31,245 research outputs found

### 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 $X$

### 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.

### 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

### Conserved cosmological structures in the one-loop superstring effective action

A generic form of low-energy effective action of superstring theories with
one-loop quantum correction is well known. Based on this action we derive the
complete perturbation equations and general analytic solutions in the
cosmological spacetime. Using the solutions we identify conserved quantities
characterizing the perturbations: the amplitude of gravitational wave and the
perturbed three-space curvature in the uniform-field gauge both in the
large-scale limit, and the angular-momentum of rotational perturbation are
conserved independently of changing gravity sector. Implications for
calculating perturbation spectra generated in the inflation era based on the
string action are presented.Comment: 5 pages, no figure, To appear in Phys. Rev.

### Development of a solar-powered residential air conditioner: System optimization preliminary specification

Investigations aimed at the optimization of a baseline Rankine cycle solar powered air conditioner and the development of a preliminary system specification were conducted. Efforts encompassed the following: (1) investigations of the use of recuperators/regenerators to enhance the performance of the baseline system, (2) development of an off-design computer program for system performance prediction, (3) optimization of the turbocompressor design to cover a broad range of conditions and permit operation at low heat source water temperatures, (4) generation of parametric data describing system performance (COP and capacity), (5) development and evaluation of candidate system augmentation concepts and selection of the optimum approach, (6) generation of auxiliary power requirement data, (7) development of a complete solar collector-thermal storage-air conditioner computer program, (8) evaluation of the baseline Rankine air conditioner over a five day period simulating the NASA solar house operation, and (9) evaluation of the air conditioner as a heat pump

### Singularities in scalar-tensor gravity

The analysis of certain singularities in scalar-tensor gravity contained in a
recent paper is completed, and situations are pointed out in which these
singularities cannot occur.Comment: 6 pages, LaTe

### Inverter-Based Low-Voltage CCII- Design and Its Filter Application

This paper presents a negative type second-generation current conveyor (CCII-). It is based on an inverter-based low-voltage error amplifier, and a negative current mirror. The CCII- could be operated in a very low supply voltage such as ±0.5V. The proposed CCII- has wide input voltage range (±0.24V), wide output voltage (±0.24V) and wide output current range (±24mA). The proposed CCII- has no on-chip capacitors, so it can be designed with standard CMOS digital processes. Moreover, the architecture of the proposed circuit without cascoded MOSFET transistors is easily designed and suitable for low-voltage operation. The proposed CCII- has been fabricated in TSMC 0.18μm CMOS processes and it occupies 1189.91 x 1178.43μm2 (include PADs). It can also be validated by low voltage CCII filters

### Travelling waves in hyperbolic chemotaxis equations

Mathematical models of bacterial populations are often written as systems of partial differential equations for the densities of bacteria and concentrations of extracellular (signal) chemicals. This approach has been employed since the seminal work of Keller and Segel in the 1970s [Keller and Segel, J. Theor. Biol., 1971]. The system has been shown to permit travelling wave solutions which correspond to travelling band formation in bacterial colonies, yet only under specific criteria, such as a singularity in the chemotactic sensitivity function as the signal approaches zero. Such a singularity generates infinite macroscopic velocities which are biologically unrealistic. In this paper, we formulate a model that takes into consideration relevant details of the intracellular processes while avoiding the singularity in the chemotactic sensitivity. We prove the global existence of solutions and then show the existence of travelling wave solutions both numerically and analytically

### The infrared conductivity of Na$_x$CoO$_2$: evidence of gapped states

We present infrared ab-plane conductivity data for the layered cobaltate
Na$_x$CoO$_2$ at three different doping levels ($x=0.25, 0.50$, and 0.75). The
Drude weight increases monotonically with hole doping, $1-x$. At the lowest
hole doping level $x$=0.75 the system resembles the normal state of underdoped
cuprate superconductors with a scattering rate that varies linearly with
frequency and temperature and there is an onset of scattering by a bosonic mode
at 600 \cm. Two higher hole doped samples ($x=0.50$ and 0.25) show two
different-size gaps (110 \cm and 200 \cm, respectively) in the optical
conductivities at low temperatures and become insulators. The spectral weights
lost in the gap region of 0.50 and 0.25 samples are shifted to prominent peaks
at 200 \cm and 800 \cm, respectively. We propose that the two gapped states of
the two higher hole doped samples ($x$=0.50 and 0.25) are pinned charge ordered
states.Comment: 4 pages, 3 figure

### Spin asymmetries in jet-hyperon production at LHC

We consider polarized Lambda hyperon production in proton-proton scattering,
p p -> (\Lambda^\uparrow jet) jet X, in the kinematical region of the LHC
experiments, in particular the ALICE experiment. We present a new Lambda
polarization observable that arises from the Sivers effect in the fragmentation
process. It can be large even at midrapidity and therefore, is of interest for
high energy hadron collider experiments. Apart from its potential to shed light
on the mechanisms behind the phenomenon of Lambda polarization arising in
unpolarized hadronic collisions, the new observable in principle also allows to
test the possible color flow dependence of single spin asymmetries and the
(non)universality of transverse momentum dependent fragmentation functions.Comment: 11 pages, 10 eps figures; minor modifications, conclusions unchanged,
version to be publishe

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