Shielding of Space Vehicles by Magnetic Fields

Spacecraft shielding by magnetic field

Bohm and Einstein-Sasaki Metrics, Black Holes and Cosmological Event Horizons

We study physical applications of the Bohm metrics, which are infinite sequences of inhomogeneous Einstein metrics on spheres and products of spheres of dimension 5 <= d <= 9. We prove that all the Bohm metrics on S^3 x S^2 and S^3 x S^3 have negative eigenvalue modes of the Lichnerowicz operator and by numerical methods we establish that Bohm metrics on S^5 have negative eigenvalues too. We argue that all the Bohm metrics will have negative modes. These results imply that higher-dimensional black-hole spacetimes where the Bohm metric replaces the usual round sphere metric are classically unstable. We also show that the stability criterion for Freund-Rubin solutions is the same as for black-hole stability, and hence such solutions using Bohm metrics will also be unstable. We consider possible endpoints of the instabilities, and show that all Einstein-Sasaki manifolds give stable solutions. We show how Wick rotation of Bohm metrics gives spacetimes that provide counterexamples to a strict form of the Cosmic Baldness conjecture, but they are still consistent with the intuition behind the cosmic No-Hair conjectures. We show how the Lorentzian metrics may be created ``from nothing'' in a no-boundary setting. We argue that Lorentzian Bohm metrics are unstable to decay to de Sitter spacetime. We also argue that noncompact versions of the Bohm metrics have infinitely many negative Lichernowicz modes, and we conjecture a general relation between Lichnerowicz eigenvalues and non-uniqueness of the Dirichlet problem for Einstein's equations.Comment: 53 pages, 11 figure

Isometric Embedding of BPS Branes in Flat Spaces with Two Times

We show how non-near horizon p-brane theories can be obtained from two embedding constraints in a flat higher dimensional space with 2 time directions. In particular this includes the construction of D3 branes from a flat 12-dimensional action, and M2 and M5 branes from 13 dimensions. The worldvolume actions are determined by constant forms in the higher dimension, reduced to the usual expressions by Lagrange multipliers. The formulation affords insight in the global aspects of the spacetime geometries and makes contact with recent work on two-time physics.Comment: 29 pages, 10 figures, Latex using epsf.sty and here.sty; v2: reference added and some small correction

HLA-B27 Positivity: associated health implications

HLA-B27 positivity makes the onset of autoimmune diseases such as uveitis, ankylosing spondylitis and Crohn's disease more likely to occur. Ankylosing spondylitis and Crohn's disease are two types of HLA-B27 positive diseases that demonstrate a direct association with uveitis. Although the possession of HLA-B27 positivity is not mandatory for autoimmune diseases such as uveitis to occur, HLA-B27 positivity not only makes it more likely but may modify the clinical picture in which a patient presents. In relation to assessment and diagnosis it is imperative that the medical history of patients is thoroughly examined to ensure pathological sequelae are appropriately treated. Nurses play an important role in assessing patients that have uveitis and should suspect ankylosing spondylitis or Crohn's disease may be present

Cosmic Acceleration from M Theory on Twisted Spaces

In a recent paper [I.P. Neupane and D.L. Wiltshire, Phys. Lett. B 619, 201 (2005).] we have found a new class of accelerating cosmologies arising from a time--dependent compactification of classical supergravity on product spaces that include one or more geometric twists along with non-trivial curved internal spaces. With such effects, a scalar potential can have a local minimum with positive vacuum energy. The existence of such a minimum generically predicts a period of accelerated expansion in the four-dimensional Einstein-conformal frame. Here we extend our knowledge of these cosmological solutions by presenting new examples and discuss the properties of the solutions in a more general setting. We also relate the known (asymptotic) solutions for multi-scalar fields with exponential potentials to the accelerating solutions arising from simple (or twisted) product spaces for internal manifolds.Comment: 23 pages, 3 figures; added a summary Table, PRD versio

Genetic Study of Alkane Production and Oxic Nitrogen Fixation in Anabaena SP. PCC 7120

Anabaena sp. PCC 7120 is a filamentous, nitrogen-fixing cyanobacterium that uses spatial separation to perform photosynthesis and nitrogen fixation simultaneously. Under fixed nitrogen limiting conditions, Anabaena 7120 forms specialized cells, called heterocysts, to fix nitrogen. In this research, I sought to add to the knowledge surrounding the process of heterocystous nitrogen fixation in Anabaena 7120. Cyanobacteria universally produce alkanes, which have been suggested to play a role in helping the organism adapt to abiotic stress, such as diazotrophic conditions. In my first study, I sought to identify the genes required for production of the hydrocarbon heptadecane (C17H36). Through using a double crossover approach and determining loss of function, I identified two genes (alr5283 and alr5284) responsible for heptadecane production in Anabaena 7120. In my second study, I employed the alkane knockout mutant to study the impact of alkanes in nitrogen-fixing conditions. My results showed that heptadecane production spikes immediately following nitrogen starvation. Correspondingly, the mutant, which is unable to produce heptadecane, exhibited a fragmented phenotype with a lower nitrogenase activity in the days immediately following nitrogen starvation. These results suggested that heptadecane may play an important role in maintaining membrane integrity for cell-to-cell connections, especially during the initial response to nitrogen starvation. In my last study, I knocked out 16 genes whose encoded proteins had been found only in heterocysts (Qiu 2018). Two of these gene mutants (for all3132 and alr0731) were unable to grow under aerobic nitrogen-fixing conditions. I was able to complement the alr0731 mutant, and it regained its ability to grow in diazotrophic conditions. Therefore, I categorized alr0731 as a Fox gene (unable to fix nitrogen in the presence of oxygen). This gene encodes an anaerobic ribocucleoside triphosphate reductase activating protein that is needed for conversion of ribonucleoside triphosphates (NTPs) to deoxyribonucleoside triphosphates (dNTPs). Because cell division (requiring DNA replication) and gene rearrangements have been linked to heterocyst differentiation, Alr0731 may be primarily important in the process of heterocyst differentiation. Future work will continue to add to the understanding of key regulators and contributors to the process of heterocyst differentiation and nitrogen fixation in cyanobacteria