From continuum mechanics to general relativity

Using ideas from continuum mechanics we construct a theory of gravity. We show that this theory is equivalent to Einstein's theory of general relativity; it is also a much faster way of reaching general relativity than the conventional route. Our approach is simple and natural: we form a very general model and then apply two physical assumptions supported by experimental evidence. This easily reduces our construction to a model equivalent to general relativity. Finally, we suggest a simple way of modifying our theory to investigate non-standard space-time symmetries.Comment: 7 pages, this essay received a honorable mention in the 2014 essay competition of the Gravity Research Foundatio

Rotational elasticity

We consider an infinite 3-dimensional elastic continuum whose material points experience no displacements, only rotations. This framework is a special case of the Cosserat theory of elasticity. Rotations of material points are described mathematically by attaching to each geometric point an orthonormal basis which gives a field of orthonormal bases called the coframe. As the dynamical variables (unknowns) of our theory we choose the coframe and a density. We write down the general dynamic variational functional for our rotational theory of elasticity, assuming our material to be physically linear but the kinematic model geometrically nonlinear. Allowing geometric nonlinearity is natural when dealing with rotations because rotations in dimension 3 are inherently nonlinear (rotations about different axes do not commute) and because there is no reason to exclude from our study large rotations such as full turns. The main result of the paper is an explicit construction of a class of time-dependent solutions which we call plane wave solutions; these are travelling waves of rotations. The existence of such explicit closed form solutions is a nontrivial fact given that our system of Euler-Lagrange equations is highly nonlinear. In the last section we consider a special case of our rotational theory of elasticity which in the stationary setting (harmonic time dependence and arbitrary dependence on spatial coordinates) turns out to be equivalent to a pair of massless Dirac equations

Is dark matter an extra-dimensional effect?

We investigate the possibility that the observed behavior of test particles outside galaxies, which is usually explained by assuming the presence of dark matter, is the result of the dynamical evolution of particles in higher dimensional space-times. Hence, dark matter may be a direct consequence of the presence of an extra force, generated by the presence of extra-dimensions, which modifies the dynamic law of motion, but does not change the intrinsic properties of the particles, like, for example, the mass (inertia). We discuss in some detail several possible particular forms for the extra force, and the acceleration law of the particles is derived. Therefore, the constancy of the galactic rotation curves may be considered as an empirical evidence for the existence of the extra dimensions.Comment: 11 pages, no figures, accepted for publication in MPLA; references adde

Dark spinor models in gravitation and cosmology

We introduce and carefully define an entire class of field theories based on non-standard spinors. Their dominant interaction is via the gravitational field which makes them naturally dark; we refer to them as Dark Spinors. We provide a critical analysis of previous proposals for dark spinors noting that they violate Lorentz invariance. As a working assumption we restrict our analysis to non-standard spinors which preserve Lorentz invariance, whilst being non-local and explicitly construct such a theory. We construct the complete energy-momentum tensor and derive its components explicitly by assuming a specific projection operator. It is natural to next consider dark spinors in a cosmological setting. We find various interesting solutions where the spinor field leads to slow roll and fast roll de Sitter solutions. We also analyse models where the spinor is coupled conformally to gravity, and consider the perturbations and stability of the spinor.Comment: 43 pages. Several new sections and details added. JHEP in prin

Space-time evolution induced by spinor fields with canonical and non-canonical kinetic terms

We study spinor field theories as an origin to induce space-time evolution. Self-interacting spinor fields with canonical and non-canonical kinetic terms are considered in a Friedman-Robertson-Walker universe. The deceleration parameter is calculated by solving the equation of motion and the Friedman equation, simultaneously. It is shown that the spinor fields can accelerate and decelerate the universe expansion. To construct realistic models we discuss the contributions from the dynamical symmetry breaking.Comment: 16 pages, 19 figure

Isolated and vulnerable: the history and future of Pacific Island terrestrial biodiversity

Islands in the tropical Pacific have a rich and unique biota produced by island biogeographic processes and modified by recent anthropogenic influences. This biota has been shaped by four overlapping phases: natural colonization and dynamics (phase 1), impacts of indigenous (phase 2) and non-indigenous (phase 3) settlers, and increasing environmental awareness (phase 4). Island ecosystems are resilient to natural disturbance regimes but highly vulnerable to invasive species and other human-related influences, due to comparatively low alpha diversity, isolated evolution and the absence of certain functional groups. Habitat loss, overexploitation, invasive alien species and pollution continue to threaten terrestrial biodiversity, compounded by limited environmental awareness, minimal conservation funding, project mismanagement, limited local capacity and inadequate and/or unsuitable conservation policies. To achieve effective conservation of terrestrial biodiversity in the region, biophysical threats need to be mitigated with improved scientific, institutional and management capacity

Directed dispersal by rotational shepherding supports landscape genetic connectivity in a calcareous grassland plant

Directed dispersal by animal vectors has been found to have large effects on the structure and dynamics of plant populations adapted to frugivory. Yet, empirical data are lacking on the potential of directed dispersal by rotational grazing of domestic animals to mediate gene flow across the landscape. Here, we investigated the potential effect of large-flock shepherding on landscape-scale genetic structure in the calcareous grassland plant Dianthus carthusianorum, whose seeds lack morphological adaptations to dispersal to animals or wind. We found a significant pattern of genetic structure differentiating population within grazed patches of three nonoverlapping shepherding systems and populations of ungrazed patches. Among ungrazed patches, we found a strong and significant effect of isolation by distance (r = 0.56). In contrast, genetic distance between grazed patches within the same herding system was unrelated to geographical distance but significantly related to distance along shepherding routes (r = 0.44). This latter effect of connectivity along shepherding routes suggests that gene flow is spatially restricted occurring mostly between adjacent populations. While this study used nuclear markers that integrate gene flow by pollen and seed, the significant difference in the genetic structure between ungrazed patches and patches connected by large-flock shepherding indicates the potential of directed seed dispersal by sheep across the landscape

Plant Diversity, Vegetation Structure and Relationship between Plant Communities and Environmental Variables in the Afromontane Forest of Ethiopia

Diversity patterns of vascular plant species were studied along geographical gradients in the Afromontane regions of Ethiopia. Vegetation data were sampled from five moist evergreen Afromontane forest fragments, namely Harenna (southeast), Bonga, Maji, Berhane-Kontir and Yayu (southwest). In each forest, quadrats of 20x20 m were laid along transects to collect vegetation data. The patterns of plant diversity were evaluated on the basis of species richness as the total number of species at each site and species change between and within sites and in relation to vegetation structure. Floristic analyses of five Afromontane forests altogether revealed 118 families and 653 vascular plant species; about 5% of the species were endemic. Species richness and densities vary considerably between the forest sites. The highest beta and gamma diversities were recorded in the Berhane-Kontir forest and the lowest in Bonga. The studied Afromontane forests were floristically different but the common feature of the geographically separated forests is the occurrences of wild coffee populations. Ordination of the study quadrats of the forests forms five groups based on the species composition. Moist evergreen Afromontane forests support a high density of woody plants, which, however, differ between sites. The Ethiopian moist evergreen Afromontane forests are highly threatened due to anthropogenic factors and, thus, need immediate conservation measures. It is recommended that conservation strategies should focus on multiple-sites conservation networking in order to include a range of forests in the conservation system

Gravitational lensing by a regular black hole

In this paper, we study a regular Bardeen black hole as a gravitational lens. We find the strong deflection limit for the deflection angle, from which we obtain the positions and magnifications of the relativistic images. As an example, we apply the results to the particular case of the supermassive black hole at the center of our galaxy.Comment: 10 pages, 4 figures. v2: Improved version, new references adde