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.

Cold Dark Matter II: Spatial and Velocity Statistics

We examine high-resolution gravitational N-body simulations of the $\Omega=1$ cold dark matter (CDM) model in order to determine whether there is any normalization of the initial density fluctuation spectrum that yields acceptable results for galaxy clustering and velocities. Dense dark matter halos in the evolved mass distribution are identified with luminous galaxies; the most massive halos are also considered as sites for galaxy groups, with a range of possibilities explored for the group mass to light ratios. We verify the earlier conclusions of White et al. (1987) for the low amplitude (high bias) CDM model --- the galaxy correlation function is marginally acceptable but that there are too many galaxies. We also show that the peak biasing method does not accurately reproduce the results obtained using dense halos identified in the simulations themselves. The COBE anisotropy implies a higher normalization, resulting in problems with excessive pairwise galaxy velocity dispersion unless a strong velocity bias is present. Although we confirm the strong velocity bias of halos reported by Couchman \& Carlberg (1992), we show that the galaxy motions are still too large on small scales. We find no amplitude for which the CDM model can reconcile simultaneously the galaxy correlation function, the low pairwise velocity dispersion, and the richness distribution of groups and clusters. With the normalization implied by COBE, the CDM spectrum has too much power on small scales if $\Omega=1$.Comment: 31 pages, plain tex, ApJ, 236, in press; postscript figures available in ftp://arcturus.mit.edu/Preprints/CDM2_figs.tar.

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

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