28,097 research outputs found
Base manifolds for fibrations of projective irreducible symplectic manifolds
Given a projective irreducible symplectic manifold of dimension , a
projective manifold and a surjective holomorphic map with
connected fibers of positive dimension, we prove that is biholomorphic to
the projective space of dimension . The proof is obtained by exploiting two
geometric structures at general points of : the affine structure arising
from the action variables of the Lagrangian fibration and the structure
defined by the variety of minimal rational tangents on the Fano manifold
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.
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.
Light bulb heat exchanger for magnetohydrodynamic generator applications - Preliminary evaluation
The light-bulb heat-exchanger concept is investigated as a possible means of using a combustion heat source to supply energy to an inert gas MHD power generator system. In this concept, combustion gases flow through a central passage which consists of a duct with transparent walls through which heat is transferred by radiation to a radiation receiver which in turn heats the inert gas by convection. The effects of combustion-gas emissivity, transparent-wall-transmissivity, radiation-receiver emissivity, and the use of fins in the inert gas coolant passage are studied. The results indicate that inert gas outlet temperatures of 2500 K are possible for combustion temperatures of 3200 K and that sufficient energy can be transferred from the combustion gas to reduce its temperature to approximately 2000 K. At this temperature more conventional heat exchangers can be used
The Origin of Structures in Generalized Gravity
In a class of generalized gravity theories with general couplings between the
scalar field and the scalar curvature in the Lagrangian, we can describe the
quantum generation and the classical evolution of both the scalar and tensor
structures in a simple and unified manner. An accelerated expansion phase based
on the generalized gravity in the early universe drives microscopic quantum
fluctuations inside a causal domain to expand into macroscopic ripples in the
spacetime metric on scales larger than the local horizon. Following their
generation from quantum fluctuations, the ripples in the metric spend a long
period outside the causal domain. During this phase their evolution is
characterized by their conserved amplitudes. The evolution of these
fluctuations may lead to the observed large scale structures of the universe
and anisotropies in the cosmic microwave background radiation.Comment: 5 pages, latex, no figur
String theoretic axion coupling and the evolution of cosmic structures
We examine the effects of the axion coupling to on the evolution
of cosmic structures. It is shown that the evolutions of the scalar- and
vector-type perturbations are not affected by this axion coupling. However the
axion coupling causes an asymmetric evolution of the two polarization states of
the tensor-type perturbation, which may lead to a sizable polarization
asymmetry in the cosmological gravitational wave if inflation involves a period
in which the axion coupling is important. The polarization asymmetry produced
during inflation are conserved over the subsequent evolution as long as the
scales remain in the large-scale limit, and thus this may lead to an observable
trace in the cosmic microwave background radiation.Comment: 10 pages, REVte
COBE constraints on inflation models with a massive non-minimal scalar field
We derive power spectra of the scalar- and tensor-type structures generated
in an inflation model based on a massive non-minimally coupled scalar field
with the strong coupling assumption. We make analyses in both the
original-frame and the conformally transformed Einstein-frame. We derive
contributions of both structures to the anisotropy of the cosmic microwave
background radiation, and compare the contributions with the four-year COBE-DMR
data. Previous study showed that sufficient amount of inflation requires a
small coupling parameter. In such a case the spectra become near Zeldovich
spectra, and the gravitational wave contribution becomes negligible compared
with the scalar-type contribution which is testable in future CMBR experiments.Comment: 4 pages, no figure, To appear in Phys. Rev.
Webs of Lagrangian Tori in Projective Symplectic Manifolds
For a Lagrangian torus A in a simply-connected projective symplectic manifold
M, we prove that M has a hypersurface disjoint from a deformation of A. This
implies that a Lagrangian torus in a compact hyperk\"ahler manifold is a fiber
of an almost holomorphic Lagrangian fibration, giving an affirmative answer to
a question of Beauville's. Our proof employs two different tools: the theory of
action-angle variables for algebraically completely integrable Hamiltonian
systems and Wielandt's theory of subnormal subgroups.Comment: 18 pages, minor latex problem fixe
Impact of g-factors and valleys on spin qubits in a silicon double quantum dot
We define single electron spin qubits in a silicon MOS double quantum dot
system. By mapping the qubit resonance frequency as a function of gate-induced
electric field, the spectrum reveals an anticrossing that is consistent with an
inter-valley spin-orbit coupling. We fit the data from which we extract an
inter-valley coupling strength of 43 MHz. In addition, we observe a narrow
resonance near the primary qubit resonance when we operate the device in the
(1,1) charge configuration. The experimental data is consistent with a
simulation involving two weakly exchanged-coupled spins with a g-factor
difference of 1 MHz, of the same order as the Rabi frequency. We conclude that
the narrow resonance is the result of driven transitions between the T- and T+
triplet states, using an ESR signal of frequency located halfway between the
resonance frequencies of the two individual spins. The findings presented here
offer an alternative method of implementing two-qubit gates, of relevance to
the operation of larger scale spin qubit systems
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