18,021 research outputs found
An effective theory of accelerated expansion
We work out an effective theory of accelerated expansion to describe general
phenomena of inflation and acceleration (dark energy) in the Universe. Our aim
is to determine from theoretical grounds, in a physically-motivated and model
independent way, which and how many (free) parameters are needed to broadly
capture the physics of a theory describing cosmic acceleration. Our goal is to
make as much as possible transparent the physical interpretation of the
parameters describing the expansion. We show that, at leading order, there are
five independent parameters, of which one can be constrained via general
relativity tests. The other four parameters need to be determined by observing
and measuring the cosmic expansion rate only, H(z). Therefore we suggest that
future cosmology surveys focus on obtaining an accurate as possible measurement
of to constrain the nature of accelerated expansion (dark energy and/or
inflation).Comment: In press; minor changes, results unchange
Dual Instantons
We show how to map the Belavin-Polyakov instantons of the O(3)-nonlinear
model to a dual theory where they then appear as nontopological
solitons. They are stationary points of the Euclidean action in the dual
theory, and moreover, the dual action and the O(3)-nonlinear model
action agree on shell.Comment: 13 page
Spin dynamics in electrochemically charged CdSe quantum dots
We use time-resolved Faraday rotation to measure coherent spin dynamics in
colloidal CdSe quantum dots charged in an electrochemical cell at room
temperature. Filling of the 1Se electron level is demonstrated by the bleaching
of the 1Se-1S3/2 absorption peak. One of the two Lande g-factors observed in
uncharged quantum dots disappears upon filling of the 1Se electron state. The
transverse spin coherence time, which is over 1 ns and is limited by
inhomogeneous dephasing, also appears to increase with charging voltage. The
amplitude of the spin precession signal peaks near the half-filling potential.
Its evolution at charging potentials without any observable bleaching of the
1Se-1S3/2 transition suggests that the spin dynamics are influenced by
low-energy surface states.Comment: 4 pages, 4 figure
Shot Noise in Anyonic Mach-Zehnder Interferometer
We show how shot noise in an electronic Mach-Zehnder interferometer in the
fractional quantum Hall regime probes the charge and statistics of quantum Hall
quasiparticles. The dependence of the noise on the magnetic flux through the
interferometer allows for a simple way to distinguish Abelian from non-Abelian
quasiparticle statistics. In the Abelian case, the Fano factor (in units of the
electron charge) is always lower than unity. In the non-Abelian case, the
maximal Fano factor as a function of the magnetic flux exceeds one.Comment: references adde
IR Kuiper Belt Constraints
We compute the temperature and IR signal of particles of radius and
albedo at heliocentric distance , taking into account the
emissivity effect, and give an interpolating formula for the result. We compare
with analyses of COBE DIRBE data by others (including recent detection of the
cosmic IR background) for various values of heliocentric distance, ,
particle radius, , and particle albedo, . We then apply these
results to a recently-developed picture of the Kuiper belt as a two-sector disk
with a nearby, low-density sector (40<R<50-90 AU) and a more distant sector
with a higher density. We consider the case in which passage through a
molecular cloud essentially cleans the Solar System of dust. We apply a simple
model of dust production by comet collisions and removal by the
Poynting-Robertson effect to find limits on total and dust masses in the near
and far sectors as a function of time since such a passage. Finally we compare
Kuiper belt IR spectra for various parameter values.Comment: 34 pages, LaTeX, uses aasms4.sty, 11 PostScript figures not embedded.
A number of substantive comments by a particularly thoughtful referee have
been addresse
Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves
Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478
(1997)], we employ semiclassical composite-fermion theory to study the effect
of a periodic density modulation on a quantum Hall system near Landau level
filling factor nu=1/2. We show that even a weak density modulation leads to
dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an
explanation for several key features of the experimental observations. We
predict that properly arranged dc transport measurements would show a structure
similar to that seen in SAW measurements.Comment: Version published in Phys. Rev. Lett. Figures changed to show SAW
velocity shift. LaTeX, 5 pages, two included postscript figure
Serotonin Syndrome after Concomitant Treatment with Linezolid and Citalopram
Linezolid, a new synthetic antimicrobial, is an important weapon against methicillin-resistant Staphylococcus aureus (MRSA). Although there are reports of serotonin syndrome developing after concomitant use of linezolid and the selective serotonin reuptake inhibitor paroxitene, this report concerns a patient receiving citalopram who developed thrombocytopenia, serotonin syndrome, and lactic acidosis and died following long-term linezolid therap
Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured
Mergers of galaxies are thought to cause significant gas inflows to the inner
parsecs, which can activate rapid accretion onto supermassive black holes
(SMBHs), giving rise to Active Galactic Nuclei (AGN). During a significant
fraction of this process, SMBHs are predicted to be enshrouded by gas and dust.
Studying 52 galactic nuclei in infrared-selected local Luminous and
Ultra-luminous infrared galaxies in different merger stages in the hard X-ray
band, where radiation is less affected by absorption, we find that the amount
of material around SMBHs increases during the last phases of the merger. We
find that the fraction of Compton-thick (CT, ) AGN in late merger galaxies is higher
() than in local hard X-ray selected AGN
(), and that obscuration reaches its maximum when the
nuclei of the two merging galaxies are at a projected distance of
kiloparsecs (). We also
find that all AGN of our sample in late merger galaxies have , which implies that the obscuring material covers
of the X-ray source. These observations show that the material
is most effectively funnelled from the galactic scale to the inner tens of
parsecs during the late stages of galaxy mergers, and that the close
environment of SMBHs in advanced mergers is richer in gas and dust with respect
to that of SMBHs in isolated galaxies, and cannot be explained by the classical
AGN unification model in which the torus is responsible for the obscuration.Comment: Final version matching the article published in MNRAS - 30 pages, 16
figure
The Calibration of the HST Kuiper Belt Object Search: Setting the Record Straight
The limiting magnitude of the HST data set used by Cochran et al. (1995) to
detect small objects in the Kuiper belt is reevaluated, and the methods used
are described in detail. It is shown, by implanting artificial objects in the
original HST images, and re-reducing the images using our original algorithm,
that the limiting magnitude of our images (as defined by the 50% detectability
limit) is . This value is statistically the same as the value found in
the original analysis. We find that of the moving Kuiper belt objects
with are detected when trailing losses are included. In the same data
in which these faint objects are detected, we find that the number of false
detections brighter than is less than one per WFPC2 image. We show
that, primarily due to a zero-point calibration error, but partly due to
inadequacies in modeling the HST'S data noise characteristics and Cochran et
al.'s reduction techniques, Brown et al. 1997 underestimate the SNR of objects
in the HST dataset by over a factor of 2, and their conclusions are therefore
invalid.Comment: Accepted to ApJ Letters; 10 pages plus 3 figures, LaTe
Scaling of excitons in carbon nanotubes
Light emission from carbon nanotubes is expected to be dominated by excitonic
recombination. Here we calculate the properties of excitons in nanotubes
embedded in a dielectric, for a wide range of tube radii and dielectric
environments. We find that simple scaling relationships give a good description
of the binding energy, exciton size, and oscillator strength.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let
- âŠ