38,344 research outputs found

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

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

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

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

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

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

A consistent treatment of $B\rightarrow \pi l \nu$ decay is given on the
light-front. The $B$ to $\pi$ 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 $B$-meson bound state is
represented effectively by the $|B^*\pi\rangle$ configuration, and its effect
is calculated in the chiral perturbation theory. Wave function renormalization
is taken into account consistently. The $|B^*\pi\rangle$ contribution dominates
near the zero-recoil point ($q^2\simeq 25$ GeV$^2$), and decreases rapidly as
the recoil momentum increases. We find that the calculated form factor
$f_+(q^2)$ follows approximately a dipole $q^2$-dependence in the entire range
of momentum transfer.Comment: Revtex, 19 pages, 9 figure

### Contamination effects of GPS Navstar solar array performance

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

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

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