3,405 research outputs found
Post-Newtonian cosmological dynamics of plane-parallel perturbations and back-reaction
We study the general relativistic non-linear dynamics of self-gravitating
irrotational dust in a cosmological setting, adopting the comoving and
synchronous gauge, where all the equations can be written in terms of the
metric tensor of spatial hyper-surfaces orthogonal to the fluid flow.
Performing an expansion in inverse powers of the speed of light, we obtain the
post-Newtonian equations, which yield the lowest-order relativistic effects
arising during the non-linear evolution. We then specialize our analysis to
globally plane-parallel configurations, i.e. to the case where the initial
perturbation field depends on a single coordinate. The leading order of our
expansion, corresponding to the "Newtonian background", is the Zel'dovich
approximation, which, for plane-parallel perturbations in the Newtonian limit,
represents an exact solution. This allows us to find the exact analytical form
for the post-Newtonian metric, thereby providing the post-Newtonian extension
of the Zel'dovich solution: this accounts for some relativistic effects, such
as the non-Gaussianity of primordial perturbations. An application of our
solution in the context of the back-reaction proposal is eventually given,
providing a post-Newtonian estimation of kinematical back-reaction, mean
spatial curvature and average scale-factor.Comment: revised to match the version accepted for publication in JCA
Current Status and Perspectives of Cosmic Microwave Background Observations
Measurements of the cosmic microwave background (CMB) radiation provide a
unique opportunity for a direct study of the primordial cosmic plasma at
redshift z ~1000. The angular power spectra of temperature and polarisation
fluctuations are powerful observational objectives as they encode information
on fundamental cosmological parameters and on the physics of the early
universe. A large number of increasingly ambitious balloon-borne and
ground-based experiments have been carried out following the first detection of
CMB anisotropies by COBE-DMR, probing the angular power spectrum up to high
multipoles. The recent data from WMAP provide a new major step forward in
measurements percision. The ESA mission Planck Surveyor, to be launched in
2007, is the third-generation satellite devoted to CMB imaging. Planck is
expected to extract the full cosmological information from temperature
anisotropies and to open up new fronteers in the CMB field.Comment: 6 pages, 1 figure, to appear in "Proc of International Symposium on
Plasmas in the Laboratory and in the Universe: new insights and new
challenges", September 16-19, 2003, Como, Ital
Documenting architecture. From 3D simulations to virtual restoration
The Casa del Mutilato (House of the Mutilated) in Ancona, Italy, is a significant example of twentieth-century architecture and the current initiatives aimed at its recovery and restoration can represent a useful and interesting test bed for experimenting with the most innovative IT methodologies for documenting the history and state of conservation of the building. This contribution therefore intends to propose some possible strategies for the design and implementation of a new concept of digital information system to preserve all the documentation relating to the Casa del Mutilato, make it accessible online also to citizens and the wider public, create a useful service for the management of the building itself, and enhance the asset, making known its history, constructive and artistic characteristics, and the possibilities of use. The results so obtained can then be usefully utilized for the documentation and planning of the recovery of monumental buildings of the twentieth century of historical and artistic interest in Europe
Documenting architecture. From 3D simulations to virtual restoration
The Casa del Mutilato (House of the Mutilated) in Ancona, Italy, is a significant example of twentieth-century architecture and the current initiatives aimed at its recovery and restoration can represent a useful and interesting test bed for experimenting with the most innovative IT methodologies for documenting the history and state of conservation of the building. This contribution therefore intends to propose some possible strategies for the design and implementation of a new concept of digital information system to preserve all the documentation relating to the Casa del Mutilato, make it accessible online also to citizens and the wider public, create a useful service for the management of the building itself, and enhance the asset, making known its history, constructive and artistic characteristics, and the possibilities of use. The results so obtained can then be usefully utilized for the documentation and planning of the recovery of monumental buildings of the twentieth century of historical and artistic interest in Europe
Data Streams from the Low Frequency Instrument On-Board the Planck Satellite: Statistical Analysis and Compression Efficiency
The expected data rate produced by the Low Frequency Instrument (LFI) planned
to fly on the ESA Planck mission in 2007, is over a factor 8 larger than the
bandwidth allowed by the spacecraft transmission system to download the LFI
data. We discuss the application of lossless compression to Planck/LFI data
streams in order to reduce the overall data flow. We perform both theoretical
analysis and experimental tests using realistically simulated data streams in
order to fix the statistical properties of the signal and the maximal
compression rate allowed by several lossless compression algorithms. We studied
the influence of signal composition and of acquisition parameters on the
compression rate Cr and develop a semiempirical formalism to account for it.
The best performing compressor tested up to now is the arithmetic compression
of order 1, designed for optimizing the compression of white noise like
signals, which allows an overall compression rate = 2.65 +/- 0.02. We find
that such result is not improved by other lossless compressors, being the
signal almost white noise dominated. Lossless compression algorithms alone will
not solve the bandwidth problem but needs to be combined with other techniques.Comment: May 3, 2000 release, 61 pages, 6 figures coded as eps, 9 tables (4
included as eps), LaTeX 2.09 + assms4.sty, style file included, submitted for
the pubblication on PASP May 3, 200
The effect of signal digitisation in CMB experiments
Signal digitisation may produce significant effects in balloon - borne or
space CMB experiments, since the limited bandwidth for downlink of data
requires imposes a large quantisation step q applied on board by the instrument
acquisition chain. In this paper we present a study of the impact of the
quantization error in CMB experiments using, as a working case, simulated data
from the Planck/LFI. At TOD level, the effect of the quantization can be
approximated as a source of nearly normally distributed noise. At map level,
the data quantization alters the noise distribution and the expectation of some
higher order moments. Finally, at the levell of power spectra, the quantization
introduces a power excess, that, although related to the instrument and mission
parameters, is weakly dependent on the multipole l at middle and large l and
can be quite accurately subtracted, leaving a residual uncertainty of few % of
the RMS uncertainty. Only for l<30 the quantization removal is less accurate.Comment: 15 pages, 5 figures, LaTeX2e, A&A style (aa.cls). Release 1, april
1st 2003. Submitted to A&A for the pubblication, april 1st 2003. Contact
author: [email protected]
New insights into foreground analysis of the WMAP five-year data using FASTICA
In this paper, we present a foreground analysis of the WMAP 5-year data using
the FASTICA algorithm, improving on the treatment of the WMAP 3-year data in
Bottino et al 2008. We revisit the nature of the free-free spectrum with the
emphasis on attempting to confirm or otherwise the spectral feature claimed in
Dobbler et al 2008b and explained in terms of spinning dust emission in the
warm ionised medium. With the application of different Galactic cuts, the index
is always flatter than the canonical value of 2.14 except for the Kp0 mask
which is steeper. Irrespective of this, we can not confirm the presence of any
feature in the free-free spectrum. We experiment with a more extensive approach
to the cleaning of the data, introduced in connection with the iterative
application of FASTICA. We confirm the presence of a residual foreground whose
spatial distribution is concentrated along the Galactic plane, with pronounced
emission near the Galactic center. This is consistent with the WMAP haze
detected in Finkbeiner 2004. Finally, we attempted to perform the same analysis
on full-sky maps. The code returns good results even for those regions where
the cross-talk among the components is high. However, slightly better results
in terms of the possibility of reconstructing a full-sky CMB map, are achieved
with a simultaneous analysis of both the five WMAP maps and foreground
templates. Nonetheless, some residuals are still present and detected in terms
of an excess in the CMB power spectrum, on small angular scales. Therefore, a
minimal mask for the brightest regions of the plane is necessary, and has been
defined.Comment: Accepted for publication in MNRAS, 25 pages, 17 figures, 4 tables.
Version with full resolution figures available at:
http://www.mpa-garching.mpg.de/~bottino/downloads/bottino_etal.pd
Symmetries, functional theory and simulation models for macromolecules and polymers
Functional techniques have proven to be very powerful tools in the treatment of integro-differential equations which rule the dynamics of crystallographic structures or of the Free Electron Laser (FEL), allowing for a derivation of the relevant solutions in closed form by means of Bessel-Clifford functions of half-integer order which are easily computable. Therefore, this approach coupled to algebraic methods can be extended to other physical and biochemical fields, where the dynamics of the condensed-matter system is determined by integro-differential equations of Volterra type, satisfying very general conditions, thus allowing calculations that are otherwise both complex and cumbersome. This paper shows that it is possible to reduce very complex problems such as those of protein folding or
polymer/dendrimer dynamics to a series of manageable steps, starting from the description of fundamental structural patterns to their mutual interaction and the computation of the dynamics up to the formation of the final equilibrated 3D structure
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