Neutron Stars as Dark Matter Probes

We examine whether the accretion of dark matter onto neutron stars could ever have any visible external effects. Captured dark matter which subsequently annihilates will heat the neutron stars, although it seems the effect will be too small to heat close neutron stars at an observable rate whilst those at the galactic centre are obscured by dust. Non-annihilating dark matter would accumulate at the centre of the neutron star. In a very dense region of dark matter such as that which may be found at the centre of the galaxy, a neutron star might accrete enough to cause it to collapse within a period of time less than the age of the Universe. We calculate what value of the stable dark matter-nucleon cross section would cause this to occur for a large range of masses.Comment: 8 pages, 7 figure

Archaeological identification of fragmented nuts and fruits from key Asia-Pacific economic tree species using anatomical criteria: comparative analysis of Canarium, Pandanus and Terminalia

The fats, protein and carbohydrates afforded by tree nuts and fruits are key resources for communities from Southeast Asia, through Melanesia, Australia and across Oceania. They are important in long-distance marine trade networks, large-scale ceremonial gatherings, and are core resources in a wide range of subsistence economies, including foraging systems, horticulture and swidden agriculture. Recent archaeobotanical evidence has also shown their deep-time importance, being amongst the earliest foods used in the colonisation of novel environments in Australia and New Guinea, as well as the later colonisation of Near and Remote Oceania. The archaeobotanical methods used to identify fruit and nut-derived plant macrofossils have been largely limited to use of morphological characters of near whole or exceptionally preserved remains, most commonly endocarps, the hard, nutshell-like interior layer of the fruit protecting the seed. Here we detail how anatomical characteristics of endocarps, visible in light and scanning electron microscopy (SEM), can be used with surviving morphological features to identify confidently the use of key Asia-Pacific economic trees, in this case, Canarium, Pandanus and Terminalia. Systematic anatomical description allows the identification of these important economic taxa, and separation from the remains of others such as Aleurites and Cocos, when found in a range of archaeological assemblages. This includes the often highly fragmented charred assemblages that can be recovered routinely from most sites with appropriate fine-sieving and flotation methods. These methods provide the basis for a more representative and nuanced understanding of ancient plant use, economy and social systems operating in the region and, being particularly useful in tropical regions, will broaden the archaeobotanical database on ancient foods globally.Introduction Background - Definition of tree fruits and nuts - Structure and use of Canarium - Structure and use of Pandanus - Structure and use of Terminalia - Archaeological visibility Methods - Archaeobotanical analysis of modern comparative material - Archaeobotanical analysis of archaeological material Results - Canarium - Pandanus - Terminalia Discussion Conclusio

Voids as Alternatives to Dark Energy and the Propagation of Gamma Rays through the Universe

We test the opacity of a void Universe to TeV energy gamma rays having obtained the extra-galactic background light in that Universe using a simple model and the observed constraints on the star formation rate history. We find that the void Universe has significantly more opacity than a Lambda-CDM Universe, putting it at odds with observations of BL-Lac objects. We argue that while this method of distinguishing between the two cosmologies contains uncertainties, it circumvents any debates over fine-tuning.Comment: 4 pages 4 figure

Extended matter coupled to BF theory

Recently, a topological field theory of membrane-matter coupled to BF theory in arbitrary spacetime dimensions was proposed [1]. In this paper, we discuss various aspects of the four-dimensional theory. Firstly, we study classical solutions leading to an interpretation of the theory in terms of strings propagating on a flat spacetime. We also show that the general classical solutions of the theory are in one-to-one correspondence with solutions of Einstein's equations in the presence of distributional matter (cosmic strings). Secondly, we quantize the theory and present, in particular, a prescription to regularize the physical inner product of the canonical theory. We show how the resulting transition amplitudes are dual to evaluations of Feynman diagrams coupled to three-dimensional quantum gravity. Finally, we remove the regulator by proving the topological invariance of the transition amplitudes.Comment: 27 pages, 7 figure

Photon-axion mixing and ultra-high-energy cosmic rays from BL Lac type objects -- Shining light through the Universe

Photons may convert into axion like particles and back in the magnetic field of various astrophysical objects, including active galaxies, clusters of galaxies, intergalactic space and the Milky Way. This is a potential explanation for the candidate neutral ultra-high-energy (E>10^18 eV) particles from distant BL Lac type objects which have been observed by the High Resolution Fly's Eye experiment. Axions of the same mass and coupling may explain also TeV photons detected from distant blazars.Comment: Revtex 10 pages, 6 figures. V.2: QED dispersion effects taken into account; principal results unchanged. V3: misprints and sqrt(4*pi) factors in Gauss to eV conversion corrected; conclusions unchange

Relaxing Cosmological Constraints on Large Extra Dimensions

We reconsider cosmological constraints on extra dimension theories from the excess production of Kaluza-Klein gravitons. We point out that, if the normalcy temperature is above 1 GeV, then graviton states produced at this temperature will decay early enough that they do not affect the present day dark matter density, or the diffuse gamma ray background. We rederive the relevant cosmological constraints for this scenario.Comment: 17 pages, latex, revtex4; added a short discussion of other constraints, reference

Observables in 3d spinfoam quantum gravity with fermions

We study expectation values of observables in three-dimensional spinfoam quantum gravity coupled to Dirac fermions. We revisit the model introduced by one of the authors and extend it to the case of massless fermionic fields. We introduce observables, analyse their symmetries and the corresponding proper gauge fixing. The Berezin integral over the fermionic fields is performed and the fermionic observables are expanded in open paths and closed loops associated to pure quantum gravity observables. We obtain the vertex amplitudes for gauge-invariant observables, while the expectation values of gauge-variant observables, such as the fermion propagator, are given by the evaluation of particular spin networks.Comment: 32 pages, many diagrams, uses psfrag