Cold Dark Matter I: The Formation of Dark Halos

We use numerical simulations of critically-closed cold dark matter (CDM) models to study the effects of numerical resolution on observable quantities. We study simulations with up to $256^3$ particles using the particle-mesh (PM) method and with up to $144^3$ particles using the adaptive particle-particle --particle-mesh (P$^3$M) method. Comparisons of galaxy halo distributions are made among the various simulations. We also compare distributions with observations and we explore methods for identifying halos, including a new algorithm that finds all particles within closed contours of the smoothed density field surrounding a peak. The simulated halos show more substructure than predicted by the Press-Schechter theory. We are able to rule out all $\Omega=1$ CDM models for linear amplitude \sigma_8\gsim 0.5 because the simulations produce too many massive halos compared with the observations. The simulations also produce too many low mass halos. The distribution of halos characterized by their circular velocities for the P$^3$M simulations is in reasonable agreement with the observations for 150\kms\lsim V_{\rm circ} \lsim 350\kms.}}Comment: 41 pages, plain tex, ApJ, 236, in press; postscript figures available in ftp://arcturus.mit.edu/Preprints/CDM1_figs.tar.

Relativistic electrons in a rotating spherical magnetic dipole: localized three-dimensional states

Paralleling a previous paper, we examine single- and many-body states of relativistic electrons in an intense, rotating magnetic dipole field. Single-body orbitals are derived semiclassically and then applied to the many-body case via the Thomas-Fermi approximation. The many-body case is reminiscent of the quantum Hall state. Electrons in a realistic neutron star crust are considered with both fixed density profiles and constant Fermi energy. In the first case, applicable to young neutron star crusts, the varying magnetic field and relativistic Coriolis correction lead to a varying Fermi energy and macroscopic currents. In the second, relevant to older crusts, the electron density is redistributed by the magnetic field.Comment: 8 pages, LaTeX, requires REVTeX, 10 PostScript figures; accepted by Int. J. Mod. Phys.

Percolation, depinning, and avalanches in capillary condensation of gases in disordered porous solids

We propose a comprehensive theoretical description of hysteresis in capillary condensation of gases in mesoporous disordered materials. Applying mean-field density functional theory to a coarse-grained lattice-gas model, we show that the morphology of the hysteresis loops is influenced by out-of-equilibrium transitions that are different on filling and on draining. In particular, desorption may be associated to a depinning process and be percolation-like without explicit pore-blocking effects.Comment: 4 pages, 5 figure

Liquid-liquid coexistence in the phase diagram of a fluid confined in fractal porous materials

Multicanonical ensemble sampling simulations have been performed to calculate the phase diagram of a Lennard-Jones fluid embedded in a fractal random matrix generated through diffusion limited cluster aggregation. The study of the system at increasing size and constant porosity shows that the results are independent from the matrix realization but not from the size effects. A gas-liquid transition shifted with respect to bulk is found. On growing the size of the system on the high density side of the gas-liquid coexistence curve it appears a second coexistence region between two liquid phases. These two phases are characterized by a different behaviour of the local density inside the interconnected porous structure at the same temperature and chemical potential.Comment: 5 pages, 4 figures. To be published in Europhys. Letter

Laser driven launch vehicles for continuous access to space

The availability of megawatt laser systems in the next century will make laser launch systems from ground to orbit feasible and useful. Systems studies indicate launch capabilities of 1 ton payload per gigawatt laser power. Recent research in ground to orbit laser propulsion has emphasized laser supported detonation wave thrusters driven by repetitively pulsed infrared lasers. In this propulsion concept each laser repetition cycle consists of two pulses. A lower energy first pulse is used to vaporize a small amount of solid propellant and then after a brief expansion period, a second and higher energy laser pulse is used to drive a detonation wave through the expanded vapor. The results are reported of numerical studies comparing the detonation wave properties of various candidate propellants, and the simulation of thruster performance under realistic conditions. Experimental measurements designed to test the theoretical predictions are also presented. Measurements are discussed of radiance and opacity in absorption waves, and mass loss and momentum transfer. These data are interpreted in terms of specific impulse and energy conversion efficiency

Information and Communication Technologies— Opportunities to Mobilize Agricultural Science for Development

Knowledge, information, and data—and the social and physical infrastructures that carry them—are widely recognized as key building blocks for more sustainable agriculture, effective agricultural science, and productive partnerships among the global research community. Through investments in e-Science infrastructure and collaboration on one hand, and rapid developments in digital devices and connectivity in rural areas, the ways that scientists, academics, and development workers create, share, and apply agricultural knowledge is being transformed through the use of information and communication technologies (ICTs). This paper examines some trends and opportunities associated with the use of these ICTs in agriculturalscience for development

Simple Model of Capillary Condensation in porous media

We employ a simple model to describe the phase behavior of 4He and Ar in a hypothetical porous material consisting of a regular array of infinitely long, solid, parallel cylinders. We find that high porosity geometries exhibit two transitions: from vapor to film and from film to capillary condensed liquid. At low porosity, the film is replaced by a ``necking'' configuration, and for a range of intermediate porosity there are three transitions: from vapor to film, from film to necking and from necking to a capillary condensed phase.Comment: 14 pages, 7 figure

A Mass Matrix for Atmospheric, Solar, and LSND Neutrino Oscillations

We construct a mass matrix for the four neutrino flavors, three active and one sterile, needed to fit oscillations in all three neutrino experiments: atmospheric, solar, and LSND, simultaneously. It organizes the neutrinos into two doublets whose central values are about 1 eV apart, and whose splittings are of the order of 10^(-3) eV. Atmospheric neutrino oscillations are described as maximal mixing within the upper doublet, and solar as the same within the lower doublet. Then LSND is a weak transition from one doublet to the other. We comment on the Majorana versus Dirac nature of the active neutrinos and show that our mass matrix can be derived from an S_2 x S_2 permutation symmetry plus an equal splitting rule.Comment: 4 pages, 0 figures, minor text change