Physics category

Quantitative Analysis of Voids in Percolating Structures in Two-Dimensional N-Body Simulations

We present in this paper a quantitative method for defining void size in large-scale structure based on percolation threshold density. Beginning with two-dimensional gravitational clustering simulations smoothed to the threshold of nonlinearity, we perform percolation analysis to determine the large scale structure. The resulting objective definition of voids has a natural scaling property, is topologically interesting, and can be applied immediately to redshift surveys.

published on 04-12-2019 - 56 विचार
Quantitative Analysis of Voids in Percolating Structures in Two-Dimensional N-Body Simulations

We present in this paper a quantitative method for defining void size in large-scale structure based on percolation threshold density. Beginning with two-dimensional gravitational clustering simulations smoothed to the threshold of nonlinearity, we perform percolation analysis to determine the large scale structure. The resulting objective definition of voids has a natural scaling property, is topologically interesting, and can be applied immediately to redshift surveys.

published on 22-11-2019 - 70 विचार
The Gravitational Lens System B1422+231: Dark Matter, Superluminal Expansion and the Hubble Constant

A gravitational lens model of the radio quasar B1422+231 is presented which can account for the image arrangement and approximately for the relative magnifications. The locations of the principal lensing mass and a more distant secondary mass concentration were predicted and subsequently luminous galaxies were found at these locations. This argues against the existence of substantial numbers of “dark” galaxies. The model suggests that if the compact radio source is intrinsically superluminal then the observed component motions may be as large as ∼ 100 c with image B moving in the opposite direction to images A and C. The prospects for a measuring the Hubble constant from a model incorporating lens galaxy locations, compact radio source expansion speeds and radio time delays, if and when these are measured, are briefly assessed.

published on 22-11-2019 - 68 विचार
Graviton Mode Function in Inflationary Cosmology

We consider the production of gravitons in an inflationary cosmology by ap- proximating each epoch of change in the equation of state as sudden, from which a simple analytic graviton mode function has been derived. We use this mode func- tion to compute the graviton spectral energy density and the tensor-induced cosmic microwave background anisotropy.

published on 22-11-2019 - 69 विचार
Solar Model Uncertainties, MSW Analysis, and Future Solar Neutrino Experiments

Various theoretical uncertainties in the standard solar model and in the Mikheyev-Smirnov-Wolfenstein (MSW) analysis are discussed. It is shown that two methods of estimating the solar neutrino flux uncertainties are equivalent:

published on 22-11-2019 - 67 विचार
A Liouville String Approach to Microscopic Time and Cosmology

In the non-critical string framework that we have proposed recently, the time t is identified with a dynamical local renormalization group scale, the Liouville mode, and behaves as a statistical evolution parameter, flowing irreversibly from an infrared fixed point - which we conjecture to be a topological string phase - to an ultraviolet one - which corresponds to a static critical string vacuum. When applied to a toy two-dimensional model of space- time singularities, this formalism yields an apparent renormalization of the velocity of light, and a t-dependent form of the uncertainty relation for position and momentum of a test string. We speculate within this framework on a stringy alternative to conventional field-theoretical inflation, and the decay towards zero of the cosmological constant in a maximally-symmetric space.

published on 22-11-2019 - 68 विचार
A Cosmic String Specific Signature on the Cosmic Microwave Background

Using an analytical model for the string network we show that the kurtosis of cosmic microwave background (CMB) temperature gradi- ent maps is a good statistic to distinguish between the cosmic string model and inflationary models of structure formation.

published on 22-11-2019 - 61 विचार
A MORPHOLOGY{COSMOLOGY CONNECTION FOR X{RAY CLUSTERS

We employ N{body/3D gas dynamic simulations of the formation of galaxy clusters to determine whether cluster X{ray morphologies can be used as cosmological constraints. Con rming the analytic expectations of Richstone, Loeb, & Turner, we demonstrate that cluster evolution is sensitive to the cosmological model in which the clusters form. We further show that evolutionary di erences are echoed in the gross morphological features of the cluster X{ray emission.

published on 22-11-2019 - 72 विचार
Do gamma-ray bursts come from the Oort cloud?

We examine the possibility that gamma-ray bursts arise from sources in the Oort comet cloud, basing most of our arguments on accepted models for the formation and spatial distribution of the cloud

published on 22-11-2019 - 63 विचार
SOME STATISTICS FOR MEASURING LARGE-SCALE STRUCTURE

Good statistics for measuring large-scale structure in the Universe must be able to distinguish between different models of structure formation. In this paper, two and three dimensional “counts in cell” statistics and a new “discrete genus statis- tic” are applied to toy versions of several popular theories of structure formation: random phase cold dark matter model, cosmic string models, and global texture scenario. All three statistics appear quite promising in terms of differentiating between the models.

published on 22-11-2019 - 54 विचार
Gamma-Ray Bursts and Topology of the Universe

In this letter we propose a physical explanation for recently reported correla- tions between pairs of close and antipodal gamma-ray bursts from publicly avail- able BATSE catalogue. Our model is based on the cosmological scenario in which bursters are located at cosmological distances of order of 0.5–2 Gpc. Observed dis- tribution of gamma-ray bursts strongly suports this assumption. If so gamma-ray bursts may provide a very good probe for investigating the topological structure of the Universe. We notice that correlation between antipodal events may in fact indicate that we live in the so called Ellis’ small universe which has Friedman- Roberston-Walker metric structure and nontrivial topology.

published on 22-11-2019 - 63 विचार
ON THE LIMITATIONS OF NEUTRINO EMISSIVITY FORMULA OF IWAMOTO

The neutrino emissivity from two and three flavour quark matter is numerically cal- culated and compared with Iwamoto’s formula

published on 22-11-2019 - 51 विचार
POTENT AND MAX-FLOW ALGORITHMS

Although Potent purports to use only radial velocities in retrieving the potential ve- locity eld of galaxies, the derivation of transverse components is implicit in the smoothing procedures. Thus the possibility of using nonradial line integrals to derive the velocity eld arises. In the case of inhomogeneous distributions of galaxies, the optimal path for integration need not be radial, and can be obtained by using max- ow algorithms. In this paper we present the results of using Dijkstra's algorithm to obtain this optimal path and velocity eld.

published on 22-11-2019 - 46 विचार
BIAS IN VELOCITY FIELD RECOVERIES

We investigate the e ect of using di erent distance estimators on the recovery of the peculiar velocity eld of galaxies using Potent. An inappropriate choice of distance estimator will give rise to spurious ows. We discuss methods of minimising these biases and the levels of accuracy required of distance estimators to retrieve velocity elds to a given standard.

published on 22-11-2019 - 55 विचार
BAYESIAN ESTIMATES OF THE LARGE-SCALE VELOCITY FIELD IN REAL SPACE AND REDSHIFT SPACE

Methods for inferring the velocity eld from the peculiar velocity data are described and applied to old and newer data. Inhomogeneous Malmquist bias and ways to avoid it are discussed and utilized. We infer that these biases are probably important in interpreting the data.

published on 22-11-2019 - 60 विचार
SELF{GRAVITY AND DISSIPATION IN POLAR RINGS

Studies of inclined rings inside galaxy potentials have mostly considered the in- uence of self{gravity and viscous dissipation separately. In this study, we construct models of highly-inclined (\polar") rings in an external potential including both self{ gravity and dissipation due to a drag force.

published on 22-11-2019 - 58 विचार
Lowered Evans Models: Analytic Distribution Functions of Oblate Halo Potentials

We describe an analytic distribution function of a nite, oblate stellar system that is useful for the practical modelling of dark halos. The function is determined by lowering Evans's (1993) distribution function of a attened, cored isothermal system in analogy to the lowering of the singular, isothermal sphere in the de nition of the King (1966) model

published on 22-11-2019 - 56 विचार
NATURAL INFLATION

A pseudo-Nambu-Goldstone boson, with a potential of the form V (φ) = Λ4 [1 + cos(φ/f)], can naturally give rise to an epoch of inflation in the early universe (Freese, Frieman, and Olinto 1990). The potential is naturally flat (as required by microwave background limits on the amplitude of density fluctuations), without any fine-tuning. Successful inflation can be achieved if f ∼ mpl and Λ ∼ mGUT . Such mass scales arise in particle physics models with a large gauge group that becomes strongly interacting at a scale ∼ Λ, e.g., as can happen in the hidden sector of superstring theories.

published on 22-11-2019 - 46 विचार
GENERAL ANALYTIC RESULTS FOR NONLINEAR WAVES AND SOLITONS IN MOLECULAR CLOUDS

We study nonlinear wave phenomena in self-gravitating fluid systems, with a partic- ular emphasis on applications to molecular clouds. This paper presents analytical results for one spatial dimension

published on 22-11-2019 - 45 विचार
Subclustering and Luminous-Dark Matter Segregation in Galaxy Clusters

We have performed a series of high resolution N-body experiments on a Connection Machine CM-5 in order to simulate the formation of galaxy clusters gravitationally dominated by a massive dark background.

published on 22-11-2019 - 49 विचार
BARYONIC CONTENT OF GALACTIC HALOS AND CONSTRAINTS ON MODELS FOR STRUCTURE FORMATION

The recent detection of microlensing of stars of LMC by compact objects in the halo of our galaxy 1 − 2 suggests that our galaxy is surrounded by a non- luminous halo made of compact objects with mass of about (0.03−0.5)M⊙.

published on 22-11-2019 - 51 विचार
Large-scale structure and motions from simulated galaxy clusters.

We use high resolution dissipationless N-body simulations to examine the spatial dis- tribution of galaxy clusters on large scales. The Standard CDM model and two of its main competitors,

published on 22-11-2019 - 44 विचार
The Correlation Function of Rich Clusters of Galaxies in CDM-like Models

We use ensembles of high-resolution CDM simulations to investigate the shape and amplitude of the two point correlation function of rich clusters. The standard scale- invariant CDM model with = 1 provides a poor description of the clustering mea- sured from the APM rich cluster redshift survey, which is better tted by models with more power at large scale

published on 22-11-2019 - 53 विचार
THE MEANING OF EROS/MACHO

Most of the mass density in the Universe—and in the halo of our own galaxy—exists in the form of dark matter. Overall, the contribution of lu- minous matter (in stars) to the mass density of the Universe is less than 1%;

published on 22-11-2019 - 41 विचार
The Nuclear Ten Micron Emission of Spiral Galaxies

We examine the 10µm emission of the central regions of 281 spiral galaxies, after having compiled all ground-based, small-aperture (∼5”) broad-band photometric observations at λ∼10µm (N magnitudes) published in the literature. We evaluate the compactness of the ∼10µm emission of galaxy nuclei by comparing these small-beam measures with the large-beam IRAS 12µm fluxes. In the analysis of different subsets of objects, we apply survival analysis techniques in order to exploit the information contained in “censored” data (i.e., upper limits on the fluxes)

published on 22-11-2019 - 61 विचार
On the Galactic Evolution of D and 3He

The determined abundances of primordial 4He and 7Li provide a basis with which to test the standard model of big bang nucleosynthesis in conjunction with the other two light element isotopes D and 3He, also produced in the big bang.

published on 22-11-2019 - 68 विचार
THIRD-ORDER LAGRANGIAN PERTURBATION THEORY - REALIZATION AT HIGH-SPATIAL RESOLUTION

The Lagrangian theory of gravitational instability of homogeneous-isotropic Friedman-Lema^tre cosmogonies investigated and solved in the series of papers by Buchert (1989), (1992), Buchert & Ehlers (1993), Buchert (1993a,b), Ehlers & Buchert (1993), is illustrated. The third-order solution of this theory for generic initial conditions is presented and realized in a special case by employing methods of high-spatial resolution of the density eld.

published on 22-11-2019 - 48 विचार
Mergers of Systems Containing Gas

Several simple mergers between model galaxy clusters containing a mixture of gas and dark matter are examined, testing the coupling of the gas to the underlying collisionless material. The gas is shocked, irreversibly dissipating the energy fed into it by the collisionless compo- nent and forms a resolved constant-density core. For the dark matter, however, admixture of phase space vacuum is not very efficient and a constant-density core is not produced. In the final state the central gas has little residual kinetic energy, indicating that streaming motions do not help to support the gas.

published on 22-11-2019 - 57 विचार