Showing papers on "Dark fluid published in 1999"

Power Spectra for Cold Dark Matter and its Variants

Abstract: The bulk of recent cosmological research has focused on the adiabatic cold dark matter model and its simple extensions. Here we present an accurate —tting formula that describes the matter transfer func- tions of all common variants, including mixed dark matter models. The result is a function of wavenum- ber, time, and six cosmological parameters: the massive neutrino density, number of neutrino species degenerate in mass, baryon density, Hubble constant, cosmological constant, and spatial curvature. We show how observational constraintse.g., the shape of the power spectrum, the abundance of clusters and damped Lya systems, and the properties of the Lya forestcan be extended to a wide range of cosmologies, which includes variations in the neutrino and baryon fractions in both high-density and low-density universes. Subject headings: cosmology: theorydark matterlarge-scale structure of universe

Environmental influences on dark matter haloes and consequences for the galaxies within them

Abstract: We use large N-body simulations of dissipationless gravitational clustering in cold dark matter (CDM) cosmologies to study whether the properties of dark matter halos are affected by their environment. We look for correlations between the masses, formation redshifts, concentrations, shapes and spins of halos and the overdensity of their local environment. We also look for correlations of these quantities with the local tidal field. Our conclusion is extremely simple. Only the mass distribution varies as a function of environment. This variation is well described by a simple analytic formula based on the conditional Press-Schechter theory. We find no significant dependence of any other halo property on environment. Our results do not depend on our choice of cosmology. According to current hierarchical models, the structure and evolutionary history of a galaxy is fully determined by the structure and evolutionary history of the dark halo in which it is embedded. If these models are correct, clustering variations between galaxies of differing morphological types, luminosities, colours and surface brightnesses, must all arise because the halo mass function is skewed towards high mass objects in overdense regions of the Universe and towards low mass objects in underdense regions.

Is the Dark Matter a Solid

Abstract: A smooth unclustered dark matter component with negative pressure could reconcile a flat universe with the many observations that find a density in ordinary, clustered matter well below the critical density and also explain the recent high red-shift supernova data suggesting that the expansion of the universe is now accelerating. For a perfect fluid negative pressure leads to instabilities that are most severe on the shortest scales. However, if instead the dark matter is a solid, with an elastic resistance to pure shear deformations, an equation of state with negative pressure can avoid these short wavelength instabilities. Such a solid may arise as the result of different kinds of microphysics. Two possible candidates for a solid dark matter component are a frustrated network of non-Abelian cosmic strings or a frustrated network of domain walls. If these networks settle down to an equilibrium configuration that gets carried along and stretched by the Hubble flow, equations of state result with $w=\ensuremath{-}1/3$ and $w=\ensuremath{-}2/3,$ respectively. One expects the sound speeds for the solid dark matter component to comprise an appreciable fraction of the speed of light. Therefore, the solid dark matter does not cluster, except on the very largest scales, accessible only through observing the large-angle CMB anisotropy. In this paper we develop a generally covariant, continuum description for the dynamics of a solid dark matter component. We derive the evolution equations for the cosmological perturbations in a flat universe with $\mathrm{C}\mathrm{D}\mathrm{M}+(\mathrm{solid})$ and compute the resulting large-angle CMB anisotropy. The formalism presented here applies to any generalized dark matter with negative pressure and a nondissipative resistance to shear.

Scalar Field as Dark Matter in the Universe

Abstract: We investigate the hypothesis that the scalar field is the dark matter and the dark energy in the Cosmos, wich comprises about 95% of the matter of the Universe. We show that this hypothesis explains quite well the recent observations on type Ia supernovae.

Noninteracting dark matter

Abstract: Since an acceptable dark matter candidate may interact only weakly with ordinary matter and radiation, it is of interest to consider the limiting case where the dark matter interacts only with gravity and itself, the matter originating by the gravitational particle production at the end of inflation. We use the bounds on the present dark mass density and the measured large-scale fluctuations in the thermal cosmic background radiation to constrain the two parameters in a self-interaction potential that is a sum of quadratic and quartic terms in a single scalar dark matter field that is minimally coupled to gravity. In quintessential inflation, where the temperature at the end of inflation is relatively low, the field starts acting like cold dark matter relatively late, shortly before the epoch of equal mass densities in matter and radiation. This could have observable consequences for galaxy formation. We respond to recent criticisms of the quintessential inflation scenario, since these issues also apply to elements of the noninteracting dark matter picture.

Constraints on a non-Gaussian χm2 cold dark matter model

Abstract: We consider constraints on the structure formation model based on non-Gaussian fluctuations generated during inflation, which have distributions. Using three data sets, the abundance of the clusters at z=0, moderate z and the correlation length, we show that constraints on the non-Gaussianity and the amplitude of fluctuations and the density parameter can be obtained. We obtain an upper bound forΩm, and a lower bound for the non-Gaussianity and the amplitude of the fluctuations. Using the abundance of clusters at z∼0.6, for the spectrum parametrized by cold dark matter (CDM) shape parameter Γ=0.23, we obtain an upper bound for the density parameter of Ωm∼0.5 and lower bounds for the amplitude of σ8∼0.7 and for the non-Gaussianity of fluctuations ofG∼2 (m∼200), where G=1 for Gaussian.

A Fundamental Test of the Nature of Dark Matter

Abstract: Dark matter may consist of weakly interacting elementary particles or of macroscopic compact objects. We show that the statistics of the gravitational lensing of high-redshift supernovae strongly discriminate between these two classes of dark matter candidates. We develop a method of calculating the magnification distribution of supernovae, which can be interpreted in terms of the properties of the lensing objects. With simulated data, we show that 50 well-measured Type Ia supernovae (Δm ~ 0.2 mag) at redshifts ~1 can clearly distinguish macroscopic from microscopic dark matter if Ω0 0.2 and all dark matter is in one form or the other.

Scalar fields as dark matter in spiral galaxies: comparison with experiments

Abstract: We present an exact solution for a static and axially symmetric spacetime, which is obtained from a scalar-tensor theory that comes from unification theories. As an attempt to model the dark matter (DM) in spiral galaxies we find that an exponential scalar potential is enough to explain the rotation curves in such galaxies. We also present the fitting to the rotation curve of six spiral galaxies and we find an excellent agreement between observational data and the results of our model.

Domain Wall Dominated Universes

Abstract: We consider a cosmogony with a dark matter component consisting of a network of frustrated domain walls. Such a network provides a solid dark matter component with $p=-(2/3)\rho $ that remains unclustered on small scales and with $\Omega_{\rm dw}\approx 0.7$ can reconcile a spatially flat universe with the many observations indicating $\Omega_{\rm m}\approx 0.3.$ Because of its large negative pressure, this component can explain the recent observations indicating an accelerating universe without recourse to a non-vanishing cosmological constant. We explore the viability of this proposal and prospects for distinguishing it from other kinds of proposed dark matter with significant negative pressure.

Mapping the Dark Matter with Weak Gravitational Lensing

Abstract: The developments summarized with the name “weak gravitational lensing” have led to exciting possibilities to study the (statistical properties of the) dark matter distribution in the Universe. Concentrating on those aspects which require deep wide-field imaging surveys, I will describe the basic principles of weak lensing and discuss its future application (a) to determine the statistical properties of the dark matter halos of individual galaxies, (b) to measure the power spectrum of the matter distribution in the Universe, (c) to measure the bias parameter and investigate its dependence on scale and/or redshift, and (d) to detect mass (dark) concentrations from their mass properties alone.

Distinguishing between cold dark matter and modified newtonian dynamics: predictions for the microwave background

Abstract: Two hypothesized solutions for the mass discrepancy problem are cold dark matter and modified Newtonian dynamics. The virtues and vices of these very different hypotheses are largely disjointed, making the process of distinguishing between them very dependent on how we weigh disparate lines of evidence. One clear difference is the nature of the principal mass constituent of the universe (cold dark matter or baryons). This difference in the baryon fraction (fb ≈ 0.1 vs. 1) should leave a distinctive signature in the spectrum of fluctuations in the cosmic microwave background. Here I discuss some of the signatures that should be detectable in the near future. The most promising appears to be the ratio of the amplitudes of the first two peaks relative to the intervening trough.

Deciphering the nature of dark matter

Abstract: The history of physics can be seen as the gradual discovery of structures invisible to the human eye and the instruments of the time. These structures are often hinted at first by indirect evidence, and new instrumentation has to be invented to fully establish their existence and to study their properties. The search for the dark matter in the universe represents an archetypal case study of such a process. It took nearly 60 years from the first evidence (Zwicki, 1933) for astronomers to reach a consensus that there is a dark component which dominates gravity but cannot be seen, as it neither emits nor absorbs light. The debate has now shifted to one about its nature, in particular whether dark matter is made of ordinary baryonic matter or whether new nonbaryonic components play a significant role. A number of innovative attempts to decipher this nature have been launched. The searches for baryonic forms of dark matter, including the evidence for massive halo compact objects (MACHOs), and for nonbaryonic forms are reviewed. The numerous attempts to detect weakly interactive, massive particles (WIMPs) are presented as an example of the novel instruments necessary to make progress in this new field of astrophysics. Because of space constraints, references are limited to recent reviews or representative works in each of the areas.

Superheavy dark matter from thermal inflation

Abstract: It is quite plausible that the mass of the dark matter particle increases significantly after its freeze-out, due to a scalar field rolling to large values. We describe a realization of this scenario in the context of thermal inflation which naturally gives a cold dark matter particle with the correct cosmological abundance and a mass around ${10}^{10}\mathrm{GeV},$ evading the conventional upper bound of ${10}^{5}\mathrm{GeV}.$ We also discuss another realization which could produce a cosmologically interesting abundance of near Planck mass, possibly electromagnetically charged, particles. The detection and observational consequences of superheavy cold dark matter or wimpzillas are briefly examined.

The Bright Side of Dark Matter

Abstract: We show that it is not possible in the absence of dark matter to construct a four-dimensional metric that explains galactic observations. In particular, by working with an effective potential it is shown that a metric which is constructed to fit flat rotation curves in spiral galaxies leads to the wrong sign for the bending of light i.e. repulsion instead of attraction. Hence, without dark matter the motion of particles on galactic scales cannot be explained in terms of geodesic motion on a four- dimensional metric. This reveals a new bright side to dark matter: it is indispensable if we wish to retain the cherished equivalence principle.

Cosmology solved? Maybe

Abstract: For two decades the hot big-bang model as been referred to as the standard cosmology - and for good reason. For just as long cosmologists have known that there are fundamental questions that are not answered by the standard cosmology and point to a grander theory. The best candidate for that grander theory is inflation + cold dark matter. It holds that the Universe is flat, that slowly moving elementary particles left over from the earliest moments provide the cosmic infrastructure, and that the primeval density inhomogeneities that seed all the structure arose from quantum fluctuations. There is now prima facie evidence that supports two basic tenets of this paradigm. An avalanche of high-quality cosmological observations will soon make this case stronger or will break it. Key questions remain to be answered; foremost among them are: identification and detection of the cold dark matter particles and elucidation of the dark-energy component. These are exciting times in cosmology!

Superheavy Dark Matter and Thermal Inflation

Abstract: The thermal inflation is the most plausible mechanism that solves the cosmological moduli problem naturally. We discuss relic abundance of superheavy particle X in the presence of the thermal inflation assuming that its lifetime is longer than the age of the universe, and show that the long-lived particle X of mass 10 12 –10 14 GeV may form a part of the dark matter in the present universe in a wide region of parameter space of the thermal inflation model. The superheavy dark matter of mass ∼ 10 13 GeV may be interesting in particular, since its decay may account for the observed ultra high-energy cosmic rays if the lifetime of the X particle is sufficiently long.

A Scalar Field as a Candidate for the Cosmological Non-baryonic Dark Matter

Abstract: We present an up-to-date report on the status ofthe cosmological model based on a massive scalar fieldnon-minimally coupled to gravity, which has beenrecently used to explain the apparent periodicity in the distribution of galaxies, and as a modelfor the missing non-baryonic component of dark matter,within the standard inflationary scenario (Ω = 1).The model agrees with most cosmological observations, however local experiments can pose seriousconstraints that indicate the necessity of a slightlymodified model.

Dark Matter and its Particle Candidates

Abstract: In these lectures we first briefly review the main observational facts which imply that most part of matter in the Universe is not visible and some recent intriguing experimental data which would point to a significant contribution to Omega due to a cosmological constant. We subsequently discuss some particle candidates for dark matter, with particular emphasis for the neutralino. We present the main properties of this particle, also in the light of the most recent experimental results in direct search for relic particles; furthermore, we discuss the perspectives for their indirect searches.

Non-Baryonic Dark Matter -- A Theoretical Perspective

Abstract: I review axions, neutralinos, axinos, gravitinos and super-massive Wimpzillas as dark matter candidates.

Cold and Hot Dark Matter from a Single Nonthermal Relic

Abstract: The origin of dark matter in the Universe may be scalar particles produced by amplification of quantum fluctuations during a period of dilaton-driven inflation. We show, for the first time, that a single species of such particles, depending on its mass and interactions, can be a source of both cold and hot dark matter simultaneously. Detection of such weakly interacting particles with masses below a fraction of an eV presents a new challenge for dark matter searches. {copyright} {ital 1999} {ital The American Physical Society}

Galactic halos of self-interacting dark matter

Abstract: Recent, very accurate simulations of galaxy formation have revealed that the standard cold dark matter model has great difficulty in explaining the detailed structure of galaxies. One of the major problems is that galactic halos are too centrally concentrated. Dark matter self-interactions have been proposed as a possible means of resolving this inconsistency. Here, we investigate quantitatively the effect of dark matter self interactions on formation of galactic halos. Our numerical framework is extremely simple, while still keeping the essential physics. We confirm that strongly self-interacting dark matter leads to less centrally concentrated structures. Interestingly, we find that for a range of different interaction strengths, the dark matter halos are unstable to particle ejection on a timescale comparable to the Hubble time.

Dark matter from Affleck-Dine baryogenesis

Abstract: Fragmentation of the Affleck-Dine condensate into Q-balls could fill the Universe with dark matter either in the form of stable baryonic balls, or LSP produced from the decay of unstable Q-balls. The dark matter and the ordinary matter in the Universe may share the same origin.

Galactic propagation of positrons from particle dark-matter annihilation

Abstract: We have made a calculation of the propagation of positrons from dark-matter particle annihilation in the Galactic halo for different models of the dark matter halo distribution using our 3D code. We show that the Green's functions are not very sensitive to the dark matter distribution for the same local dark matter energy density. We compare our predictions with computed cosmic ray positron spectra ("background") for the "conventional" cosmic-ray nucleon spectrum which matches the local measurements, and a modified spectrum which respects the limits imposed by measurements of diffuse Galactic gamma-rays, antiprotons, and positrons. We conclude that significant detection of a dark matter signal requires favourable conditions and precise measurements unless the dark matter is clumpy which would produce a stronger signal. Although our conclusion qualitatively agrees with that of previous authors, it is based on a more realistic model of particle propagation and thus reduces the scope for future speculations. Reliable background evaluation requires new accurate positron measurements and further developments in modelling production and propagation of cosmic ray species in the Galaxy.

Derivation of the Tully-Fisher Law from General Relativity Theory: Doubts about the Existence of Halo Dark Matter

Abstract: Observations show that for disk galaxies the fourth power of the circular velocity of stars around the core of the galaxy is proportional to the luminosity L. This is known as the Tully-Fisher law. Since L is proportional to the mass M of the galaxy, it follows that the fourth power of the circular velocity is proportional to M. Newtonian mechanics, however, tells that the square of the circular velocity is proportional to M. In order to rectify this big difference, astronomers assume the existence of halo dark matter. In this paper we show that general relativity theory yields a term of the Tully-Fisher form. This puts doubts about the necessity and existence of halo dark matter for galaxies.