330,890 research outputs found
A Measurement of the Cosmic Microwave Background Damping Tail from the 2500-square-degree SPT-SZ survey
We present a measurement of the cosmic microwave background (CMB) temperature
power spectrum using data from the recently completed South Pole Telescope
Sunyaev-Zel'dovich (SPT-SZ) survey. This measurement is made from observations
of 2540 deg of sky with arcminute resolution at GHz, and improves
upon previous measurements using the SPT by tripling the sky area. We report
CMB temperature anisotropy power over the multipole range . We
fit the SPT bandpowers, combined with the seven-year Wilkinson Microwave
Anisotropy Probe (WMAP7) data, with a six-parameter LCDM cosmological model and
find that the two datasets are consistent and well fit by the model. Adding SPT
measurements significantly improves LCDM parameter constraints; in particular,
the constraint on tightens by a factor of 2.7. The impact of
gravitational lensing is detected at , the most significant
detection to date. This sensitivity of the SPT+WMAP7 data to lensing by
large-scale structure at low redshifts allows us to constrain the mean
curvature of the observable universe with CMB data alone to be
. Using the SPT+WMAP7 data, we measure the
spectral index of scalar fluctuations to be in the LCDM
model, a preference for a scale-dependent spectrum with .
The SPT measurement of the CMB damping tail helps break the degeneracy that
exists between the tensor-to-scalar ratio and in large-scale CMB
measurements, leading to an upper limit of (95%,C.L.) in the LCDM+
model. Adding low-redshift measurements of the Hubble constant () and the
baryon acoustic oscillation (BAO) feature to the SPT+WMAP7 data leads to
further improvements. The combination of SPT+WMAP7++BAO constrains
in the LCDM model, a detection of , ... [abridged]Comment: 21 pages, 10 figures. Replaced with version accepted by ApJ. Data
products are available at http://pole.uchicago.edu/public/data/story12
The Dark Energy Survey
We describe the Dark Energy Survey (DES), a proposed optical-near infrared
survey of 5000 sq. deg of the South Galactic Cap to ~24th magnitude in SDSS
griz, that would use a new 3 sq. deg CCD camera to be mounted on the Blanco 4-m
telescope at Cerro Telolo Inter-American Observatory (CTIO). The survey data
will allow us to measure the dark energy and dark matter densities and the dark
energy equation of state through four independent methods: galaxy clusters,
weak gravitational lensing tomography, galaxy angular clustering, and supernova
distances. These methods are doubly complementary: they constrain different
combinations of cosmological model parameters and are subject to different
systematic errors. By deriving the four sets of measurements from the same data
set with a common analysis framework, we will obtain important cross checks of
the systematic errors and thereby make a substantial and robust advance in the
precision of dark energy measurements.Comment: White Paper submitted to the Dark Energy Task Force, 42 page
Improving Orbit Estimates for Incomplete Orbits with a New Approach to Priors -- with Applications from Black Holes to Planets
We propose a new approach to Bayesian prior probability distributions
(priors) that can improve orbital solutions for low-phase-coverage orbits,
where data cover less than approximately 40% of an orbit. In instances of low
phase coverage such as with stellar orbits in the Galactic center or with
directly-imaged exoplanets, data have low constraining power and thus priors
can bias parameter estimates and produce under-estimated confidence intervals.
Uniform priors, which are commonly assumed in orbit fitting, are notorious for
this. We propose a new observable-based prior paradigm that is based on
uniformity in observables. We compare performance of this observable-based
prior and of commonly assumed uniform priors using Galactic center and
directly-imaged exoplanet (HR 8799) data. The observable-based prior can reduce
biases in model parameters by a factor of two and helps avoid under-estimation
of confidence intervals for simulations with less than about 40% phase
coverage. Above this threshold, orbital solutions for objects with sufficient
phase coverage such as S0-2, a short-period star at the Galactic center with
full phase coverage, are consistent with previously published results. Below
this threshold, the observable-based prior limits prior influence in regions of
prior dominance and increases data influence. Using the observable-based prior,
HR 8799 orbital analyses favor lower eccentricity orbits and provide stronger
evidence that the four planets have a consistent inclination around 30 degrees
to within 1-sigma. This analysis also allows for the possibility of
coplanarity. We present metrics to quantify improvements in orbital estimates
with different priors so that observable-based prior frameworks can be tested
and implemented for other low-phase-coverage orbits.Comment: Published in AJ. 23 pages, 14 figures. Monte Carlo chains are
available in the published article, or are available upon reques
Measurements of the Temperature and E-Mode Polarization of the CMB from 500 Square Degrees of SPTpol Data
We present measurements of the -mode polarization angular auto-power
spectrum () and temperature--mode cross-power spectrum () of the
cosmic microwave background (CMB) using 150 GHz data from three seasons of
SPTpol observations. We report the power spectra over the spherical harmonic
multipole range , and detect nine acoustic peaks in the
spectrum with high signal-to-noise ratio. These measurements are the most
sensitive to date of the and power spectra at and , respectively. The observations cover 500 deg, a fivefold increase
in area compared to previous SPTpol analyses, which increases our sensitivity
to the photon diffusion damping tail of the CMB power spectra enabling tighter
constraints on \LCDM model extensions. After masking all sources with
unpolarized flux mJy we place a 95% confidence upper limit on residual
polarized point-source power of at , suggesting that the damping tail
dominates foregrounds to at least with modest source masking. We
find that the SPTpol dataset is in mild tension with the model
(), and different data splits prefer parameter values that differ
at the level. When fitting SPTpol data at we
find cosmological parameter constraints consistent with those for
temperature. Including SPTpol data at results in a preference for
a higher value of the expansion rate (H_0 = 71.3 \pm
2.1\,\mbox{km}\,s^{-1}\mbox{Mpc}^{-1} ) and a lower value for present-day
density fluctuations ().Comment: Updated to match version accepted to ApJ. 34 pages, 17 figures, 6
table
Constraints on Cosmological Parameters from the 500 deg² SPTPOL Lensing Power Spectrum
We present cosmological constraints based on the cosmic microwave background (CMB) lensing potential power spectrum measurement from the recent 500 deg² SPTPOL survey, the most precise CMB lensing measurement from the ground to date. We fit a flat ΛCDM model to the reconstructed lensing power spectrum alone and in addition with other data sets: baryon acoustic oscillations (BAO), as well as primary CMB spectra from Planck and SPTPOL. The cosmological constraints based on SPTPOL and Planck lensing band powers are in good agreement when analyzed alone and in combination with Planck full-sky primary CMB data. With weak priors on the baryon density and other parameters, the SPTPOL CMB lensing data alone provide a 4% constraint on σ₈Ω^(0.25)_m = 0.593 ± 0.025. Jointly fitting with BAO data, we find σ₈ = 0.779±0.023, Ω_m = 0.368^(+0.032)_(−0.037), and H₀ = 72.0^(+2.1)_(−2.5)kms⁻¹ Mpc⁻¹, up to 2σ away from the central values preferred by Planck lensing + BAO. However, we recover good agreement between SPTPOL and Planck when restricting the analysis to similar scales. We also consider single-parameter extensions to the flat ΛCDM model. The SPTPOL lensing spectrum constrains the spatial curvature to be Ω_K = −0.0007±0.0025 and the sum of the neutrino masses to be ∑m_ν < 0.23 eV at 95% C.L. (with Planck primary CMB and BAO data), in good agreement with the Planck lensing results. With the differences in the signal-to-noise ratio of the lensing modes and the angular scales covered in the lensing spectra, this analysis represents an important independent check on the full-sky Planck lensing measurement
Qualitative System Identification from Imperfect Data
Experience in the physical sciences suggests that the only realistic means of
understanding complex systems is through the use of mathematical models.
Typically, this has come to mean the identification of quantitative models
expressed as differential equations. Quantitative modelling works best when the
structure of the model (i.e., the form of the equations) is known; and the
primary concern is one of estimating the values of the parameters in the model.
For complex biological systems, the model-structure is rarely known and the
modeler has to deal with both model-identification and parameter-estimation. In
this paper we are concerned with providing automated assistance to the first of
these problems. Specifically, we examine the identification by machine of the
structural relationships between experimentally observed variables. These
relationship will be expressed in the form of qualitative abstractions of a
quantitative model. Such qualitative models may not only provide clues to the
precise quantitative model, but also assist in understanding the essence of
that model. Our position in this paper is that background knowledge
incorporating system modelling principles can be used to constrain effectively
the set of good qualitative models. Utilising the model-identification
framework provided by Inductive Logic Programming (ILP) we present empirical
support for this position using a series of increasingly complex artificial
datasets. The results are obtained with qualitative and quantitative data
subject to varying amounts of noise and different degrees of sparsity. The
results also point to the presence of a set of qualitative states, which we
term kernel subsets, that may be necessary for a qualitative model-learner to
learn correct models. We demonstrate scalability of the method to biological
system modelling by identification of the glycolysis metabolic pathway from
data
Testing the performance and accuracy of the RELXILL model for the relativistic X-ray reflection from accretion disks
The reflection spectroscopic model RELXILL is commonly implemented in
studying relativistic X-ray reflection from accretion disks around black holes.
We present a systematic study of the model's capability to constrain the
dimensionless spin and ionization parameters from 6,000 NuSTAR
simulations of a bright X-ray source employing the lamppost geometry. We employ
high count spectra to show the limitations in the model without being confused
with limitations in signal-to-noise. We find that both parameters are
well-recovered at 90% confidence with improving constraints at higher
reflection fraction, high spin, and low source height. We test spectra across a
broad range - first at 1010 and then 10 total source counts
across the effective 3-79 keV band of NuSTAR, and discover a strong dependence
of the results on how fits are performed around the starting parameters, owing
to the complexity of the model itself. A blind fit chosen over an approach that
carries some estimates of the actual parameter values can lead to significantly
worse recovery of model parameters. We further stress on the importance to span
the space of nonlinear-behaving parameters like carefully and
thoroughly for the model to avoid misleading results. In light of selecting
fitting procedures, we recall the necessity to pay attention to the choice of
data binning and fit statistics used to test the goodness of fit by
demonstrating the effect on the photon index . We re-emphasize and
implore the need to account for the detector resolution while binning X-ray
data and using Poisson fit statistics instead while analyzing Poissonian data.Comment: 9 pages, 6 figures, accepted for publication in Ap
The Sunyaev-Zeldovich effect in CMB-calibrated theories applied to the Cosmic Background Imager anisotropy power at l > 2000
We discuss the nature of the possible high-l excess in the Cosmic Microwave
Background (CMB) anisotropy power spectrum observed by the Cosmic Background
Imager (CBI). We probe the angular structure of the excess in the CBI deep
fields and investigate whether it could be due to the scattering of CMB photons
by hot electrons within clusters, the Sunyaev-Zeldovich (SZ) effect. We
estimate the density fluctuation parameters for amplitude, sigma_8, and shape,
Gamma, from CMB primary anisotropy data and other cosmological data. We use the
results of two separate hydrodynamical codes for Lambda-CDM cosmologies,
consistent with the allowed sigma_8 and Gamma values, to quantify the expected
contribution from the SZ effect to the bandpowers of the CBI experiment and
pass simulated SZ effect maps through our CBI analysis pipeline. The result is
very sensitive to the value of sigma_8, and is roughly consistent with the
observed power if sigma_8 ~ 1. We conclude that the CBI anomaly could be a
result of the SZ effect for the class of Lambda-CDM concordance models if
sigma_8 is in the upper range of values allowed by current CMB and Large Scale
Structure (LSS) data.Comment: Accepted by The Astrophysical Journal; 17 pages including 12 color
figures. v2 matches accepted version. Additional information at
http://www.astro.caltech.edu/~tjp/CBI
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