Are stealth scalar fields stable?

Non-gravitating (stealth) scalar fields associated with Minkowski space in scalar-tensor gravity are examined. Analytical solutions for both non-minimally coupled scalar field theory and for Brans-Dicke gravity are studied and their stability with respect to tensor perturbations is assessed using a covariant and gauge-invariant formalism developed for alternative gravity. For Brans-Dicke solutions, the stability with respect to homogeneous perturbations is also studied. There are regions of parameter space corresponding to stability and other regions corresponding to instability.Comment: 10 pages, 1 table, no figures, to appear in Phys. Rev,

Self-Deployable Membrane Structures

Currently existing approaches for deployment of large, ultra-lightweight gossamer structures in space rely typically upon electromechanical mechanisms and mechanically expandable or inflatable booms for deployment and to maintain them in a fully deployed, operational configuration. These support structures, with the associated deployment mechanisms, launch restraints, inflation systems, and controls, can comprise more than 90 percent of the total mass budget. In addition, they significantly increase the stowage volume, cost, and complexity. A CHEM (cold hibernated elastic memory) membrane structure without any deployable mechanism and support booms/structure is deployed by using shape memory and elastic recovery. The use of CHEM micro-foams reinforced with carbon nanotubes is considered for thin-membrane structure applications. In this advanced structural concept, the CHEM membrane structure is warmed up to allow packaging and stowing prior to launch, and then cooled to induce hibernation of the internal restoring forces. In space, the membrane remembers its original shape and size when warmed up. After the internal restoring forces deploy the structure, it is then cooled to achieve rigidization. For this type of structure, the solar radiation could be utilized as the heat energy used for deployment and space ambient temperature for rigidization. The overall simplicity of the CHEM self-deployable membrane is one of its greatest assets. In present approaches to space-deployable structures, the stow age and deployment are difficult and challenging, and introduce a significant risk, heavy mass, and high cost. Simple procedures provided by CHEM membrane greatly simplify the overall end-to-end process for designing, fabricating, deploying, and rigidizing large structures. The CHEM membrane avoids the complexities associated with other methods for deploying and rigidizing structures by eliminating deployable booms, deployment mechanisms, and inflation and control systems that can use up the majority of the mass budge

Theory for the ultrafast ablation of graphite films

The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re

Spectroscopy with the Engineering Development Array: cold H+^{+} at 63 MHz towards the Galactic Centre

The Engineering Development Array (EDA) is a single test station for Square Kilometre Array (SKA) precursor technology. We have used the EDA to detect low-frequency radio recombination lines (RRLs) from the Galactic Centre region. Low-frequency RRLs are an area of interest for future low-frequency SKA work as these lines provide important information on the physical properties of the cold neutral medium. In this project we investigate the EDA, its bandpass and the radio frequency interference environment for low-frequency spectroscopy. We present line spectra from 30 to 325 MHz for the Galactic Centre region. The decrease in sensitivity for the EDA at the low end of the receiver prevents carbon and hydrogen RRLs to be detected below 40 and 60 MHz respectively. RFI strongly affects frequencies in the range 276-292, 234-270, 131-138, 95-102 and below 33 MHz. Cn$\alpha$ RRLs were detected in absorption for quantum levels n = 378 to 550 (39-121 MHz) and in emission for n = 272 to 306 (228-325 MHz). Cn$\beta$ lines were detected in absorption for n = 387 to 696 (39-225 MHz). Hn$\alpha$ RRLs were detected in emission for n = 272 to 480 (59-325 MHz). Hn$\beta$ lines were detected for n = 387 to 453 (141-225 MHz). The stacked Hn$\alpha$ detection at 63 MHz is the lowest frequency detection made for hydrogen RRLs and shows that a cold (partially) ionized medium exists along the line of sight to the Galactic Centre region. The size and velocity of this cold H$^{+}$ gas indicates that it is likely associated with the nearby Riegel-Crutcher cloud.Comment: 18 pages, 6 figures and 5 table

The Universality of Einstein Equations

It is shown that for a wide class of analytic Lagrangians which depend only on the scalar curvature of a metric and a connection, the application of the so--called ``Palatini formalism'', i.e., treating the metric and the connection as independent variables, leads to ``universal'' equations. If the dimension $n$ of space--time is greater than two these universal equations are Einstein equations for a generic Lagrangian and are suitably replaced by other universal equations at bifurcation points. We show that bifurcations take place in particular for conformally invariant Lagrangians $L=R^{n/2} \sqrt g$ and prove that their solutions are conformally equivalent to solutions of Einstein equations. For 2--dimensional space--time we find instead that the universal equation is always the equation of constant scalar curvature; the connection in this case is a Weyl connection, containing the Levi--Civita connection of the metric and an additional vectorfield ensuing from conformal invariance. As an example, we investigate in detail some polynomial Lagrangians and discuss their bifurcations.Comment: 15 pages, LaTeX, (Extended Version), TO-JLL-P1/9

Adolescent spinal pain: The pediatric orthopedist's point of view

AbstractIntroductionTen to twenty percent of persons experience spinal pain during growth. Causes are diverse in adolescents, and it is essential to determine etiology rapidly so as to guide optimal management.HypothesisIt is important for the pediatric orthopedist to understand the natural history of conditions inducing spinal pain.Material and methodsA retrospective study included 116 adolescents presenting with spinal pain at the Hôpital Nord (Marseille, France) between January 1, 2009 and January 1, 2014. Malignant tumoral etiologies were excluded. Mean patient age was 13.6 years. Risser ranged between >0 and <5. Interview and clinical examination (skin, spine, neurologic examination, general clinical examination) were systematic; depending on results, complementary examinations (imaging, biology, biopsy) were prescribed.ResultsThere were 32 cases of non-specific adolescent low back pain, 31 of lumbar or thoracolumbar scoliosis, 23 of spinal growth dystrophy (Scheuermann's disease), 13 of isthmic lysis, 5 of spondylolisthesis, 8 of transitional lumbosacral hinge abnormality, 2 of discal hernia, 1 of osteoid osteoma and 1 of eosinophil granuloma. Treatment was often non-operative when diagnosis was sufficiently early. In case of failure, surgery could generally be considered.DiscussionCorrectly indicated non-operative management or surgery changes the natural history of these pathologies. The aim of treatment is to resolve pain in adolescence, as it risks becoming chronic and disabling by adulthood.Level of evidenceIV

Low Temperature Properties of Selected Kramers Rare Earth Oxychlorides

In this work, we present low temperature magnetic and electronic properties measured on selected Kramers rare earth oxychlorides REOCl, RE Nd, Gd, Dy which adopt the PbFCl type of structure. Prepared powder samples were characterized by means of standard structural, magnetic and electronic methods as X ray diffraction 300 K , heat capacity 0.3 K 12 K and susceptibility measurements 2 K 300 K, at ambient pressure and hydrostatic pressures up to 0.68 0.01 GPa . Our results indicate new transition to the ordered magnetic state for GdOCl and NdOCl compound at temperatures of 5 K and 1.5 K, respectively. We found small increase of magnetization saturation value of dysprosium oxychloride with an applied hydrostatic pressure, but no remarkable changes occur to antiferromagnetic transition temperature TN 9.2 K when a moderate hydrostatic pressure p 0.68 0.01 GPa was applied. Observed deviations from the CurieWeiss behavior below 26 K can be caused by the vicinity of the magnetic ordering temperature, or another magnetic effects. The single crystal experiments which will solve this opened question are in progres

Pre-big bang model has Planck problem

The pre-big bang's kinetic driven inflationary mechanism is not an adequate form of inflation: the Planck length grows more rapidly than the scale factor. In order to explain our large universe, the resulting post-big bang universe requires the same unnatural constants (Planck problem) as those of any other non-inflationary big bang model.Comment: figures not include