24,915 research outputs found
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Continuous-variable quantum teleportation of entanglement
Entangled coherent states can be used to determine the entanglement fidelity
for a device that is designed to teleport coherent states. This entanglement
fidelity is universal, in that the calculation is independent of the use of
entangled coherent states and applies generally to the teleportation of
entanglement using coherent states. The average fidelity is shown to be a poor
indicator of the capability of teleporting entanglement; i.e., very high
average fidelity for the quantum teleportation apparatus can still result in
low entanglement fidelity for one mode of the two-mode entangled coherent
state.Comment: 5 pages, 1 figure, published versio
Entanglement as a signature of quantum chaos
We explore the dynamics of entanglement in classically chaotic systems by
considering a multiqubit system that behaves collectively as a spin system
obeying the dynamics of the quantum kicked top. In the classical limit, the
kicked top exhibits both regular and chaotic dynamics depending on the strength
of the chaoticity parameter in the Hamiltonian. We show that the
entanglement of the multiqubit system, considered for both bipartite and
pairwise entanglement, yields a signature of quantum chaos. Whereas bipartite
entanglement is enhanced in the chaotic region, pairwise entanglement is
suppressed. Furthermore, we define a time-averaged entangling power and show
that this entangling power changes markedly as moves the system from
being predominantly regular to being predominantly chaotic, thus sharply
identifying the edge of chaos. When this entangling power is averaged over
initial states, it yields a signature of global chaos. The qualitative behavior
of this global entangling power is similar to that of the classical Lyapunov
exponent.Comment: 8 pages, 8 figure
Using a simple expert system to assist a powered wheelchair user
A simple expert system is described that helps wheelchair users to drive their wheelchairs. The expert system takes data in from sensors and a joystick, identifies obstacles and then recommends a safe route. Wheelchair users were timed while driving around a variety of routes and using a joystick controlling their wheelchair via the simple expert system. Ultrasonic sensors are used to detect the obstacles. The simple expert system performed better than other recently published systems. In more difficult situations, wheelchair drivers did better when there was help from a sensor system. Wheelchair users completed routes with the sensors and expert system and results are compared with the same users driving without any assistance. The new systems show a significant improvement
Resolving the Submillimeter Background: the 850-micron Galaxy Counts
Recent deep blank field submillimeter surveys have revealed a population of
luminous high redshift galaxies that emit most of their energy in the
submillimeter. The results suggest that much of the star formation at high
redshift may be hidden to optical observations. In this paper we present
wide-area 850-micron SCUBA data on the Hawaii Survey Fields SSA13, SSA17, and
SSA22. Combining these new data with our previous deep field data, we establish
the 850-micron galaxy counts from 2 mJy to 10 mJy with a >3-sigma detection
limit. The area coverage is 104 square arcmin to 8 mJy and 7.7 square arcmin to
2.3 mJy. The differential 850-micron counts are well described by the function
n(S)=N_0/(a+S^3.2), where S is the flux in mJy, N_0=3.0 x 10^4 per square
degree per mJy, and a=0.4-1.0 is chosen to match the 850-micron extragalactic
background light. Between 20 to 30 per cent of the 850-micron background
resides in sources brighter than 2 mJy. Using an empirical fit to our >2 mJy
data constrained by the EBL at lower fluxes, we argue that the bulk of the
850-micron extragalactic background light resides in sources with fluxes near 1
mJy. The submillimeter sources are plausible progenitors of the present-day
spheroidal population.Comment: 5 pages, accepted by The Astrophysical Journal Letter
Mid-infrared and optical spectroscopy of ultraluminous infrared galaxies: A comparison
New tools from Infrared Space Observatory (ISO) mid-infrared spectroscopy
have recently become available to determine the power sources of dust-obscured
ultraluminous infrared galaxies (ULIRGs). We compare ISO classifications -
starburst or active galactic nucleus (AGN) - with classifications from optical
spectroscopy, and with optical/near-infrared searches for hidden broad-line
regions. The agreement between mid-infrared and optical classification is
excellent if optical LINER spectra are assigned to the starburst group. The
starburst nature of ULIRG LINERs strongly supports the suggestion that LINER
spectra in infrared-selected galaxies, rather than being an expression of the
AGN phenomenon, are due to shocks that are probably related to galactic
superwinds. Differences between ISO and optical classification provide clues on
the evolution of ULIRGs and on the configuration of obscuring dust. We find few
ISO AGN with optical HII or LINER identification, suggesting that highly
obscured AGN exist but are not typical for the ULIRG phenomenon in general.
Rather, our results indicate that strong AGN activity, once triggered, quickly
breaks the obscuring screen at least in certain directions, thus becoming
detectable over a wide wavelength range.Comment: aastex, 1 eps figure. Accepted by ApJ (Letters
Bars from the Inside Out: An HST Study of their Dusty Circumnuclear Regions
The results of bar-driven mass inflow are directly observable in
high-resolution HST observations of their circumnuclear regions. These
observations reveal a wealth of structures dominated by dust lanes, often with
a spiral-like morphology, and recent star formation. Recent work has shown that
some of these structures are correlated with the presence or absence of a bar.
I extend this work with an investigation of circumnuclear morphology as a
function of bar strength for a sample of 48 galaxies with both measured bar
strengths and ``structure maps'' computed from HST images. The structure maps
for these galaxies, which have projected spatial resolutions of 2 - 15 pc, show
that the fraction of galaxies with grand-design (GD) circumnuclear dust spirals
increases significantly with bar strength, while tightly wound dust spirals are
only present in the most axisymmetric galaxies. GD structure is only found at
the centers of galaxies classified as SB(s) or SB(rs) and not SB(r). SB(s)
galaxies on average have stronger bars than SB(r) galaxies. There is also a
modest increase in the fraction of loosely wound dust spirals at later
morphological types, which may reflect an increase in the fraction of galaxies
with circumnuclear, gaseous disks. (abridged)Comment: 10 pages, 6 figures. To appear in "Penetrating Bars through Masks of
Cosmic Dust: The Hubble Tuning Fork strikes a New Note" held June 7-12th,
2004 in Pilanesberg National Park, South Africa. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/safrica04.pd
Modified gravity without dark matter
On an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
RelativityComment: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy, minor errors corrected,
references update
Patient Choice for Older People in English NHS Primary Care: Theory and Practice
In the English National Health Service (NHS), patients are now expected to choose the time and place of treatment and even choose the actual treatment. However, the theory on which patient choice is based and the implementation of patient choice are controversial. There is evidence to indicate that attitudes and abilities to make choices are relatively sophisticated and not as straightforward as policy developments suggest. In addition, and surprisingly, there is little research on whethermaking individual choices about care is regarded as a priority by the largest NHS patient group and the single largest group for most GPsâolder people.This conceptual paper examines the theory of patient choice concerning accessing and engaging with healthcare provision and reviews existing evidence on older people and patient choice in primary care
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