38,344 research outputs found

    Gauge-invariant gravitational wave modes in pre-big bang cosmology

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    The t<0 branch of pre-big bang cosmological scenarios is subject to a gravitational wave instability. The unstable behaviour of tensor perturbations is derived in a very simple way in Hwang's covariant and gauge-invariant formalism developed for extended theories of gravity. A simple interpretation of this instability as the effect of an "antifriction" is given, and it is argued that a universe must eventually enter the expanding phase.Comment: 4 pages, latex, to appear in Eur. Phys. J.

    Base manifolds for fibrations of projective irreducible symplectic manifolds

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    Given a projective irreducible symplectic manifold MM of dimension 2n2n, a projective manifold XX and a surjective holomorphic map f:MXf:M \to X with connected fibers of positive dimension, we prove that XX is biholomorphic to the projective space of dimension nn. The proof is obtained by exploiting two geometric structures at general points of XX: the affine structure arising from the action variables of the Lagrangian fibration ff and the structure defined by the variety of minimal rational tangents on the Fano manifold XX

    Why Newton's gravity is practically reliable in the large-scale cosmological simulations

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    Until now, it has been common to use Newton's gravity to study the non-linear clustering properties of the large-scale structures. Without confirmation from Einstein's theory, however, it has been unclear whether we can rely on the analysis, for example, near the horizon scale. In this work we will provide a confirmation of using Newton's gravity in cosmology based on relativistic analysis of weakly non-linear situations to the third order in perturbations. We will show that, except for the gravitational wave contribution, the relativistic zero-pressure fluid equations perturbed to the second order in a flat Friedmann background coincide exactly with the Newtonian results. We will also present the pure relativistic correction terms appearing in the third order. The third-order correction terms show that these are the linear-order curvature perturbation strength higher than the second-order relativistic/Newtonian terms. Thus, the pure general relativistic corrections in the third order are independent of the horizon scale and are small in the large-scale due to the low-level temperature anisotropy of the cosmic microwave background radiation. Since we include the cosmological constant, our results are relevant to currently favoured cosmology. As we prove that the Newtonian hydrodynamic equations are valid in all cosmological scales to the second order, and that the third-order correction terms are small, our result has a practically important implication that one can now use the large-scale Newtonian numerical simulation more reliably as the simulation scale approaches and even goes beyond the horizon.Comment: 8 pages, no figur

    B>πlνB -> \pi l \nu Form Factors Calculated on the Light-Front

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    A consistent treatment of BπlνB\rightarrow \pi l \nu decay is given on the light-front. The BB to π\pi transition form factors are calculated in the entire physical range of momentum transfer for the first time. The valence-quark contribution is obtained using relativistic light-front wave functions. Higher quark-antiquark Fock-state of the BB-meson bound state is represented effectively by the Bπ|B^*\pi\rangle configuration, and its effect is calculated in the chiral perturbation theory. Wave function renormalization is taken into account consistently. The Bπ|B^*\pi\rangle contribution dominates near the zero-recoil point (q225q^2\simeq 25 GeV2^2), and decreases rapidly as the recoil momentum increases. We find that the calculated form factor f+(q2)f_+(q^2) follows approximately a dipole q2q^2-dependence in the entire range of momentum transfer.Comment: Revtex, 19 pages, 9 figure

    Contamination effects of GPS Navstar solar array performance

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    The solar arrays on Navstars 1 to 6 have been limiting the electrical power capabilities in the extended life (beyond the 5 year design life). The departure from predicted performance consists of an extra 2.5 percent per year degradation beyond the radiation model estimates. This degradation is unusual in showing a linear rather than exponential decay with time. The performance of the arrays on these satellites has been examined in order to predict future behavior and to make refined projections on the Navstar 7 to 11 solar arrays. Evidence obtained from flight experiments on Navstar 5 and 6, and from laboratory experiments, suggests that contamination of the solar arrays while on orbit may be responsible. The evidence for photo-induced contamination of spacecraft surfaces is presented, and the effect on solar array output in the case of the GPS satellites is shown to be consistent with the observed anomalies

    Mounting technique for pressure transducers minimizes measurement interferences

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    Miniaturized transducers are fabricated from commercially available four-arm semiconductor gages; transducers are connected as bridge circuit and mounted on internal face of small diaphragm. Jacket made of conductive plastic may be needed to avoid buildup or static charges