1,307 research outputs found
Cross-Correlation Detection of Point Sources in WMAP First Year Data
We apply a Cross-correlation (CC) method developed previously for detecting
gamma-ray point sources to the WMAP first year data by using the Point-Spread
Function of WMAP and obtain a full sky CC coefficient map. Analyzing this map,
we find that the CC method is a powerful tool to examine the WMAP foreground
residuals which can be further cleaned accordingly. Evident foreground signals
are found in WMAP foreground cleaned maps and Tegmark cleaned map. In this
process 101 point-sources are detected, and 26 of them are new sources besides
the originally listed WMAP 208 sources. We estimate the flux of these new
sources and verify them by another method. As a result, a revised mask file
based on the WMAP first year data is produced by including these new sources.Comment: 14 pages, 10 figures; accepted for publication by ChJA
Rigid Chiral Membranes
Statistical ensembles of flexible two-dimensional fluid membranes arise
naturally in the description of many physical systems. Typically one encounters
such systems in a regime of low tension but high stiffness against bending,
which is just the opposite of the regime described by the Polyakov string. We
study a class of couplings between membrane shape and in-plane order which
break 3-space parity invariance. Remarkably there is only {\it one} such
allowed coupling (up to boundary terms); this term will be present for any
lipid bilayer composed of tilted chiral molecules. We calculate the
renormalization-group behavior of this relevant coupling in a simplified model
and show how thermal fluctuations effectively reduce it in the infrared.Comment: 11 pages, UPR-518T (This replaced version has fonts not used
removed.
The MAP Satellite Feed Horns
We present the design, manufacturing methods, and characterization of 20
microwave feed horns currently in use on the Microwave Anisotropy Probe (MAP)
satellite. The nature of the cosmic microwave background (CMB) anisotropy
requires a detailed understanding of the properties of every optical component
of a microwave telescope. In particular, the properties of the feeds must be
known so that the forward gain and sidelobe response of the telescope can be
modeled and so that potential systematic effects may be computed. MAP requires
low emissivity, azimuthally symmetric, low-sidelobe feeds in five microwave
bands (K, Ka, Q, V, and W) that fit within a constrained geometry. The beam
pattern of each feed is modeled and compared with measurements; the agreement
is generally excellent to the -60 dB level (80 degrees from the beam peak).
This agreement verifies the beam-predicting software and the manufacturing
process. The feeds also affect the properties and modeling of the microwave
receivers. To this end, we show that the reflection from the feeds is less than
-25 dB over most of each band and that their emissivity is acceptable. The
feeds meet their multiple requirements.Comment: 9 pages with 7 figures, of which 2 are in low-resolution versions;
paper is available with higher quality figures at
http://map.gsfc.nasa.gov/m_mm/tp_links.htm
Filling in the Gaps in the 4.85 GHz Sky
We describe a 4.85 GHz survey of bright, flat-spectrum radio sources
conducted with the Effelsberg 100 m telescope in an attempt to improve the
completeness of existing surveys, such as CRATES. We report the results of
these observations and of follow-up 8.4 GHz observations with the VLA of a
subset of the sample. We comment on the connection to the WMAP point source
catalog and on the survey's effectiveness at supplementing the CRATES sky
coverage.Comment: 13 pages, 3 figures, 2 tables. Accepted for publication in the
Astronomical Journal. Tables available in electronic form:
http://astro.stanford.edu/gaps
Nonsingular and accelerated expanding universe from effective Yang-Mills theory
The energy-momentum tensor coming from one-parameter effective Yang- Mills
theory is here used to describe the matter-energy content of the homogeneous
and isotropic Friedmann cosmology in its early stages. The behavior of all
solutions is examined. Particularly, it is shown that only solutions
corresponding to an open model allow the universe to evolve into an accelerated
expansion. This result appears as a possible mechanism for an inflationary
phase produced by a vector field. Further, depending on the value of some
parameters characterizing the system, the resulting models are classified as
singular or nonsingular.Comment: 15 pages, 7 figures, some discussions were simplified and new remarks
were introduce
Sky maps without anisotropies in the cosmic microwave background are a better fit to WMAP's uncalibrated time ordered data than the official sky maps
The purpose of this reanalysis of the WMAP uncalibrated time ordered data
(TOD) was two fold. The first was to reassess the reliability of the detection
of the anisotropies in the official WMAP sky maps of the cosmic microwave
background (CMB). The second was to assess the performance of a proposed
criterion in avoiding systematic error in detecting a signal of interest. The
criterion was implemented by testing the null hypothesis that the uncalibrated
TOD was consistent with no anisotropies when WMAP's hourly calibration
parameters were allowed to vary. It was shown independently for all 20 WMAP
channels that sky maps with no anisotropies were a better fit to the TOD than
those from the official analysis. The recently launched Planck satellite should
help sort out this perplexing result.Comment: 11 pages with 1 figure and 2 tables. Extensively rewritten to explain
the research bette
Probing for cosmological parameters with LAMOST measurement
In this paper we study the sensitivity of the Large Sky Area Multi-Object
Fiber Spectroscopic Telescope (LAMOST) project to the determination of
cosmological parameters, employing the Monte Carlo Markov Chains (MCMC) method.
For comparison, we first analyze the constraints on cosmological parameters
from current observational data, including WMAP, SDSS and SN Ia. We then
simulate the 3D matter power spectrum data expected from LAMOST, together with
the simulated CMB data for PLANCK and the SN Ia from 5-year Supernovae Legacy
Survey (SNLS). With the simulated data, we investigate the future improvement
on cosmological parameter constraints, emphasizing the role of LAMOST. Our
results show the potential of LAMOST in probing for the cosmological
parameters, especially in constraining the equation-of-state (EoS) of the dark
energy and the neutrino mass.Comment: 7 pages and 3 figures. Replaced with version accepted for publication
in JCA
Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid Crystals Under an Electric Field
Chiral liquid crystals often exhibit periodic modulations in the molecular
director; in particular, thin films of the smectic-C* phase show a chiral
striped texture. Here, we investigate whether similar chiral modulations can
occur in the induced molecular tilt of the smectic-A phase under an applied
electric field. Using both continuum elastic theory and lattice simulations, we
find that the state of uniform induced tilt can become unstable when the system
approaches the smectic-A--smectic-C* transition, or when a high electric field
is applied. Beyond that instability point, the system develops chiral stripes
in the tilt, which induce corresponding ripples in the smectic layers. The
modulation persists up to an upper critical electric field and then disappears.
Furthermore, even in the uniform state, the system shows chiral fluctuations,
including both incipient chiral stripes and localized chiral vortices. We
compare these predictions with observed chiral modulations and fluctuations in
smectic-A liquid crystals.Comment: 11 pages, including 9 postscript figures, uses REVTeX 3.0 and
epsf.st
Theory of Cylindrical Tubules and Helical Ribbons of Chiral Lipid Membranes
We present a general theory for the equilibrium structure of cylindrical
tubules and helical ribbons of chiral lipid membranes. This theory is based on
a continuum elastic free energy that permits variations in the direction of
molecular tilt and in the curvature of the membrane. The theory shows that the
formation of tubules and helical ribbons is driven by the chirality of the
membrane. Tubules have a first-order transition from a uniform state to a
helically modulated state, with periodic stripes in the tilt direction and
ripples in the curvature. Helical ribbons can be stable structures, or they can
be unstable intermediate states in the formation of tubules.Comment: 43 pages, including 12 postscript figures, uses REVTeX 3.0 and
epsf.st
PAPPA: Primordial Anisotropy Polarization Pathfinder Array
The Primordial Anisotropy Polarization Pathfinder Array (PAPPA) is a
balloon-based instrument to measure the polarization of the cosmic microwave
background and search for the signal from gravity waves excited during an
inflationary epoch in the early universe. PAPPA will survey a 20 x 20 deg patch
at the North Celestial Pole using 32 pixels in 3 passbands centered at 89, 212,
and 302 GHz. Each pixel uses MEMS switches in a superconducting microstrip
transmission line to combine the phase modulation techniques used in radio
astronomy with the sensitivity of transition-edge superconducting bolometers.
Each switched circuit modulates the incident polarization on a single detector,
allowing nearly instantaneous characterization of the Stokes I, Q, and U
parameters. We describe the instrument design and status.Comment: 12 pages, 9 figures. Proceedings of the Fundamental Physics With CMB
workshop, UC Irvine, March 23-25, 2006, to be published in New Astronomy
Review
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