33,508 research outputs found
On the absence of gravitational lensing of the cosmic microwave background
The magnification of distant sources by mass clumps at lower ()
redshifts is calculated analytically. The clumps are initially assumed to be
galaxy group isothermal spheres with properties inferred from an extensive
survey. The average effect, which includes strong lensing, is exactly
counteracted by the beam divergence in between clumps (more precisely, the
average reciprocal magnification cancels the inverse Dyer-Roeder
demagnification). This conclusion is in fact independent of the matter density
function within each clump, and remains valid for arbitrary densities of matter
and dark energy. When tested against the CMB, a rather large lensing induced
{\it dispersion} in the angular size of the primary acoustic peaks of the TT
power spectrum is inconsistent with WMAP observations. The situation is
unchanged by the use of NFW profiles for the density distribution of groups.
Finally, our formulae are applied to an ensemble of NFW mass clumps or
isothermal spheres having the parameters of galaxy {\it clusters}. The acoustic
peak size dispersion remains unobservably large, and is also excluded by WMAP.
For galaxy groups, two possible ways of reconciling with the data are proposed,
both exploiting maximally the uncertainties in our knowledge of group
properties. The same escape routes are not available in the case of clusters,
however, because their properties are well understood. Here we have a more
robust conclusion: neither of the widely accepted models are good description
of clusters, or important elements of physics responsible for shaping zero
curvature space are missing from the standard cosmological model. When all the
effects are accrued, it is difficult to understand how WMAP could reveal no
evidence whatsoever of lensing by groups and clusters.Comment: ApJ v628, pp. 583-593 (August 1, 2005
The Counting of Generalized Polarizabilities
We demonstrate a concise method to enumerate the number of generalized
polarizabilities---quantities characterizing the independent observables in
singly-virtual Compton scattering---for a target particle of arbitrary spin s.
By using crossing symmetry and J^{PC} conservation, we show that this number is
(10s+1+delta_{s,0}).Comment: 10 pages, revtex4, no figures. Version to appear in Phys. Rev. D.
Paper now divided into sections and clarifying comments added, but physics
content unchange
Visualization of hydrogen injection in a scramjet engine by simultaneous PLIF imaging and laser holographic imaging
Flowfield characterization has been accomplished for several fuel injector configurations using simultaneous planar laser induced fluorescence (PLIF) and laser holographic imaging (LHI). The experiments were carried out in the GASL-NASA HYPULSE real gas expansion tube facility, a pulsed facility with steady test times of about 350 microsec. The tests were done at simulated Mach numbers 13.5 and 17. The focus of this paper is on the measurement technologies used and their application in a research facility. The HYPULSE facility, the models used for the experiments, and the setup for the LHI and PLIF measurements are described. Measurement challenges and solutions are discussed. Results are presented for experiments with several fuel injector configurations and several equivalence ratios
Substituting Quantum Entanglement for Communication
We show that quantum entanglement can be used as a substitute for
communication when the goal is to compute a function whose input data is
distributed among remote parties. Specifically, we show that, for a particular
function among three parties (each of which possesses part of the function's
input), a prior quantum entanglement enables one of them to learn the value of
the function with only two bits of communication occurring among the parties,
whereas, without quantum entanglement, three bits of communication are
necessary. This result contrasts the well-known fact that quantum entanglement
cannot be used to simulate communication among remote parties.Comment: 4 pages REVTeX, no figures. Minor correction
ALMA detection of [CII] 158 micron emission from a strongly lensed z=2 star-forming galaxy
Our objectives are to determine the properties of the interstellar medium
(ISM) and of star-formation in typical star-forming galaxies at high redshift.
Following up on our previous multi-wavelength observations with HST, Spitzer,
Herschel, and the Plateau de Bure Interferometer (PdBI), we have studied a
strongly lensed z=2.013 galaxy, the arc behind the galaxy cluster MACS
J0451+0006, with ALMA to measure the [CII] 158 micron emission line, one of the
main coolants of the ISM. [CII] emission from the southern part of this galaxy
is detected at 10 . Taking into account strong gravitational lensing,
which provides a magnification of , the intrinsic lensing-corrected
[CII]158 micron luminosity is . The observed
ratio of [CII]-to-IR emission, , is found to be similar to that in nearby galaxies. The same also
holds for the observed ratio , which is
comparable to that of star-forming galaxies and active galaxy nuclei (AGN) at
low redshift. We utilize strong gravitational lensing to extend diagnostic
studies of the cold ISM to an order of magnitude lower luminosity () and SFR than previous work at high redshift.
While larger samples are needed, our results provide evidence that the cold ISM
of typical high redshift galaxies has physical characteristics similar to
normal star forming galaxies in the local Universe.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysics, Letter
Monitoring stimulated emission at the single photon level in one-dimensional atoms
We theoretically investigate signatures of stimulated emission at the single
photon level for a two-level atom interacting with a one-dimensional light
field. We consider the transient regime where the atom is initially excited,
and the steady state regime where the atom is continuously driven with an
external pump. The influence of pure dephasing is studied, clearly showing that
these effects can be evidenced with state of the art solid state devices. We
finally propose a scheme to demonstrate the stimulation of one optical
transition by monitoring another one, in three-level one-dimensional atoms.Comment: 4 pages, 4 figures. Improved introduction; Comments adde
Distributed computing system with dual independent communications paths between computers and employing split tokens
This is a distributed computing system providing flexible fault tolerance; ease of software design and concurrency specification; and dynamic balance of the loads. The system comprises a plurality of computers each having a first input/output interface and a second input/output interface for interfacing to communications networks each second input/output interface including a bypass for bypassing the associated computer. A global communications network interconnects the first input/output interfaces for providing each computer the ability to broadcast messages simultaneously to the remainder of the computers. A meshwork communications network interconnects the second input/output interfaces providing each computer with the ability to establish a communications link with another of the computers bypassing the remainder of computers. Each computer is controlled by a resident copy of a common operating system. Communications between respective ones of computers is by means of split tokens each having a moving first portion which is sent from computer to computer and a resident second portion which is disposed in the memory of at least one of computer and wherein the location of the second portion is part of the first portion. The split tokens represent both functions to be executed by the computers and data to be employed in the execution of the functions. The first input/output interfaces each include logic for detecting a collision between messages and for terminating the broadcasting of a message whereby collisions between messages are detected and avoided
Ultraviolet Spectra of CV Accretion Disks with Non-Steady T(r) Laws
An extensive grid of synthetic mid- and far-ultraviolet spectra for accretion
disks in cataclysmic variables has been presented by Wade and Hubeny (1998). In
those models, the disk was assumed to be in steady-state, that is T_eff(r) is
specified completely by the mass M_WD and radius R_WD of the accreting white
dwarf star and the mass transfer rate M_dot which is constant throughout the
disk. In these models, T_eff(r) is proportional to r^{-3/4} except as modified
by a cutoff term near the white dwarf.
Actual disks may vary from the steady-state prescription for T_eff(r),
however, e.g. owing to outburst cycles in dwarf novae M_dot not constant with
radius) or irradiation (in which case T_eff in the outer disk is raised above
T_steady). To show how the spectra of such disks might differ from the steady
case, we present a study of the ultraviolet (UV) spectra of models in which
power-law temperature profiles T_eff(r) is proportional to r^{-gamma} with
gamma < 3/4 are specified. Otherwise, the construction of the models is the
same as in the Wade & Hubeny grid, to allow comparison. We discuss both the UV
spectral energy distributions and the appearance of the UV line spectra. We
also briefly discuss the eclipse light curves of the non-standard models.
Comparison of these models with UV observations of novalike variables suggests
that better agreement may be possible with such modified T_eff(r) profiles.Comment: 13 pages, 6 figures (one reduced quality), ApJ in pres
Pure point diffraction implies zero entropy for Delone sets with uniform cluster frequencies
Delone sets of finite local complexity in Euclidean space are investigated.
We show that such a set has patch counting and topological entropy 0 if it has
uniform cluster frequencies and is pure point diffractive. We also note that
the patch counting entropy is 0 whenever the repetitivity function satisfies a
certain growth restriction.Comment: 16 pages; revised and slightly expanded versio
On the Existence of Heavy Pentaquarks: The large Nc and Heavy Quark Limits and Beyond
We present a very general argument that the analogue of a heavy pentaquark (a
state with the quantum numbers of a baryon combined with an additional light
quark and a heavy antiquark) must exist as a particle stable under strong
interactions in the combined heavy quark and large Nc limits of QCD. Moreover,
in the combined limit these heavy pentaquark states fill multiplets of
SU(4)xO(8)xSU(2). We explore the question of whether corrections in the
combined 1/Nc and 1/mQ expansions are sufficiently small to maintain this
qualitative result. Since no model-independent way is known to answer this
question, we use a class of ``realistic'' hadronic models in which a pentaquark
can be formed via nucleon-heavy meson binding through a pion-exchange
potential. These models have the virtue that they necessarily yield the correct
behavior in the combined limit, and the long-distance parts of the interactions
are model independent. If the long-distance attraction in these models were to
predict bound states in a robust way (i.e., largely insensitive to the details
of the short-range interaction), then one could safely conclude that heavy
pentaquarks do exist. However, in practice the binding does depend very
strongly on the details of the short-distance physics, suggesting that the real
world is not sufficiently near the combined large Nc, mQ limit to use it as a
reliable guide. Whether stable heavy pentaquarks exist remains an open
question.Comment: 11 pages; references adde
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