15,917 research outputs found
The Peculiar Velocities of Local Type Ia Supernovae and their Impact on Cosmology
We quantify the effect of supernova Type Ia peculiar velocities on the
derivation of cosmological parameters. The published distant and local Ia SNe
used for the Supernova Legacy Survey first-year cosmology report form the
sample for this study. While previous work has assumed that the local SNe are
at rest in the CMB frame (the No Flow assumption), we test this assumption by
applying peculiar velocity corrections to the local SNe using three different
flow models. The models are based on the IRAS PSCz galaxy redshift survey, have
varying beta = Omega_m^0.6/b, and reproduce the Local Group motion in the CMB
frame. These datasets are then fit for w, Omega_m, and Omega_Lambda using
flatness or LambdaCDM and a BAO prior. The chi^2 statistic is used to examine
the effect of the velocity corrections on the quality of the fits. The most
favored model is the beta=0.5 model, which produces a fit significantly better
than the No Flow assumption, consistent with previous peculiar velocity
studies. By comparing the No Flow assumption with the favored models we derive
the largest potential systematic error in w caused by ignoring peculiar
velocities to be Delta w = +0.04. For Omega_Lambda, the potential error is
Delta Omega_Lambda = -0.04 and for Omega_m, the potential error is Delta
Omega_m < +0.01. The favored flow model (beta=0.5) produces the following
cosmological parameters: w = -1.08 (+0.09,-0.08), Omega_m = 0.27 (+0.02,-0.02)
assuming a flat cosmology, and Omega_Lambda = 0.80 (+0.08,-0.07) and Omega_m =
0.27 (+0.02,-0.02) for a w = -1 (LambdaCDM) cosmology.Comment: 4 pages, 2 figures, 1 table, accepted for publication in ApJ Letter
Swept-wavelength mid-infrared fiber laser for real-time ammonia gas sensing
The mid-infrared (mid-IR) spectral region holds great promise for new
laser-based sensing technologies, based on measuring strong mid-IR molecular
absorption features. Practical applications have been limited to date, however,
by current low-brightness broadband mid-IR light sources and slow
acquisition-time detection systems. Here, we report a new approach by
developing a swept-wavelength mid-infrared fiber laser, exploiting the broad
emission of dysprosium and using an acousto-optic tunable filter to achieve
electronically controlled swept-wavelength operation from 2.89 to 3.25 {\mu}m
(3070-3460 cm^-1). Ammonia (NH3) absorption spectroscopy is demonstrated using
this swept source with a simple room-temperature single-pixel detector, with
0.3 nm resolution and 40 ms acquisition time. This creates new opportunities
for real-time high-sensitivity remote sensing using simple, compact mid-IR
fiber-based technologies.Comment: Invited article for APL Photonic
Ten Years of Solar Change as Monitored by SBUV and SBUV/2
Observations of the Sun by the Solar Backscatter Ultraviolet (SBUV) instrument aboard Nimbus 7 and the SBUV/2 instrument aboard NOAA-9 reveal variations in the solar irradiance from 1978, to 1988. The maximum to minimum solar change estimated from the Heath and Schlesinger Mg index and wavelength scaling factors is about 4 percent from 210 to 260 nm and 8 percent for 180 to 210 nm; direct measurements of the solar change give values of 1 to 3 percent and 5 to 7 percent, respectively, for the same wavelength range. Solar irradiances were high from the start of observations, late in 1978, until 1983, declined until early 1985, remained approximately constant until mid-1987, and then began to rise. Peak-to-peak 27-day rotational modulation amplitudes were as large as 6 percent at solar maximum and 1 to 2 percent at solar minimum. During occasional intervals of the 1979 to 1983 maximum and again during 1988, the dominant rotational modulation period was 13.5 days. Measurements near 200 to 205 nm show the same rotational modulation behavior but cannot be used to track long-term changes in the Sun because of uncertainties in the characterization of long-term instrument sensitivity changes
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
A Populist Paradox? How Brexit Softened Anti-Immigration Attitudes
Recent political contests across Europe and North America have been propelled by a wave of populist, anti-immigrant resentment, and it was widely expected that these populist victories would further fan the flames of xenophobia. We implemented an experimental design around the Brexit referendum to test how populist victories shape anti-immigrant attitudes. We find that anti-immigrant attitudes actually softened after the Brexit referendum, among both Leave and Remain supporters, and these effects persisted for several months. How could a right- wing, populist victory soften anti-immigrant attitudes? We use causal mediation analysis to understand this ‘populist paradox.’ Among Leavers, a greater sense of control over immigration channelled the effects of the Brexit outcome onto anti-immigrant attitudes. But it is individuals’ efforts to distance themselves from accusations of xenophobia and racism that explains why we see a softening of attitudes towards immigration among both Leavers and Remainers
Analysis of the Station No. 2 seismoscope record—1966, Parkfield, California, earthquake
During the 1966 Parkfield, California, earthquake a strong motion accelerograph and an adjacent seismoscope were located essentially at the causative fault. The accelerograph component parallel to the fault malfunctioned and left no recorded trace. The seismoscope record and the component of the ground motion perpendicular to the fault are used to reconstruct the acceleration component in the direction parallel to the fault. The details of the derived acceleration parallel to the fault differ from the past recorded strong ground motions in that higher frequency components are present. The peak ground accelerations transverse and along the fault are found to be of the same order of magnitude
A robust floating nanoammeter
A circuit capable of measuring nanoampere currents while floating at voltages
up to at least 25kV is described. The circuit relays its output to ground
potential via an optical fiber. We particularly emphasize the design and
construction techniques which allow robust operation in the presence of high
voltage spikes and discharges.Comment: 5 pages, 2 figure
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