45,170 research outputs found
Relationships between geographic and inertial coordinates of position
Relationships between geographic and inertial coordinates of positio
The shape of primordial non-Gaussianity and the CMB bispectrum
We present a set of formalisms for comparing, evolving and constraining
primordial non-Gaussian models through the CMB bispectrum. We describe improved
methods for efficient computation of the full CMB bispectrum for any general
(non-separable) primordial bispectrum, incorporating a flat sky approximation
and a new cubic interpolation. We review all the primordial non-Gaussian models
in the present literature and calculate the CMB bispectrum up to l <2000 for
each different model. This allows us to determine the observational
independence of these models by calculating the cross-correlation of their CMB
bispectra. We are able to identify several distinct classes of primordial
shapes - including equilateral, local, warm, flat and feature (non-scale
invariant) - which should be distinguishable given a significant detection of
CMB non-Gaussianity. We demonstrate that a simple shape correlator provides a
fast and reliable method for determining whether or not CMB shapes are well
correlated. We use an eigenmode decomposition of the primordial shape to
characterise and understand model independence. Finally, we advocate a
standardised normalisation method for based on the shape
autocorrelator, so that observational limits and errors can be consistently
compared for different models.Comment: 32 pages, 20 figure
Primordial non-Gaussianity and the CMB bispectrum
We present a new formalism, together with efficient numerical methods, to
directly calculate the CMB bispectrum today from a given primordial bispectrum
using the full linear radiation transfer functions. Unlike previous analyses
which have assumed simple separable ansatze for the bispectrum, this work
applies to a primordial bispectrum of almost arbitrary functional form, for
which there may have been both horizon-crossing and superhorizon contributions.
We employ adaptive methods on a hierarchical triangular grid and we establish
their accuracy by direct comparison with an exact analytic solution, valid on
large angular scales. We demonstrate that we can calculate the full CMB
bispectrum to greater than 1% precision out to multipoles l<1800 on reasonable
computational timescales. We plot the bispectrum for both the superhorizon
('local') and horizon-crossing ('equilateral') asymptotic limits, illustrating
its oscillatory nature which is analogous to the CMB power spectrum
The XMM-Newton spectral-fit database
The XMM-Newton spectral-fit database is an ongoing ESA funded project aimed
to construct a catalogue of spectral-fitting results for all the sources within
the XMM-Newton serendipitous source catalogue for which spectral data products
have been pipeline-extracted (~ 120,000 X-ray source detections). The
fundamental goal of this project is to provide the astronomical community with
a tool to construct large and representative samples of X-ray sources by
allowing source selection according to spectral properties.Comment: Conference proceedings of IAU Symposium 304: Multiwavelength AGN
surveys and studie
String windings in the early universe
We study string dynamics in the early universe. Our motivation is the
proposal of Brandenberger and Vafa, that string winding modes may play a key
role in decompactifying three spatial dimensions. We model the universe as a
homogeneous but anisotropic 9-torus filled with a gas of excited strings. We
adopt initial conditions which fix the dilaton and the volume of the torus, but
otherwise assume all states are equally likely. We study the evolution of the
system both analytically and numerically to determine the late-time behavior.
We find that, although dynamical evolution can indeed lead to three large
spatial dimensions, such an outcome is not statistically favored.Comment: 26 pages, LaTeX, 4 eps figure
Information extraction and transmission techniques for spaceborne synthetic aperture radar images
Information extraction and transmission techniques for synthetic aperture radar (SAR) imagery were investigated. Four interrelated problems were addressed. An optimal tonal SAR image classification algorithm was developed and evaluated. A data compression technique was developed for SAR imagery which is simple and provides a 5:1 compression with acceptable image quality. An optimal textural edge detector was developed. Several SAR image enhancement algorithms have been proposed. The effectiveness of each algorithm was compared quantitatively
The NASA Astrophysics Data System: Overview
The NASA Astrophysics Data System Abstract Service has become a key component
of astronomical research. It provides bibliographic information daily, or near
daily, to a majority of astronomical researchers worldwide.
We describe the history of the development of the system and its current
status.
We show several examples of how to use the ADS, and we show how ADS use has
increased as a function of time. Currently it is still increasing
exponentially, with a doubling time for number of queries of 17 months.
Using the ADS logs we make the first detailed model of how scientific
journals are read as a function of time since publication.
The impact of the ADS on astronomy can be calculated after making some simple
assumptions. We find that the ADS increases the efficiency of astronomical
research by 333 Full Time Equivalent (2000 hour) research years per year, and
that the value of the early development of the ADS for astronomy, compared with
waiting for mature technologies to be adopted, is 2332 FTE research years.
The ADS is available at http://adswww.harvard.edu/.Comment: 19 pages, 22 figure
Three-dimensional theory of stimulated Raman scattering
We present a three-dimensional theory of stimulated Raman scattering
(SRS) or superradiance. In particular we address how the spatial and temporal
properties of the generated SRS beam, or Stokes beam, of radiation depends on
the spatial properties of the gain medium. Maxwell equations for the Stokes
field operators and of the atomic operators are solved analytically and a
correlation function for the Stokes field is derived. In the analysis we
identify a superradiating part of the Stokes radiation that exhibit beam
characteristics. We show how the intensity in this beam builds up in time and
at some point largely dominates the total Stokes radiation of the gain medium.
We show how the SRS depends on geometric factors such as the Fresnel number and
the optical depth, and that in fact these two factors are the only factors
describing the coherent radiation.Comment: 21 pages 14 figure
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