41,044 research outputs found
Are Black Holes Elementary Particles?
Quantum black holes are the smallest and heaviest conceivable elementary
particles. They have a microscopic size but a macroscopic mass. Several
fundamental types have been constructed with some remarkable properties.
Quantum black holes in the neighborhood of the Galaxy could resolve the paradox
of ultra-high energy cosmic rays detected in Earth's atmosphere. They may also
play a role as dark matter in cosmology.Comment: Lecture delivered in Conference on Particle Physics, Astrophysics and
Quantum Field Theory: 75 Years since Solvay, 27 -29 November 2008, Nanyang
Executive Centre, Singapore. 10 page
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Three-dimensional simulation of a new cooling strategy for proton exchange membrane fuel cell stack using a non-isothermal multiphase model
In this study, a new cooling strategy for a proton exchange membrane (PEM) fuel cell stack is investigated using a three-dimensional (3D) multiphase non-isothermal model. The new cooling strategy follows that of the Honda's Clarity design and further extends to a cooling unit every five cells in stacks. The stack consists of 5 fuel cells sharing the inlet and outlet manifolds for reactant gas flows. Each cell has 7-path serpentine flow fields with a counter-flow configuration arranged for hydrogen and air streams. The coolant flow fields are set at the two sides of the stack and are simplified as the convective heat transfer thermal boundary conditions. This study also compares two thermal boundary conditions, namely limited and infinite coolant flow rates, and their impacts on the distributions of oxygen, liquid water, current density and membrane hydration. The difference of local temperature between these two cooling conditions is as much as 6.9 K in the 5-cell stack, while it is only 1.7 K in a single cell. In addition, the increased vapor concentration at high temperature (and hence water saturation pressure) dilutes the oxygen content in the air flow, reducing local oxygen concentration. The higher temperature in the stack also causes low membrane hydration, and consequently poor cell performance and non-uniform current density distribution, as disclosed by the simulation. The work indicates the new cooling strategy can be optimized by increasing the heat transfer coefficient between the stack and coolant to mitigate local overheating and cell performance reduction
Horizon Mass Theorem
A new theorem for black holes is found. It is called the horizon mass
theorem. The horizon mass is the mass which cannot escape from the horizon of a
black hole. For all black holes: neutral, charged or rotating, the horizon mass
is always twice the irreducible mass observed at infinity. Previous theorems on
black holes are: 1. the singularity theorem, 2. the area theorem, 3. the
uniqueness theorem, 4. the positive energy theorem. The horizon mass theorem is
possibly the last general theorem for classical black holes. It is crucial for
understanding Hawking radiation and for investigating processes occurring near
the horizon.Comment: A new theorem for black holes is establishe
Modeling the Non-Thermal X-ray Tail Emission of Anomalous X-ray Pulsars
The paradigm for Anomalous X-ray Pulsars (AXPs) has evolved recently with the
discovery by INTEGRAL and RXTE of flat, hard X-ray components in three AXPs.
These non-thermal spectral components differ dramatically from the steeper
quasi-power-law tails seen in the classic X-ray band in these sources, and can
naturally be attributed to activity in the magnetosphere. Resonant, magnetic
Compton upscattering is a candidate mechanism for generating this new
component, since it is very efficient in the strong fields present near AXP
surfaces. In this paper, results from an inner magnetospheric model for
upscattering of surface thermal X-rays in AXPs are presented, using a kinetic
equation formalism and employing a QED magnetic scattering cross section.
Characteristically flat and strongly-polarized emission spectra are produced by
non-thermal electrons injected in the emission region. Spectral results depend
strongly on the observer's orientation and the magnetospheric locale of the
scattering, which couple directly to the angular distributions of photons
sampled. Constraints imposed by the Comptel upper bounds for these AXPs are
mentioned.Comment: 8 pages, 2 embedded figures, in Proc. of the Huangshan conference
"Astrophysics of Compact Objects," (2008) eds. Y.-F. Yuan, X.-D. Li and D.
Lai, (AIP Conf. Proc. 968, New York) p. 9
Single Top Quark Production and Decay at Next-to-leading Order in Hadron Collision
We present a calculation of the next-to-leading order QCD corrections, with
one-scale phase space slicing method, to single top quark production and decay
process at hadron colliders.
Using the helicity amplitude method, the angular correlation of the final state
partons and the spin correlation of the top quark are preserved. The effect of
the top quark width is also examined.Comment: 47 pages, 9 figure
Mechanisms of vascular smooth muscle contraction and the basis for pharmacologic treatment of smooth muscle disorders
The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.Accepted manuscrip
XMM-Newton Observations of High Redshift Quasars
We report on our XMM observations of the high redshift quasars BR 2237--0607
(z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found
in the XMM public archive. Contrary to former reports, we do not find high
redshift radio-loud quasars to be more absorbed than their radio-quiet
counterparts. We find that the optical to X-ray spectral index alpha-ox is
correlated with the luminosity density at 2500 A, but does not show a
correlation with redshift. The mean 2-10 keV power-law slope of the 9 high
redshift radio-quiet quasars in our sample for which a spectral analysis can be
performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA
observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and
significantly different from alpha-x=0.78 found for low redshift Broad-Line
Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars
look remarkably similar, we find a first indication of a difference in their
X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios
L/L_Edd. These observations give credence to the hypothesis of Mathur (2000)
that NLS1s are low luminosity cousins of high redshift quasars, both likely to
be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006
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