Showing papers on "Dark fluid published in 1987"

Estimates of the density of dark matter near the center of the Galaxy.

Abstract: It was recently pointed out by Silk and Bloemen that the COS-B satellite observations of the cosmic ..gamma..-ray flux are sensitive to ..gamma.. rays from dark-matter annihilation at the center of the Galaxy if the dark matter is composed of stable weakly interacting particles such as photinos, Higgs fermions, scalar neutrinos, or heavy neutrinos and if the dark-matter content of the central component of the Galaxy is more than about 2%. Motivated by this remark, we have studied how much dark matter is expected to be near the center of the Galaxy under two different assumptions which we call the isothermal and adiabatic models. In the first model, the dark matter is assumed to be isothermally distributed. In the second model, it is assumed that while the central component forms the adiabatic invariant for the radial motion of each dark-matter particle, as well as its angular momentum, is conserved. In the isothermal model, the typical dark-matter density enhancement near the center of the Galaxy is large enough to make the COS-B data sensitive to the hypothesis that the dark matter is composed of stable weakly interacting particles, whereas in the adiabatic model it is about a factor of 5 toomore » small. We show that in the adiabatic model the density of dark-matter particles near the center of the Galaxy is proportional to (-phi/sub c/(r))/sup 3/2/ where phi/sub c/(r) is the gravitational potential due to the central component.« less

Formation and evolution of dwarf galaxies and Lyman-alpha clouds - Two-component dark matter universe. I

Abstract: The formation and evolution of subgalactic objects formed prior to pancaking of hot particles are studied along with the formation and evolution of associated Lyman-alpha absorbing clouds and of dwarf galaxies in which cold dark matter dominates. The scenario for the formation of astronomical objects of various scales and their evolution based upon a two-component dark matter-dominated universe is presented, and the gravitational equilibrium of a baryon cloud embedded in a cold dark matter system is examined. The evolution of collapsed clouds, especially star formation bursts and the resulting gas expulsion, is explored. Various correlation parameters for dwarf galaxies are derived, and ionization and heating of the IGM by energy input from these subgalactic objects are analyzed. Predicted properties of Lyman-alpha clouds are derived from these models. 47 references.

New limits to bias and the amount of dark matter in the universe

Abstract: The observed thermodynamic distribution function f(N) for galaxies places significant constraints on the amount of structured dark matter in the universe. The simplest models of cold dark matter require the cosmological density parameter Omega(0) of less than about 0.4. Biased galaxy formation in more complicated models must have specific forms which depend on the amount and structure of dark matter in the model. 20 references.

A lower limit to the mass of neutral leptons as constituents of the dark halo around galaxies

Abstract: We have found that the structural equations governing the hydrostatic equilibrium of a thermally relaxed, spherically-symmetric neutral lepton system with a density profile that can account for the dark matter distribution around spiral galaxies donot admit any physically reasonable solutions (namely the temperature should be positive definite and monotonically decreasing with distance) if the lepton mass is ≲17 eV

Dark Matter in the Universe

Abstract: It is a prime goal of cosmology to un­ derstand how the Universe has evolved from an initial dense fireball about 1010 years ago to its present state, where galaxies, grouped into clusters, domi­ nate the large-scale cosmic scene. An impediment to understanding ga­ laxies and their spatial distribution is that the stars and gas we observe may be little more than a tracer for the mate­ rial that is dynamically dominant. The evidence for 'dark matter' dates back more than fifty years, to the time when Fritz Zwicky realised that the aggregate mass of the observed galaxies in the Coma cluster was inadequate, by a wide margin, to prevent the system from fly­ ing apart. The case is now quite compell­ ing (for a recent review 1), see the pro­ ceedings of IAU Symposium 117); what the dark matter consists of is, however, still a mystery. The masses inferred from relative mo­ tions of galaxies in apparently-bound groups and clusters exceed by a factor about 10 those inferred from the internal dynamics of the luminous parts of ga­ laxies. This apparent discrepancy could be resolved if galaxies were embedded in extensive dark 'halos'. This hypothesis can be checked in some edge-on disc galaxies, where emission from gas can be observed at radii far exceeding the ex­ tent of the conspicuous stellar disc. The mass of this gas is itself negligible, but rotation velocities derived from its spec­ tral lines do not fall off as r-1/2, as would be expected if the gas were orbiting a mass distribution concentrated at much smaller radii. Instead, the velocity re­ mains almost constant, implying that M(< r) r, out to ≈ 80 kpc in some cases (see Fig. 1). Estimates of the masses of galaxy clusters come from the virial theorem : the gravitational binding energy must be half the internal kinetic energy if they are to be in equilibrium. This method is now complemented by X-ray studies of ther­ mal emission from hot gas in clusters, which probe the depth of the gravita­ tional potential well. In well-studied clusters which appear to have reached virial equilibrium, the mass-to-light ratio M/L is typically 200 solar units. The dark matter must mainly be in some un­ known form — neither ordinary stars nor the gas that emits X-rays suffices. The data are summarised in Fig. 2. All things considered, the existence of dark matter is quite unsurprising — there are all too many forms it could take, and the aim of observers and theorists must be to narrow down the options. The present Hubble timescale tH is still uncertain, so in quoting numerical values I shall follow a widespread con­ vention and introduce a useful quantity h = (3 x 1017 s/tH). The experts advo­ cate values of h in the range 0.5-1. I shall also refer to average densities of matter in terms of Ω, fractions of the cri­ tical density ρcrit (kg/m3) = (8πGt2H/3)-1 above which the Universe would just close. This density were it all in baryonic form would correspond to a particle den­ sity of 11 h2/m3 so the real average den­ sity nb becomes nb = 11 Ωbh2/m3. An important number which is perhaps of fundamental significance is the ratio of the baryon density to the density nγ of photons in the microwave background, apparently a black body with T ≈ 2.7 K. This is then 3 x 10-8 (Ωbh2); it is a num­ ber that grand unified theories (GUTs) must attempt to explain in terms of an in­ herent favouritism of matter over anti­ matter at the ultra-early epoch when symmetry-breaking occurred. A direct lower limit on Ωb of order 10-2 can be set from the observed mean den­ sity of baryons in conspicuously 'lumi­ nous' form (visible stars and gas in ga­ laxies, and intergalactic gas revealed by its X-ray emission), implying a baryonphoton ratio of not less than 3 x 10-10h2. The inferred dark matter in the halos of individual galaxies and in clusters of galaxies apparently contributes a frac­ tion Ω = 0.1-0.2 of the critical cosmo­ logical density. Its smoother and less clumped distribution suggests that it underwent less dissipation during the processes of galaxy formation than the luminous stars and gas. Three strands of evidence could even­ tually pin down what the dark matter in halos and clusters of galaxies really is. (i) Particle physics. When our theories of high-energy physics become less speculative, and we can calculate how many particles of each species (with known mass) should have survived as relics of the big bang.

Gravitational lenses and dark matter - Theory

Abstract: Theoretical models are presented for guiding the application of gravitational lenses to probe the characteristics of dark matter in the universe. Analytical techniques are defined for quantifying the mass associated with lensing galaxies (in terms of the image separation), determining the quantity of dark mass of the lensing bodies, and estimating the mass density of the lenses. The possibility that heavy halos are made of low mass stars is considered, along with the swallowing of central images of black holes or cusps in galactic nuclei and the effects produced on a lensed quasar image by nonbaryonic halos. The observable effects of dense groups and clusters and the characteristics of dark matter strings are discussed, and various types of images which are possible due to lensing phenomena and position are described.

Fundamental Physics and Dark Matter

Abstract: For the past ten years or so there has been a lot of speculation about the existence and behavior of nonbaryonic dark matter. In this talk I would like to re-examine the evidence for a non-baryonic dark component, contrasting the theoretical prejudice that Ω = 1 with the Big Bang Nucleosynthesis constraints on the amount of baryonic matter, Ω b ≤ 0.2. Assuming then that such a component does exist, I will present a list of candidates from particle physics to explain this dark matter, and illustrate what I find to be a most exciting development, the fact that all the most likely candidates stand to be ruled out or even detected in the next five to ten years.

Gravitational lenses and dark matter - Observations

Abstract: Following a few general comments on gravitational lenses from an observer's perspective, the currently available observations of the six known gravitational lenses are summarized Attention is called to some regularities and peculiarities of the properties of the known lenses and to how they might be interpreted The most important conclusions, relevant to the dark matter problem, which can be obtained from current observations are that the distributions of mass and light appear to be quite different in at least some of the lensing objects and that objects with projected mass/brightness values about 10 times larger than those ordinarily associated with galaxies exist and are not too rare

Axions in the Universe

Abstract: The observational evidence for the presence of dark matter is now generally accepted, with no lack of possible candidates (see e. g. Dekel, Einasto, and Rees 1986). The proposed candidates are devided into two groups, baryonic and non-baryonic. The latter is further devided to hot and cold dark matter. For the cold dark matter, among the first to be proposed is the axion. In this paper we shall not dwell on numerous cold dark matter candidates offered by particle physicists, for there are review articles on the subject (see e. g. Turner 1986, Primack 1986). The main purpose of the present report is to suggest that neutron star cooling theory and future space satellite programs (e. g. AXAF, XAO, LXAO) have a potential for offering the best astrophysical constraint on the axion mass and hence, giving valuable insight to some cosmological problems

Unstable Dark Matter and Galaxy Formation

Abstract: Inflation has become a very attractivecosmological model because it can naturally resolve major cosmological puzzles that the standard Big Bang Cosmology leaves unanswered, such as the flatness, isotropy and homogeneity of the Universe and the origin of density perturbations; inflation naturally produces the scale-invariant, Harrison-Zeldovich spectrum of density perturbations as well. Because of the tremendous expansion that the Universe undergoes at very early times in the inflationary scenario, it is predicted to be extremely flat at present; if the cosmological constant vanishes, this implies that the mean energy density of the Universe ρ, in units of the critical density ρ c, is Ω ≡ ρ/ρ c = 1 to a very high accuracy. Observations, on the hand, indicate that the clustered matter (including dark matter) on scales of up to a few megaparsecs amounts only to Ω cl = 0.1 - 0.3 [1,2]. Several solutions to this puzzle have been proposed, such as a relic cosmological constant Λ [3] such that Ω + Λ/8πGρ c = 1, or “biased” galaxy formation [4], in which it is postulated that galaxies are rare events, resulting only from large fluctuations of the density field -thus they would not be good tracers of mass in the Universe.