Dark Matter with Time-Dependent Mass

We propose a simple model in which the cosmological dark matter consists of particles whose mass increases with the scale factor of the universe. The particle mass is generated by the expectation value of a scalar field which does not have a stable vacuum state, but which is effectively stabilized by the rest energy of the ambient particles. As the universe expands, the density of particles decreases, leading to an increase in the vacuum expectation value of the scalar (and hence the mass of the particle). The energy density of the coupled system of variable-mass particles (``vamps'') redshifts more slowly than that of ordinary matter. Consequently, the age of the universe is larger than in conventional scenarios.Comment: 14 pages, 2 figures; based on a talk by SMC at Cosmo-97, September 1997, Ambleside, England. Important references adde

The Effect of Irradiating AlN on its Dielectric Properties

Aluminum Nitride is an active element of sensors that monitor the performance and well-being of the nuclear reactors due to its piezoelectric properties. Yet, the variations of its properties under irradiation are largely unexplored. Here, we report the results of the molecular dynamics simulations of the structural changes in AlN under irradiation via the knock-on atom technique. By creating and evolving the irradiation cascades due to energetic particle interaction with the atom of the crystalline lattice we determine the rate of the defect production as a function of the deposited energy. Further, we determine a displacement energy, a key characteristic that describes how efficient the defect production in the given material is. Comparison with the isostructural GaN is provided

A linguistic model of minimalist syntax composes Tebe Poem

HonorsCognitive ScienceUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/167886/1/seanpaul.pd

The changing functions of fungi in forest succession.

General EcologyThe ecological roles of fungi in forest settings are incredibly diverse, but two of the most crucial services provided by members of this kingdom include fixation of nitrogen into forms useable by plants and decomposition of organic matter to allow the recycling of certain nutrients into the soil. As forests recover from disturbances such as burning, different nutrients become limiting to plant growth at different stages of succession—for example, nitrogen tends to be a limiting factor in younger forests. Our group hypothesized that nitrogen-fixing species of fungi (i.e. mycorrhizae) should be more abundant than decomposing (i.e. saprophytic) species in younger forests due to the competitive advantage in resource availability given to plants who had formed mutualistic relationships with mycorrhizae; in other words, we predicted that the primary function of fungi in a forest ecosystem would shift from mycorrhizal to decomposing as the forest increased in age. To test this, we performed a transect of three differently aged forests, represented by “burned plots” in various stages of secondary succession, and categorized each species of fungi found as either mycorrhizal or saprophytic. Additionally, we took soil samples from all forests to establish the relative ratio of carbon to nitrogen in each. Though no significant differences in soil nutrient percentages were found between the different plots, we found a significantly higher proportion of nitrogen-fixing:decomposing fungi in the youngest plot sampled. This result indicates that the primary function of fungi in a successive forest may indeed change from mycorrhizal to saprophytic (or, at least, less mycorrhizal) as the forest reaches different stages of recovery and that this change is strongly reflected in the composition of the fungal community, though several factors limited our ability to wholly assess the dominant functional group of fungi present in any particular forest. The capability to understand the changing dynamics of fungal function and distribution as compared to the specific needs of a particular ecosystem could be valuable for researchers attempting to describe the vast majority of fungal species that remain unknown yet still play unique, critical ecological roles in their environment.http://deepblue.lib.umich.edu/bitstream/2027.42/110213/1/Anderson_Sean_2014.pd