25,352 research outputs found
Viscous dark fluid universe
We investigate the cosmological perturbation dynamics for a universe
consisting of pressureless baryonic matter and a viscous fluid, the latter
representing a unified model of the dark sector. In the homogeneous and
isotropic background the \textit{total} energy density of this mixture behaves
as a generalized Chaplygin gas. The perturbations of this energy density are
intrinsically non-adiabatic and source relative entropy perturbations. The
resulting baryonic matter power spectrum is shown to be compatible with the
2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also
Hubble-function and supernovae Ia data, shows that, different from other
studies, there exists a maximum in the probability distribution for a negative
present value of the deceleration parameter. Moreover,
while previous descriptions on the basis of generalized Chaplygin gas models
were incompatible with the matter power spectrum data since they required a
much too large amount of pressureless matter, the unified model presented here
favors a matter content that is of the order of the baryonic matter abundance
suggested by big-bang nucleosynthesis.Comment: 19 pages, 6 figure
Radio signatures of solar energetic particles during the 23rd solar cycle
We present the association rates between solar energetic particles (SEPs) and
the radio emission signatures in the corona and IP space during the entire
solar cycle 23. We selected SEPs associated with X and M-class flares from the
visible solar hemisphere. All SEP events are also accompanied by coronal mass
ejections. Here, we focus on the correlation between the SEP events and the
appearance of radio type II, III and IV bursts on dynamic spectra. For this we
used the available radio data from ground-based stations and the Wind/WAVES
spacecraft. The associations are presented separately for SEP events
accompanying activity in the eastern and western solar hemisphere. We find the
highest association rate of SEP events to be with type III bursts, followed by
types II and IV. Whereas for types III and IV no longitudinal dependence is
noticed, these is a tendency for a higher SEP-association rate with type II
bursts in the eastern hemisphere. A comparison with reports from previous
studies is briefly discussed.Comment: http://adsabs.harvard.edu.ezproxy.obspm.fr/abs/2013CEAB...37..541
Frequency and risk factors for incident and redetected Chlamydia trachomatis infection in sexually active, young, multi-ethnic women: a community based cohort study.
OBJECTIVE: To investigate the frequency and risk factors for incident and redetected Chlamydia trachomatis infection in sexually active, young, multi-ethnic women in the community. DESIGN: Cohort study. SETTING: 20 London universities and Further Education colleges. PARTICIPANTS: 954 sexually experienced women, mean age 21.5 years (range 16-27), 26% from ethnic minorities, who were recruited to the Prevention of Pelvic Infection (POPI) chlamydia screening trial between 2004 and 2006, and returned repeat postal self-taken vaginal swabs 11-32 (median 16) months after recruitment. RESULTS: The estimated annual incidence of chlamydia infection among 907 women who tested negative at baseline was 3.4 per 100 person-years (95% CI 2.5 to 4.6 per 100 person-years), but 6.6 per 100 person-years (95% CI 4.5 to 9.3 per 100 person-years) in the 326 teenagers (<20 years). Predictors of incident chlamydia infection were age <20 years (relative risk (RR) 4.0, 95% CI 2.1 to 7.5), and (after adjusting for age) a new sexual partner during 12 months follow-up (RR 4.4, 95% CI 2.0 to 9.9), smoking (RR 2.2 95% CI 1.2 to 3.9), concurrent bacterial vaginosis (RR 2.0 95% CI 1.1 to 3.9) and high risk carcinogenic human papillomavirus (RR 2.2, 95% CI 1.1 to 4.3). Of 47 women positive for chlamydia at baseline, 12 (25.5%, 95% CI 13.9% to 40.3%) had redetected infection at a median of 16 months follow-up. Taking into account follow-up time (65 person-years), the annual redetection rate was 18.5 per 100 person-years (95% CI 9.9 to 30.0 per 100 person-years). CONCLUSIONS: One in four women with chlamydia infection at baseline retested positive, supporting recent recommendations to routinely retest chlamydia positives
On the Computation of Power in Volume Integral Equation Formulations
We present simple and stable formulas for computing power (including
absorbed/radiated, scattered and extinction power) in current-based volume
integral equation formulations. The proposed formulas are given in terms of
vector-matrix-vector products of quantities found solely in the associated
linear system. In addition to their efficiency, the derived expressions can
guarantee the positivity of the computed power. We also discuss the application
of Poynting's theorem for the case of sources immersed in dissipative
materials. The formulas are validated against results obtained both with
analytical and numerical methods for scattering and radiation benchmark cases
Effect of heat treatment and aging on the mechanical loss and strength of hydroxide catalysis bonds between fused silica samples
Hydroxide catalysis bonds are used in the aLIGO gravitational wave detectors and are an essential technology within the mirror suspensions which allowed for detector sensitivities to be reached that enabled the first direct detections of gravitational waves. Methods aimed at further improving hydroxide catalysis bonds for future upgrades to these detectors, in order to increase detection rates and the number of detectable sources, are explored. Also, the effect on the bonds of an aLIGO suspension construction procedure involving heat, the fibre welding process, is investigated. Here we show that thermal treatments can be beneficial to improving some of the bond properties important to the mirror suspensions in interferometric gravitational wave detectors. It was found that heat treating bonds at 150\,^\circC increases bond strength by a factor of approximately 1.5 and a combination of bond ageing and heat treatment of the optics at 150\,\circC reduces the mechanical loss of a bond from 0.10 to 0.05. It is also shown that current construction procedures do not reduce bond strength
Constraining the History of the Sagittarius Dwarf Galaxy Using Observations of its Tidal Debris
We present a comparison of semi-analytic models of the phase-space structure
of tidal debris with observations of stars associated with the Sagittarius
dwarf galaxy (Sgr). We find that many features in the data can be explained by
these models. The properties of stars 10-15 degrees away from the center of Sgr
--- in particular, the orientation of material perpendicular to Sgr's orbit
(c.f. Alard 1996) and the kink in the velocity gradient (Ibata et al 1997) ---
are consistent with those expected for unbound material stripped during the
most recent pericentric passage ~50 Myrs ago. The break in the slope of the
surface density seen by Mateo, Olszewski & Morrison (1998) at ~ b=-35 can be
understood as marking the end of this material. However, the detections beyond
this point are unlikely to represent debris in a trailing streamer, torn from
Sgr during the immediately preceding passage ~0.7 Gyrs ago, but are more
plausibly explained by a leading streamer of material that was lost more that 1
Gyr ago and has wrapped all the way around the Galaxy. The observations
reported in Majewski et al (1999) also support this hypothesis. We determine
debris models with these properties on orbits that are consistent with the
currently known positions and velocities of Sgr in Galactic potentials with
halo components that have circular velocities v_circ=140-200 km/s. The best
match to the data is obtained in models where Sgr currently has a mass of ~10^9
M_sun and has orbited the Galaxy for at least the last 1 Gyr, during which time
it has reduced its mass by a factor of 2-3, or luminosity by an amount
equivalent to ~10% of the total luminosity of the Galactic halo. These numbers
suggest that Sgr is rapidly disrupting and unlikely to survive beyond a few
more pericentric passages.Comment: 19 pages, 5 figures, accepted to Astronomical Journa
Brown dwarfs in the Hyades and beyond?
We have used both the Low-Resolution Imaging Spectrograph and the HIRES
echelle spectrograph on the Keck telescopes to obtain spectra of twelve
candidate members of the Hyades cluster identified by Leggett and Hawkins
(1988, 1989). All of the objects are chromospherically-active, late-type
M-dwarfs, with H equivalent widths varying from 1 to 30\AA. Based on
our measured radial velocities, the level of stellar activity and other
spectroscopic features, only one of the twelve stars has properties consistent
with cluster membership. We consider how this result affects estimates of the
luminosity and mass function of the Hyades cluster. Five of the eleven field
stars have weak K I 7665/7699\AA and CaH absorption as compared with M-dwarf
standards of the same spectral type, suggesting a lower surface gravity. Two of
these sources, LH0416+14 and LH0419+15, exhibit significant lithium 6708 \AA
absorption. Based partly on parallax measurements by the US Naval Observatory
(Harris et al, 1998), we identify all five as likely to be young, pre-main
sequence objects in or near the Taurus-Auriga association at distances of
between 150 and 250 parsecs. A comparison with theoretical models of pre-main
sequence stars indicates masses of less than 0.05 M.Comment: to appear in AJ, January 1999; 34 pages, (Latex format), including 10
embedded postscript figures and two table
Size-resolved aerosol and cloud condensation nuclei (CCN) properties in the remote marine South China Sea - Part 1: Observations and source classification
Abstract. Ship-based measurements of aerosol and cloud condensation nuclei (CCN) properties are presented for 2 weeks of observations in remote marine regions of the South China Sea/East Sea during the southwestern monsoon (SWM) season. Smoke from extensive biomass burning throughout the Maritime Continent advected into this region during the SWM, where it was mixed with anthropogenic continental pollution and emissions from heavy shipping activities. Eight aerosol types were identified using a k-means cluster analysis with data from a size-resolved CCN characterization system. Interpretation of the clusters was supplemented by additional onboard aerosol and meteorological measurements, satellite, and model products for the region. A typical bimodal marine boundary layer background aerosol population was identified and observed mixing with accumulation mode aerosol from other sources, primarily smoke from fires in Borneo and Sumatra. Hygroscopicity was assessed using the κ parameter and was found to average 0.40 for samples dominated by aged accumulation mode smoke; 0.65 for accumulation mode marine aerosol; 0.60 in an anthropogenic aerosol plume; and 0.22 during a short period that was characterized by elevated levels of volatile organic compounds not associated with biomass burning impacts. As a special subset of the background marine aerosol, clean air masses substantially scrubbed of particles were observed following heavy precipitation or the passage of squall lines, with changes in observed aerosol properties occurring on the order of minutes. Average CN number concentrations, size distributions, and κ values are reported for each population type, along with CCN number concentrations for particles that activated at supersaturations between 0.14 and 0.85 %
Violations of Bell Inequalities for Measurements with Macroscopic Uncertainties: What does it Mean to Violate Macroscopic Local Realism?
We suggest to test the premise of ``macroscopic local realism'' which is
sufficient to derive Bell inequalities when measurements of photon number are
only accurate to an uncertainty of order photons, where is macroscopic.
Macroscopic local realism is only sufficient to imply, in the context of the
original Einstein-Podolsky-Rosen argument, fuzzy ``elements of reality'' which
have a macroscopic indeterminacy. We show therefore how the violation of local
realism in the presence of macroscopic uncertainties implies the failure of
``macroscopic local realism''. Quantum states violating this macroscopic local
realism are presented.Comment: 28 pages, 5 figures- new version is unchanged but tightened-20 pages,
5 figure
Controlling entanglement by direct quantum feedback
We discuss the generation of entanglement between electronic states of two
atoms in a cavity using direct quantum feedback schemes. We compare the effects
of different control Hamiltonians and detection processes in the performance of
entanglement production and show that the quantum-jump-based feedback proposed
by us in Phys. Rev. A {\bf 76} 010301(R) (2007) can protect highly entangled
states against decoherence. We provide analytical results that explain the
robustness of jump feedback, and also analyse the perspectives of experimental
implementation by scrutinising the effects of imperfections and approximations
in our model.Comment: 10 pages, 8 figures. To appear in PR
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