General properties of cosmological models with an Isotropic Singularity

Much of the published work regarding the Isotropic Singularity is performed under the assumption that the matter source for the cosmological model is a barotropic perfect fluid, or even a perfect fluid with a $\gamma$-law equation of state. There are, however, some general properties of cosmological models which admit an Isotropic Singularity, irrespective of the matter source. In particular, we show that the Isotropic Singularity is a point-like singularity and that vacuum space-times cannot admit an Isotropic Singularity. The relationships between the Isotropic Singularity, and the energy conditions, and the Hubble parameter is explored. A review of work by the authors, regarding the Isotropic Singularity, is presented.Comment: 18 pages, 1 figur

Natural scene statistics and the structure of orientation maps in the visual cortex

Visual activity after eye-opening influences feature map structure in primary visual cortex (V1). For instance, rearing cats in an environment of stripes of one orientation yields an over-representation of that orientation in V1. However, whether such changes also affect the higher-order statistics of orientation maps is unknown. A statistical bias of orientation maps in normally raised animals is that the probability of the angular difference in orientation preference between each pair of points in the cortex depends on the angle of the line joining those points relative to a fixed but arbitrary set of axes. Natural images show an analogous statistical bias; however, whether this drives the development of comparable structure in V1 is unknown. We examined these statistics for normal, stripe-reared and dark-reared cats, and found that the biases present were not consistently related to those present in the input, or to genetic relationships. We compared these results with two computational models of orientation map development, an analytical model and a Hebbian model. The analytical model failed to reproduce the experimentally observed statistics. In the Hebbian model, while orientation difference statistics could be strongly driven by the input, statistics similar to those seen in experimental maps arose only when symmetry breaking was allowed to occur spontaneously. These results suggest that these statistical biases of orientation maps arise primarily spontaneously, rather than being governed by either input statistics or genetic mechanisms

Application of seminal plasma in sex-sorting and sperm cryopreservation

Substantial dilution of boar semen during processing decreased the concentration of seminal plasma, perhaps contributing to the decline in sperm quality after cryopreservation and sex-sorting. Results of replacing seminal plasma in investigations from many laboratories have been contradictory. Results and discussion here suggest that whereas membrane status can be influenced by seminal plasma, the action of its various components, both positive and negative, is determined in part by the membrane status of the spermatozoa to which it is being exposed. Although progress has been made in identifying components of seminal plasma responsible for its protective effect (notably PSP-I/II spermadhesin for sex-sorted boar spermatozoa), little is known (in any species) regarding how external factors may influence their levels, and their functionality, in seminal plasma. It is noteworthy that seminal plasma is beneficial to post-thaw quality of sex-sorted ram spermatozoa only when added before freezing, not after thawing. Therefore, the action of seminal plasma and its components is dependent on sperm-related factors, in particular the type of processing to which they have been previously exposed. Further research is needed to unravel these biological complexities, and then characterise and synthesise useful proteins within seminal plasma. © 2008

The role of biophysical cohesion on subaqueous bed form size

Biologically active, fine-grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud-sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross-stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record

Isotropic Singularities in Shear-free Perfect Fluid Cosmologies

We investigate barotropic perfect fluid cosmologies which admit an isotropic singularity. From the General Vorticity Result of Scott, it is known that these cosmologies must be irrotational. In this paper we prove, using two different methods, that if w

General Properties of Cosmological Models with an Isotropic Singularity

Much of the published work regarding the Isotropic Singularity is performed under the assumption that the matter source for the cosmological model is a barotropic perfect fluid, or even a perfect fluid with a γ-law equation of state. There are, however

The Laplacian of Gaussian and arbitrary z-crossings approach applied to automated single particle reconstruction

The automation of single particle selection and tomographic segmentation of asymmetric particles and objects is facilitated by continuing improvement of methods based on the detection of pixel discontinuity. Here, we present the new arbitrary z-crossings approach which can be employed to enhance the accuracy of edge detection algorithms that are based on the second derivative. This is demonstrated using the Laplacian of Gaussian (LoG) filter. In its normal implementation the LoG filter uses a z value of zero to define edge contours. In contrast, the arbitrary z-crossings approach allows the user to adjust z, which causes the subsequently generated contours to tend towards lighter or darker image objects, depending on the sign of z. This functionality has been coupled with an additional feature: the ability to use the major and minor axes of bounding contours to hone automated object selection. In combination, these features significantly enhance the accuracy of particle selection and the speed of tomographic segmentation. Both features have been incorporated into the software package SwarmPS in which parameters are automatically adjusted based on user defined target selection