1,192 research outputs found
Spin Dynamics of the LAGEOS Satellite in Support of a Measurement of the Earth's Gravitomagnetism
LAGEOS is an accurately-tracked, dense spherical satellite covered with 426
retroreflectors. The tracking accuracy is such as to yield a medium term (years
to decades) inertial reference frame determined via relatively inexpensive
observations. This frame is used as an adjunct to the more difficult and data
intensive VLBI absolute frame measurements. There is a substantial secular
precession of the satellite's line of nodes consistent with the classical,
Newtonian precession due to the non-sphericity of the earth. Ciufolini has
suggested the launch of an identical satellite (LAGEOS-3) into an orbit
supplementary to that of LAGEOS-1: LAGEOS-3 would then experience an equal and
opposite classical precession to that of LAGEOS-1. Besides providing a more
accurate real-time measurement of the earth's length of day and polar wobble,
this paired-satellite experiment would provide the first direct measurement of
the general relativistic frame-dragging effect. Of the five dominant error
sources in this experiment, the largest one involves surface forces on the
satellite, and their consequent impact on the orbital nodal precession. The
surface forces are a function of the spin dynamics of the satellite.
Consequently, we undertake here a theoretical effort to model the spin
ndynamics of LAGEOS. In this paper we present our preliminary results.Comment: 16 pages, RevTeX, LA-UR-94-1289. (Part I of II, postscript figures in
Part II
On the observational determination of squeezing in relic gravitational waves and primordial density perturbations
We develop a theory in which relic gravitational waves and primordial density
perturbations are generated by strong variable gravitational field of the early
Universe. The generating mechanism is the superadiabatic (parametric)
amplification of the zero-point quantum oscillations. The generated fields have
specific statistical properties of squeezed vacuum quantum states.
Macroscopically, squeezing manifests itself in a non-stationary character of
variances and correlation functions of the fields, the periodic structures of
the metric power spectra, and, as a consequence, in oscillatory behavior of the
higher order multipoles C_l of the cosmic microwave background anisotropy. We
start with the gravitational wave background and then apply the theory to
primordial density perturbations. We derive an analytical formula for the
positions of peaks and dips in the angular power spectrum l(l+1)C_l as a
function of l. This formula shows that the values of l at the peak positions
are ordered in the proportion 1:3:5:..., whereas at the dips they are ordered
as 1:2:3:.... We compare the derived positions with the actually observed
features, and find them to be in reasonably good agreement. It appears that the
observed structure is better described by our analytical formula based on the
(squeezed) metric perturbations associated with the primordial density
perturbations, rather than by the acoustic peaks reflecting the existence of
plasma sound waves at the last scattering surface. We formulate a forecast for
other features in the angular power spectrum, that may be detected by the
advanced observational missions, such as MAP and PLANCK. We tentatively
conclude that the observed structure is a macroscopic manifestation of
squeezing in the primordial metric perturbations.Comment: 34 pages, 3 figures; to appear in Phys. Rev. D66, 0435XX (2002);
includes Note Added in Proofs: "The latest CBI observations (T.J.Pearson et
al., astro-ph/0205388) have detected four peaks, at l ~ 550, 800, 1150, 1500,
and four dips, at l ~ 400, 700, 1050, 1400. These positions are in a very
good agreement with the theoretical formula (6.35) of the present paper. We
interpret this data as confirmation of our conclusion that it is gravity, and
not acoustics, that is responsible for the observed structure.
Designing multiplayer games to facilitate emergent social behaviours online
This paper discusses an exploratory case study of the design of games that facilitate spontaneous social interaction and group behaviours among distributed individuals, based largely on symbolic presence 'state' changes. We present the principles guiding the design of our game environment: presence as a symbolic phenomenon, the importance of good visualization and the potential for spontaneous self-organization among groups of people. Our game environment, comprising a family of multiplayer 'bumper-car' style games, is described, followed by a discussion of lessons learned from observing users of the environment. Finally, we reconsider and extend our design principles in light of our observations
Non-parametric modeling of the intra-cluster gas using APEX-SZ bolometer imaging data
We demonstrate the usability of mm-wavelength imaging data obtained from the
APEX-SZ bolometer array to derive the radial temperature profile of the hot
intra-cluster gas out to radius r_500 and beyond. The goal is to study the
physical properties of the intra-cluster gas by using a non-parametric
de-projection method that is, aside from the assumption of spherical symmetry,
free from modeling bias. We use publicly available X-ray imaging data from the
XMM-Newton observatory and our Sunyaev-Zel'dovich Effect (SZE) imaging data
from the APEX-SZ experiment at 150 GHz to de-project the density and
temperature profiles for the relaxed cluster Abell 2204. We derive the gas
density, temperature and entropy profiles assuming spherical symmetry, and
obtain the total mass profile under the assumption of hydrostatic equilibrium.
For comparison with X-ray spectroscopic temperature models, a re-analysis of
the recent Chandra observation is done with the latest calibration updates.
Using the non-parametric modeling we demonstrate a decrease of gas temperature
in the cluster outskirts, and also measure the gas entropy profile. These
results are obtained for the first time independently of X-ray spectroscopy,
using SZE and X-ray imaging data. The contribution of the SZE systematic
uncertainties in measuring T_e at large radii is shown to be small compared to
the Chandra systematic spectroscopic errors. The upper limit on M_200 derived
from the non-parametric method is consistent with the NFW model prediction from
weak lensing analysis.Comment: Replaced with the published version; A&A 519, A29 (2010
Galaxy clusters discovered with a Sunyaev-Zel'dovich effect survey
The South Pole Telescope (SPT) is conducting a Sunyaev-Zel'dovich (SZ) effect
survey over large areas of the southern sky, searching for massive galaxy
clusters to high redshift. In this preliminary study, we focus on a 40
square-degree area targeted by the Blanco Cosmology Survey (BCS), which is
centered roughly at right ascension 5h30m, declination -53 degrees. Over two
seasons of observations, this entire region has been mapped by the SPT at 95
GHz, 150 GHz, and 225 GHz. We report the four most significant SPT detections
of SZ clusters in this field, three of which were previously unknown and,
therefore, represent the first galaxy clusters discovered with an SZ survey.
The SZ clusters are detected as decrements with greater than 5-sigma
significance in the high-sensitivity 150 GHz SPT map. The SZ spectrum of these
sources is confirmed by detections of decrements at the corresponding locations
in the 95 GHz SPT map and non-detections at those locations in the 225 GHz SPT
map. Multiband optical images from the BCS survey demonstrate significant
concentrations of similarly colored galaxies at the positions of the SZ
detections. Photometric redshift estimates from the BCS data indicate that two
of the clusters lie at moderate redshift (z ~ 0.4) and two at high redshift (z
>~ 0.8). One of the SZ detections was previously identified as a galaxy cluster
using X-ray data from the ROSAT All-Sky Survey (RASS). Potential RASS
counterparts (not previously identified as clusters) are also found for two of
the new discoveries. These first four galaxy clusters are the most significant
SZ detections from a subset of the ongoing SPT survey. As such, they serve as a
demonstration that SZ surveys, and the SPT in particular, can be an effective
means for finding galaxy clusters.Comment: 11 pages, 3 figures, revised to match published version, uses
emulateap
Cosmology with CMB anisotropy
Measurements of CMB anisotropy and, more recently, polarization have played a
very important role allowing precise determination of various parameters of the
`standard' cosmological model. The expectation of the paradigm of inflation and
the generic prediction of the simplest realization of inflationary scenario in
the early universe have also been established -- `acausally' correlated initial
perturbations in a flat, statistically isotropic universe, adiabatic nature of
primordial density perturbations. Direct evidence for gravitational instability
mechanism for structure formation from primordial perturbations has been
established. In the next decade, future experiments promise to strengthen these
deductions and uncover the remaining crucial signature of inflation -- the
primordial gravitational wave background.Comment: Plenary talk at the IXth. International Workshop on High Energy
Physics Phenomenology (WHEPP-9), Institute of Physics, Bhubaneshwar, India.
Jan 3-14, 2006; To appear in the Proceedings to be published in Pramana; 12
pages, 2 figure
The new generation CMB B-mode polarization experiment: POLARBEAR
We describe the Cosmic Microwave Background (CMB) polarization experiment
called Polarbear. This experiment will use the dedicated Huan Tran Telescope
equipped with a powerful 1,200-bolometer array receiver to map the CMB
polarization with unprecedented accuracy. We summarize the experiment, its
goals, and current status
Fast and precise map-making for massively multi-detector CMB experiments
Future cosmic microwave background (CMB) polarisation experiments aim to
measure an unprecedentedly small signal - the primordial gravity wave component
of the polarisation field B-mode. To achieve this, they will analyse huge
datasets, involving years worth of time-ordered data (TOD) from massively
multi-detector focal planes. This creates the need for fast and precise methods
to complement the M-L approach in analysis pipelines. In this paper, we
investigate fast map-making methods as applied to long duration, massively
multi-detector, ground-based experiments, in the context of the search for
B-modes. We focus on two alternative map-making approaches: destriping and TOD
filtering, comparing their performance on simulated multi-detector polarisation
data. We have written an optimised, parallel destriping code, the DEStriping
CARTographer DESCART, that is generalised for massive focal planes, including
the potential effect of cross-correlated TOD 1/f noise. We also determine the
scaling of computing time for destriping as applied to a simulated full-season
data-set for a realistic experiment. We find that destriping can out-perform
filtering in estimating both the large-scale E and B-mode angular power
spectra. In particular, filtering can produce significant spurious B-mode power
via EB mixing. Whilst this can be removed, it contributes to the variance of
B-mode bandpower estimates at scales near the primordial B-mode peak. For the
experimental configuration we simulate, this has an effect on the possible
detection significance for primordial B-modes. Destriping is a viable
alternative fast method to the full M-L approach that does not cause the
problems associated with filtering, and is flexible enough to fit into both M-L
and Monte-Carlo pseudo-Cl pipelines.Comment: 16 pages, 14 figures. MNRAS accepted. Typos corrected and computing
time/memory requirement orders-of-magnitude numbers in section 4 replaced by
precise number
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