robustness of the result is checked against changing the observer's position in the simulations and the galaxy identification in the evolved density field. Therefore, while the void statistics is sensitive to the passage from CDM to CHDM (different spectrum and different nature of dark matter), it is not to the passage from CDM to BSI (different spectrum but same dark matter). On such a basis, we conjecture that the distribution of voids could be directly sensitive to the nature of dark matter, but scarcely sensitive to the shape of the transfer function.

Can Baryonic Dark Matter BE Solid Hydrogen

Abstract: Some requirements are discussed for solid hydrogen formation in cold dark dense clouds in galaxies. If temperatures in the clouds are near the microwave background temperature of 2.7 K and molecular hydrogen densities are 3×105 cm−3 or higher, as suggested by recent observations, it may be possible for solid hydrogen objects to form. Comet size hydrogen solids could build from molecular hydrogen condensation on grains and by collisions. Heated primarily by cosmic rays, objects with 100 km radii could last billions of years. The larger objects may be detectable, in the future, by sensitive gravitational lensing or eclipsing observations. Other possibilities are discussed for future detection of the cold dark dense molecular hydrogen regions. In our model, helium is added along with the hydrogen to preserve the primordial helium to hydrogen mass ratio,Y p , of the standard model. In the hot regions of the universe the solid hydrogen objects sublime and melt so our model predictsY p =0.250, the same as other baryonic dark matter models with identical values of ω=0.1,H o =50 and η=6.8×10−10. This value cannot be ruled out at present because of the large systematic uncertainties in the observed value of 0.232. In the cold dark regions where solid hydrogen objects exist, we predict thatY p will be greater than 0.250. Observations are not yet sensitive enough to measure this ratio.

The Distribution of Dark Mass in Galaxies

Abstract: Gravitational lensing is one of a number of methods used to probe the distribution of dark mass in the Universe. On galactic scales, complementary techniques include the use of stellar kinematics, the kinematics and morphology of the neutral gas layer, kinematics of satellites, and the morphology and temperature profile of X-ray halos. These methods are compared, with emphasis on their relative strengths and weaknesses in constraining the distribution and extent of dark matter in the Milky Way and other galaxies. It is concluded that (1) the extent of dark halos remains ill-constrained, (2) halos need not be isothermal, and (3) the dark mass is probably quite flattened.

Dark Matter Halos

Abstract: The observational evidence for dark halos around galaxies is shortly reviewed. New and old techniques and results constraining the mass, the distribution, the shape and the nature of dark halos are discussed.

The epoch of structure formation in blue mixed dark matter models

Abstract: Recent data on the high–redshift abundance of damped Lyα systems are compared withtheoretical predictions for ‘blue’ (i.e. n > 1) Mixed Dark Matter models. The resultsshow that decreasing the hot component fraction Ω ν and/or increasing the primordialspectral index n implies an earlier epoch of cosmic structure formation. However, wealso show that varying Ω ν and n in these directions makes the models barely consistentwith the observed abundance of galaxy clusters. Therefore, requiring at the same timeobservational constraints on damped Lyα systems and cluster abundance to be satisfiedrepresents a challenge for the Mixed Dark Matter class of models. 1 INTRODUCTIONSince a long time observations of high–redshift objects havebecome a potentially powerful constraint for models of cos-mic structure formation. The availability of statistically re-liable samples of quasars allowed to address this problem ina quantitative way in the framework of the Cold Dark Mat-ter cosmogony (Efstathiou & Rees 1988; Haehnelt 1993).Moreover, the comparison of predictions and observations ofquasar abundance at different redshifts has been used as atest for model reliability (e.g. Nusser & Silk 1993; Pogosyan& Starobinsky 1993).Recently, damped Lyα systems (DLAS) have been rec-ognized as a promising way to trace the presence of highredshift collapsed structures, thanks to the possibility ofidentifying them as protogalaxies and to their detectabil-ity at high z (see Wolfe 1993 for a comprehensive review).DLAS are seen as wide absorbtion features in quasar spec-tra. The associated absorbing systems have a neutral hydro-gen column density ≥ 10

Conformally coupled dark matter

Abstract: Dark matter is obtained from a scalar field coupled conformally to gravitation, the scalar being a relict of Dirac's gauge function. This conformally coupled dark matter includes a gas of very light (m ≈ 2.25 × 10−34 eV) neutral bosons having spin 0, as well as a time-dependent global scalar field, both pervading all of the cosmic space. The time-development of this dark matter in the expanding F-R-W universe is investigated, and an acceptable cosmological behaviour is obtained.

The Shapes of Dark Halos

Abstract: Galactic and extragalactic astronomers often mean different things by “halo”; the former mean the spheroidal distribution of metal poor stars around the Milky Way (MW), the latter mean the massive, dark component providing the large dynamically inferred mass in and around galaxies. This review will focus on massive, dark halos. Despite the title of this conference, we cannot restrict our attention to the shape of the MW halo: very little would be to say. We must turn to cosmological simulations and to observations of external disk galaxies in the hope of deriving a representative picture.

Flat Rotation Curves, Inverse Cascade, and Magnetic Fields

Abstract: We critically examine the recently suggested alternatives to dark matter in spiral galaxies. We look into the inverse cascade in turbulent galactic disks and the magnetic alternative. We conclude that some amount of dark matter is necessary on galactic scales. However, we speculate the possibility that the constancy of the H I-dark matter ratio may have a magnetohydrodynamic explanation.

Galaxy Structure, Dark Matter, and Galaxy Formation

Abstract: The structure of galaxies, the nature of dark matter, and the physics of galaxy formation were the interlocking themes of DM 1996: Dark and Visible Matter in Galaxies and Cosmological Implications. In this conference summary report, I review recent observational and theoretical advances in these areas, then describe highlights of the meeting and discuss their implications. I include as an appendix the lyrics of The Dark Matter Rap: A Cosmological History for the MTV Generation.

Evolution of galaxies along the Hubble sequence

Abstract: Today we have numerous pieces of evidence suggesting that galaxies evolve dynamically along the Hubble sequence through various processes, sometimes over much shorter periods than the standard galaxy age of 10–15 Gyr. Linking this to the known mass components provides new indications on the nature of the galaxy dark matter. Bounds on the amount of dark stars can be given along the spiral sequence, and the existence of large quantities of yet undetected dark gas appears as the most natural option. The recent recognition of the fractal structure of the cold interstellar gas turns out to provide not only a possible solution why much gas can be very cold and clumpy down to very small scales, so hard to detect, but also points toward the generally ignored but fundamental problem of applying statistical mechanics concepts to systems with long range interactions.

Is dark matter created in the gravitation field of galaxies

Abstract: Using the heuristic arguments of quantum physics we describe a new mechanism of the creation of short-living particles from the virtual ones in a stationary gravitation field. The mass of these particles is a function of the intensity of gravitation field. We suppose that the particles created in the gravitation field form a part of the non-baryonic dark matter. Having the intensity of gravitation field in a galaxy we can calculate the density of dark matter created in it by the vacuum quantum fluctuation. We calculate the distribution of this dark matter in a model galaxy and show that its total mass is comparable with the visible mass of the galaxy.

Experimental searches for Dark Matter

Abstract: A brief reminder of the Dark Matter problem, from the Galactic scale to the Universe scale, is done in the Introduction. Then both baryonic (Astrophysics) and non-baryonic (Particle Physics) candidates are discussed. A review of Dark Matter experiments, designed to search for the different types of candidates, represents the bulk of the lecture. Finally an outlook introduces to some concluding remarks.

High-z Constraints on the Cosmic String Models with Hot Dark Matter

Abstract: The number densities of quasars and high-redshift galaxies strictly constrain theories of large-scale structure formation. We compute the comoving space densities of these objects with a cosmic string plus hot dark matter model and compare results with observations. We show that the cosmic string model works considerably better than the standard inflationary hot dark matter theory. We also show that non-Gaussian initial conditions contribute to the number density, and because of this the abundance of objects is higher than expected by the conventional Gaussian Press-Schechter analysis. The cosmic string model with hot dark matter is consistent with the high-redshift observations. However, there might be problems with large-scale structure formation when the power spectrum of strings is compared with the three-dimensional APM spectrum.

Deviations from the Harrison-Zel'dovich spectrum due to the Quark-Gluon to Hadron Transition

Abstract: We investigate the effect of the quark-gluon to hadron transition on the evolution of cosmological perturbations. If the phase transition is first order, the sound speed vanishes during the transition, and density perturbations fall freely. The primordial Harrison-Zel'dovich density fluctuations for scales below the Hubble radius at the transition develop peaks, which grow linearly with the wavenumber, both for the hadron-photon-lepton fluid and for cold dark matter. The large peaks in the spectrum produce cold dark matter clumps of $10^{-8}$ to $10^{-11} M_\odot$.

Majorana Neutrinos as the Dark Matters in the Cold plus Hot Dark Matter Model

Abstract: A simple model of the Majorana neutrino with the see-saw mechanism is studied, assuming that two light neutrinos are the hot dark matters with equal mass of 2.4 eV in the cold plus hot dark matter model of cosmology. We find that the heavy neutrino, which is the see-saw partner with the remaining one light neutrino, can be the cold dark matter, if the light neutrino is exactly massless. This cold dark matter neutrino is allowed to have the mass of the wide range from 5.9 x 10^2 eV to 2.2 x 10^7 eV.

Mixed dark matter with low-mass bosons.

Abstract: We calculate the linear power spectrum for a range of mixed dark matter (MDM) models assuming a massive (few eV) boson, $\phi$, instead of a neutrino as the hot component. We consider both the case where the hot dark matter (HDM) particle is a boson and the cold component is some other unknown particle, and the case where there is only one dark matter particle, a boson, with the cold dark matter (CDM) component in a Bose condensate. Models resembling the latter type could arise from neutrino decays - we discuss some variants of this idea. The power spectra for MDM models with massive bosons are almost identical to neutrino MDM models for a given mass fraction of HDM if the bosons are distinct from their antiparticles ($\phi eq\bar\phi$) and have a temperature like that of neutrinos, whereas models with $\phi=\bar\phi$ tend to overproduce small-scale structure.

Favored Variants of Cold Dark Matter Cosmologies

Abstract: We discuss variants of Cold Dark Matter (CDM) dominated cosmological models that give good agreement with a range of observations. We consider models with hot dark matter, tilt, $\Omega < 1$, or a cosmological constant. We also discuss the sensitivity of the results to other parameters, such as the Hubble parameter and the baryon fraction. We obtain constraints by combining the COBE data, cluster abundances, abundance of damped Lyman-$\alpha$ systems at $z\sim3$, the small-angle Cosmic Microwave Background anisotropy, and the small-scale non-linear power spectrum. We present non-linear power spectra from a new suite of N-body simulations for the ``best-bet'' models from each category.