5,756 research outputs found
Effective Dynamics, Big Bounces and Scaling Symmetry in Bianchi Type I Loop Quantum Cosmology
The detailed formulation for loop quantum cosmology (LQC) in the Bianchi I
model with a scalar massless field has been constructed. In this paper, its
effective dynamics is studied in two improved strategies for implementing the
LQC discreteness corrections. Both schemes show that the big bang is replaced
by the big bounces, which take place up to three times, once in each diagonal
direction, when the area or volume scale factor approaches the critical values
in the Planck regime measured by the reference of the scalar field momentum.
These two strategies give different evolutions: In one scheme, the effective
dynamics is independent of the choice of the finite sized cell prescribed to
make Hamiltonian finite; in the other, the effective dynamics reacts to the
macroscopic scales introduced by the boundary conditions. Both schemes reveal
interesting symmetries of scaling, which are reminiscent of the relational
interpretation of quantum mechanics and also suggest that the fundamental
spatial scale (area gap) may give rise to a temporal scale.Comment: 19 pages, 6 figures, 1 table; one reference added; version to appear
in PR
Cosmological inflation driven by holonomy corrections of loop quantum cosmology
At the level of heuristic effective dynamics, we investigate the cosmological
inflation with holonomy corrections of loop quantum cosmology (LQC) in the
Friedmann-Robertson-Walker model with a single inflaton field subject to
a simple potential. In the symmetric bouncing scenario of LQC, the condition
for occurrence of the quantum bounce naturally and uniquely fixes the initial
conditions at the bouncing epoch. Around the quantum bounce, the universe
undergoes a short super-inflationary phase, which drives the inflaton field to
its potential hill and thus sets the proper initial conditions for the standard
slow-roll inflation. Between the super-inflation and the standard inflation,
there is a non-inflationary phase, which violates the slow-roll condition. The
violation of slow roll is expected to give some suppression on the low angular
power spectrum of the cosmic microwave background and different orders of
holonomy corrections shall yield different suppressions.Comment: 7 pages, 2 figures, 1 table; one reference added
Loop Quantum Cosmology in Bianchi Type I Models: Analytical Investigation
The comprehensive formulation for loop quantum cosmology in the spatially
flat, isotropic model was recently constructed. In this paper, the methods are
extended to the anisotropic Bianchi I cosmology. Both the precursor and the
improved strategies are applied and the expected results are established: (i)
the scalar field again serves as an internal clock and is treated as emergent
time; (ii) the total Hamiltonian constraint is derived by imposing the
fundamental discreteness and gives the evolution as a difference equation; and
(iii) the physical Hilbert space, Dirac observables and semi-classical states
are constructed rigorously. It is also shown that the state in the kinematical
Hilbert space associated with the classical singularity is decoupled in the
difference evolution equation, indicating that the big bounce may take place
when any of the area scales undergoes the vanishing behavior. The investigation
affirms the robustness of the framework used in the isotropic model by
enlarging its domain of validity and provides foundations to conduct the
detailed numerical analysis.Comment: 53 pages, 2 figures; more typos corrected; HyperTeX enable
Phenomenological loop quantum geometry of the Schwarzschild black hole
The interior of a Schwarzschild black hole is investigated at the level of
phenomenological dynamics with the discreteness corrections of loop quantum
geometry implemented in two different improved quantization schemes. In one
scheme, the classical black hole singularity is resolved by the quantum bounce,
which bridges the black hole interior with a white hole interior. In the other
scheme, the classical singularity is resolved and the event horizon is also
diffused by the quantum bounce. Jumping over the quantum bounce, the black hole
gives birth to a baby black hole with a much smaller mass. This lineage
continues as each classical black hole brings forth its own descendant in the
consecutive classical cycle, giving the whole extended spacetime fractal
structure, until the solution eventually descends into deep Planck regime,
signaling a breakdown of the semiclassical description. The issues of scaling
symmetry and no-hair theorem are also discussed.Comment: 26 pages, 6 figures, 1 table; version to appear in PR
Hmga2 is dispensable for pancreatic cancer development, metastasis, and therapy resistance.
Expression of the chromatin-associated protein HMGA2 correlates with progression, metastasis and therapy resistance in pancreatic ductal adenocarcinoma (PDAC). Hmga2 has also been identified as a marker of a transient subpopulation of PDAC cells that has increased metastatic ability. Here, we characterize the requirement for Hmga2 during growth, dissemination, and metastasis of PDAC in vivo using conditional inactivation of Hmga2 in well-established autochthonous mouse models of PDAC. Overall survival, primary tumour burden, presence of disseminated tumour cells in the peritoneal cavity or circulating tumour cells in the blood, and presence and number of metastases were not significantly different between mice with Hmga2-wildtype or Hmga2-deficient tumours. Treatment of mice with Hmga2-wildtype and Hmga2-deficient tumours with gemcitabine did not uncover a significant impact of Hmga2-deficiency on gemcitabine sensitivity. Hmga1 and Hmga2 overlap in their expression in both human and murine PDAC, however knockdown of Hmga1 in Hmga2-deficient cancer cells also did not decrease metastatic ability. Thus, Hmga2 remains a prognostic marker which identifies a metastatic cancer cell state in primary PDAC, however Hmga2 has limited if any direct functional impact on PDAC progression and therapy resistance
Specific and heritable genetic interference by double-stranded RNA in Arabidopsis thaliana
We investigated the potential of double-stranded RNA interference (RNAi) with gene activity in Arabidopsis thaliana. To construct transformation vectors that produce RNAs capable of duplex formation, gene-specific sequences in the sense and antisense orientations were linked and placed under the control of a strong viral promoter. When introduced into the genome of A. thaliana by Agrobacterium-mediated transformation, double-stranded RNA-expressing constructs corresponding to four genes, AGAMOUS (AG), CLAVATA3, APETALA1, and PERIANTHIA, caused specific and heritable genetic interference. The severity of phenotypes varied between transgenic lines. In situ hybridization revealed a correlation between a declining AG mRNA accumulation and increasingly severe phenotypes in AG (RNAi) mutants, suggesting that endogenous mRNA is the target of double-stranded RNA-mediated genetic interference. The ability to generate stably heritable RNAi and the resultant specific phenotypes allows us to selectively reduce gene function in A. thaliana
Foldy-Wouthuysen transformation for a Dirac-Pauli dyon and the Thomas-Bargmann-Michel-Telegdi equation
The classical dynamics for a charged point particle with intrinsic spin is
governed by a relativistic Hamiltonian for the orbital motion and by the
Thomas-Bargmann-Michel-Telegdi equation for the precession of the spin. It is
natural to ask whether the classical Hamiltonian (with both the orbital and
spin parts) is consistent with that in the relativistic quantum theory for a
spin-1/2 charged particle, which is described by the Dirac equation. In the
low-energy limit, up to terms of the 7th order in ( and
is the particle mass), we investigate the Foldy-Wouthuysen (FW) transformation
of the Dirac Hamiltonian in the presence of homogeneous and static
electromagnetic fields and show that it is indeed in agreement with the
classical Hamiltonian with the gyromagnetic ratio being equal to 2. Through
electromagnetic duality, this result can be generalized for a spin-1/2 dyon,
which has both electric and magnetic charges and thus possesses both intrinsic
electric and magnetic dipole moments. Furthermore, the relativistic quantum
theory for a spin-1/2 dyon with arbitrary values of the gyromagnetic and
gyroelectric ratios can be described by the Dirac-Pauli equation, which is the
Dirac equation with augmentation for the anomalous electric and anomalous
magnetic dipole moments. The FW transformation of the Dirac-Pauli Hamiltonian
is shown, up to the 7th order again, to be also in accord with the classical
Hamiltonian.Comment: 18 page
On the unitarity of higher-dervative and nonlocal theories
We consider two simple models of higher-derivative and nonlocal quantu
systems.It is shown that, contrary to some claims found in literature, they can
be made unitary.Comment: 8 pages, no figure
Data fusion with artificial neural networks (ANN) for classification of earth surface from microwave satellite measurements
A data fusion system with artificial neural networks (ANN) is used for fast and accurate classification of five earth surface conditions and surface changes, based on seven SSMI multichannel microwave satellite measurements. The measurements include brightness temperatures at 19, 22, 37, and 85 GHz at both H and V polarizations (only V at 22 GHz). The seven channel measurements are processed through a convolution computation such that all measurements are located at same grid. Five surface classes including non-scattering surface, precipitation over land, over ocean, snow, and desert are identified from ground-truth observations. The system processes sensory data in three consecutive phases: (1) pre-processing to extract feature vectors and enhance separability among detected classes; (2) preliminary classification of Earth surface patterns using two separate and parallely acting classifiers: back-propagation neural network and binary decision tree classifiers; and (3) data fusion of results from preliminary classifiers to obtain the optimal performance in overall classification. Both the binary decision tree classifier and the fusion processing centers are implemented by neural network architectures. The fusion system configuration is a hierarchical neural network architecture, in which each functional neural net will handle different processing phases in a pipelined fashion. There is a total of around 13,500 samples for this analysis, of which 4 percent are used as the training set and 96 percent as the testing set. After training, this classification system is able to bring up the detection accuracy to 94 percent compared with 88 percent for back-propagation artificial neural networks and 80 percent for binary decision tree classifiers. The neural network data fusion classification is currently under progress to be integrated in an image processing system at NOAA and to be implemented in a prototype of a massively parallel and dynamically reconfigurable Modular Neural Ring (MNR)
The Effect of Intangible Resources on Selling, General, and Administrative Cost Behavior of Young and Established Firms
In this paper we investigate the effect of intangible resources on the relationship between activities and SG&A costs, and examine this effect on young and established firms. Prior research shows that costs are sticky in that costs decrease to a lesser extent following decrease in activities than they do increase following increase in activities of the same magnitude. We hypothesize that firms relying on intangible resources will exhibit stronger sticky cost behavior because (i) intangible resources are strategic resources and (ii) they possess unique properties that constrain managers’ ability to selectively cut resources. Using a large sample of US firms, we show that costs are more sticky with increase in intangibles. We also show that the effect of intangibles on cost behavior is present only among young firms. These results are consistent with the notion that managers of young firms focus on building capacity, learning, and maintaining flexibilities whereas managers of established firms focus on efficiency
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