110 research outputs found
Effects of electrical charging on the mechanical Q of a fused silica disk
We report on the effects of an electrical charge on mechanical loss of a
fused silica disk. A degradation of Q was seen that correlated with charge on
the surface of the sample. We examine a number of models for charge damping,
including eddy current damping and loss due to polarization. We conclude that
rubbing friction between the sample and a piece of dust attracted by the
charged sample is the most likely explanation for the observed loss.Comment: submitted to Review of Scientific Instrument
Reionization and the large-scale 21 cm-cosmic microwave background cross correlation
Of the many probes of reionization, the 21 cm line and the cosmic microwave
background (CMB) are among the most effective. We examine how the
cross-correlation of the 21 cm brightness and the CMB Doppler fluctuations on
large angular scales can be used to study this epoch. We employ a new model of
the growth of large scale fluctuations of the ionized fraction as reionization
proceeds. We take into account the peculiar velocity field of baryons and show
that its effect on the cross correlation can be interpreted as a mixing of
Fourier modes. We find that the cross-correlation signal is strongly peaked
toward the end of reionization and that the sign of the correlation should be
positive because of the inhomogeneity inherent to reionization. The signal
peaks at degree scales (l~100) and comes almost entirely from large physical
scales (k~0.01 Mpc). Since many of the foregrounds and noise that plague low
frequency radio observations will not correlate with CMB measurements, the
cross correlation might appear to provide a robust diagnostic of the
cosmological origin of the 21 cm radiation around the epoch of reionization.
Unfortunately, we show that these signals are actually only weakly correlated
and that cosmic variance dominates the error budget of any attempted detection.
We conclude that the detection of a cross-correlation peak at degree-size
angular scales is unlikely even with ideal experiments.Comment: 15 pages, 4 figures, submitted to MNRA
Detection of relic gravitational waves in the CMB: Prospects for CMBPol mission
Detection of relic gravitational waves, through their imprint in the cosmic
microwave background radiation, is one of the most important tasks for the
planned CMBPol mission. In the simplest viable theoretical models the
gravitational wave background is characterized by two parameters, the
tensor-to-scalar ratio and the tensor spectral index . In this paper,
we analyze the potential joint constraints on these two parameters, and
, using the potential observations of the CMBPol mission, which is
expected to detect the relic gravitational waves if . The
influence of the contaminations, including cosmic weak lensing, various
foreground emissions, and systematical errors, is discussed.Comment: 26 pages, 19 figures, 4 tables; JCAP in pres
Probing the Circumgalactic Medium at High-Redshift Using Composite BOSS Spectra of Strong Lyman-alpha Forest Absorbers
We present composite spectra constructed from a sample of 242,150 Lyman-alpha
(Lya) forest absorbers at redshifts 2.4<z<3.1 identified in quasar spectra from
the Baryon Oscillation Spectroscopic Survey (BOSS) as part of Data Release 9 of
the Sloan Digital Sky Survey III. We select forest absorbers by their flux in
bins 138 km/s wide (approximately the size of the BOSS resolution element). We
split these absorbers into five samples spanning the range of flux -0.05 <
F<0.45. Tests on a smaller sample of high-resolution spectra show that our
three strongest absorption bins would probe circumgalactic regions (projected
separation < 300 proper kpc and |Delta v| < 300km/s) in about 60% of cases for
very high signal-to-noise ratio. Within this subset, weakening Lya absorption
is associated with decreasing purity of circumgalactic selection once BOSS
noise is included. Our weaker two Lya absorption samples are dominated by the
intergalactic medium.
We present composite spectra of these samples and a catalogue of measured
absorption features from HI and 13 metal ionization species, all of which we
make available to the community. We compare measurements of seven Lyman series
transitions in our composite spectra to single line models and obtain further
constraints from their associated excess Lyman limit opacity. This analysis
provides results consistent with column densities over the range 14.4 <~ Log
(N_HI) <~ 16.45. We compare our measurements of metal absorption to a variety
of simple single-line, single-phase models for a preliminary interpretation.
Our results imply clumping on scales down to ~30 pc and near-solar
metallicities in the circumgalactic samples, while high-ionization metal
absorption consistent with typical IGM densities and metallicities is visible
in all samples.Comment: 23 pages, 15 figures, 5 tables, link to downloadable data included.
Accepted by MNRAS 2014 March 20. New sections 3.4 and 6.1 limiting the
occurrence and impact of Lyman limit system
The strongest gravitational lenses: II. Is the large Einstein radius of MACS J0717.5+3745 in conflict with LCDM?
Can the standard cosmological model be questioned on the basis of a single
observed extreme galaxy cluster? Usually, the word extreme refers directly to
cluster mass, which is not a direct observable and thus subject to substantial
uncertainty. Hence, it is desirable to extend studies of extreme clusters to
direct observables, such as the Einstein radius (ER). We aim to evaluate the
occurrence probability of the large observed ER of MACS J0717.5 within the
standard LCDM cosmology. In particular, we want to model the distribution
function of the single largest ER in a given cosmological volume and to study
which underlying assumptions and effects have the strongest impact on the
results. We obtain this distribution by a Monte Carlo approach, based on the
semi-analytic modelling of the halo population on the past lightcone. After
sampling the distribution, we fit the results with the general extreme value
(GEV) distribution which we use for the subsequent analysis. We find that the
distribution of the maximum ER is particularly sensitive to the precise choice
of the halo mass function, lens triaxiality, the inner slope of the halo
density profile and the mass-concentration relation. Using the distributions so
obtained,we study the occurrence probability of the large ER of MACS J0717.5,
finding that this system is not in tension with LCDM. We also find that the GEV
distribution can be used to fit very accurately the sampled distributions and
that all of them can be described by a Frechet distribution. With a multitude
of effects that strongly influence the distribution of the single largest ER,
it is more than doubtful that the standard LCDM cosmology can be ruled out on
the basis of a single observation. If, despite the large uncertainties in the
underlying assumptions, one wanted to do so, a much larger ER (> 100 arcsec)
than that of MACS J0717.5 would have to be observed.Comment: 15 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics, minor corrections to match the accepted version, added
discussion of the distribution of the largest Einstein radii for the MACS
survey area, extended Fig.
Analytic Spectra of CMB Anisotropies and Polarization Generated by Scalar Perturbations in Synchronous Gauge
We present a detailed analytical calculation of CMB temperature anisotropies
\alpha_k and polarization \beta_k generated by scalar metric perturbations in
synchronous gauge, parallel to our previous work with RGW as a generating
source. This is realized primarily by an analytic time-integration of
Boltzmann's equation, yielding the closed forms of \alpha_k and \beta_k.
Approximations, such as the tight-coupling approximation for photons a prior to
the recombination and the long wavelength limit for scalar perturbations are
used. The residual gauge modes in scalar perturbations are analyzed and a
proper joining condition of scalar perturbations at the radiation-matter
equality is chosen, ensuring the continuity of energy perturbation.
The resulting analytic expressions of the multipole moments of polarization
a^E_l, and of temperature anisotropies a^T_l are explicit functions of the
scalar perturbations, recombination time, recombination width, photon free
streaming damping factor, baryon fraction, initial amplitude, primordial scalar
spectral index, and the running index. These results show that a longer
recombination width yields higher amplitudes of polarization on large scales
and more damping on small scales, and that a late recombination time shifts the
peaks of C^{XX'}_l to larger angular scales.
The analytic spectra C^{XX'}_l agree with the numerical ones and with those
observed by WMAP on large scales (l \lesssim 500), but deviate considerably
from the numerical results on smaller scales, showing the limitations of our
approximate analytic calculations. Several possible improvements are pointed
out for further studies.Comment: Accepted by Classical and Quantum Gravity. 43 pages, 15 figure
Constraining the dark energy equation of state with double source plane strong lenses
We investigate the possibility of constraining the dark energy equation of
state by measuring the ratio of Einstein radii in a strong gravitational lens
system with two source planes. This quantity is independent of the Hubble
parameter and directly measures the growth of angular diameter distances as a
function of redshift. We investigate the prospects for a single double source
plane system and for a forecast population of systems discovered by
re-observing a population of single source lenses already known from a
photometrically selected catalogue such as CASSOWARY or from a
spectroscopically selected catalogue such as SLACS. We find that constraints
comparable to current data-sets (15% uncertainty on the dark equation of state
at 68%CL) are possible with a handful of double source plane systems. We also
find that the method's degeneracy between Omega_M and w is almost orthogonal to
that of CMB and BAO measurements, making this method highly complimentary to
current probes.Comment: 13 Page
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