342 research outputs found
Probing non-Gaussianities on Large Scales in WMAP5 and WMAP7 Data using Surrogates
Probing Gaussianity represents one of the key questions in modern cosmology,
because it allows to discriminate between different models of inflation. We
test for large-scale non-Gaussianities in the cosmic microwave background (CMB)
in a model-independent way. To this end, so-called first and second order
surrogates are generated by first shuffling the Fourier phases belonging to the
scales not of interest and then shuffling the remaining phases for the length
scales under study. Using scaling indices as test statistics we find highly
significant signatures for both non-Gaussianities and asymmetries on large
scales for the WMAP data of the CMB. We find remarkably similar results when
analyzing different ILC-maps based on the WMAP five and seven year data. Such
features being independent from the map-making procedure would disfavor the
fundamental principle of isotropy as well as canonical single-field slow-roll
inflation - unless there is some undiscovered systematic error in the
collection or reduction of the CMB data or yet unknown foreground
contributions.Comment: 4 pages, 3 figures, to appear in the Proceedings of Moriond Cosmology
201
Probing non-Gaussianities in the CMB on an incomplete sky using surrogates
We demonstrate the feasibility to generate surrogates by Fourier-based
methods for an incomplete data set. This is performed for the case of a CMB
analysis, where astrophysical foreground emission, mainly present in the
Galactic plane, is a major challenge. The shuffling of the Fourier phases for
generating surrogates is now enabled by transforming the spherical harmonics
into a new set of basis functions that are orthonormal on the cut sky. The
results show that non-Gaussianities and hemispherical asymmetries in the CMB as
identified in several former investigations, can still be detected even when
the complete Galactic plane (|b| < 30{\deg}) is removed. We conclude that the
Galactic plane cannot be the dominant source for these anomalies. The results
point towards a violation of statistical isotropy.Comment: 9 pages, 13 figures, accepted by Physical Review
A model-independent test for scale-dependent non-Gaussianities in the CMB
We present a model-independent method to test for scale-dependent
non-Gaussianities in combination with scaling indices as test statistics.
Therefore, surrogate data sets are generated, in which the power spectrum of
the original data is preserved, while the higher order correlations are partly
randomised by applying a scale-dependent shuffling procedure to the Fourier
phases. We apply this method to the WMAP data of the cosmic microwave
background (CMB) and find signatures for non-Gaussianities on large scales.
Further tests are required to elucidate the origin of the detected anomalies.Comment: accepted for publication in PRL, minor revisions, results unchanged,
l(cut)-dependency adde
Search for non-Gaussianities in the WMAP data with the Scaling Index Method
In the recent years, non-Gaussianity and statistical isotropy of the Cosmic
Microwave Background (CMB) was investigated with various statistical measures,
first and foremost by means of the measurements of the WMAP satellite. In this
Review, we focus on the analyses that were accomplished with a measure of local
type, the so-called Scaling Index Method (SIM). The SIM is able to detect
structural characteristics of a given data set, and has proven to be highly
valuable in CMB analysis. It was used for comparing the data set with
simulations as well as surrogates, which are full sky maps generated by
randomisation of previously selected features of the original map. During these
investigations, strong evidence for non-Gaussianities as well as asymmetries
and local features could be detected. In combination with the surrogates
approach, the SIM detected the highest significances for non-Gaussianity to
date.Comment: 19 pages, 15 figures, Review Article, Accepted for publication in
Advances in Astronom
Deleting edges to restrict the size of an epidemic in temporal networks.
Spreading processes on graphs are a natural model for a wide variety of real-world phenomena, including information or behaviour spread over social networks, biological diseases spreading over contact or trade networks, and the potential flow of goods over logistical infrastructure. Often, the networks over which these processes spread are dynamic in nature, and can be modeled with graphs whose structure is subject to discrete changes over time, i.e. with temporal graphs. Here, we consider temporal graphs in which edges are available at specified timesteps, and study the problem of deleting edges from a given temporal graph in order to reduce the number of vertices (temporally) reachable from a given starting point. This could be used to control the spread of a disease, rumour, etc. in a temporal graph. In particular, our aim is to find a temporal subgraph in which a process starting at any single vertex can be transferred to only a limited number of other vertices using a temporally-feasible path (i.e. a path, along which the times of the edge availabilities increase). We introduce a natural deletion problem for temporal graphs and we provide positive and negative results on its computational complexity, both in the traditional and the parameterised sense (subject to various natural parameters), as well as addressing the approximability of this problem
A Laser Driven Grating Linac
The fields induced over a grating exposed to plane parallel light are explored. It is shown that acceleration is possible if either the particles travel skew to the grating lines, or if the radiation is falling at a skew angle onto the grating. A general theory of diffraction in this skew case is given. In one particular case numerical solutions are worked out for some deep grating. It is found that accelerating fields larger even than the initial fields can be obtained, the limit being set by resistive losses on the grating surface. Simple calculations are made to see what accelerating fields might be obtained using CO/sub 2/ lasers. Accelerations of 2 or 20 GeV per meter seem possible depending on whether the grating is allowed to be destroyed or not. Power requirements, injection and focussing are briefly discussed and no obvious difficulties are seen. It is concluded, therefore, that the proposed mechanism should be considered as a good candidate for the next generation of particle accelerators
Magnetic Characterization of a Superconducting Transverse Gradient Undulator for Compact Laser Wakefield Accelerator-Driven FELs
A transverse gradient undulator (TGU) is a key component compensating for the relatively large energy spread of Laser Wakefield Accelerator (LWFA)-generated electron beams for realizing a compact Free Electron Laser (FEL). A superconducting TGU with 40 periods has been fabricated at the Karlsruhe Institute of Technology (KIT). In this contribution, we report that the superconducting TGU has been commissioned with nominal operational parameters at an off-line test bench. An experimental set-up for mapping the magnetic field on a two-dimensional grid in the TGU gap has been employed for the magnetic characterization. We show the first preliminary results of these measurements showing the longitudinal quality, the transverse gradient and the transient behaviour of the superconducting TGU field
Proposed Beam Test of a Transverse Gradient Undulator at the SINBAD Facility at DESY
While Laser Plasma Accelerators produce beams with the high output energy required for FELs, up to now the relatively high energy spread has prohibited FEL lasing. Therefore it was proposed to replace the normal FEL undulators by Transverse Gradient Undulators (TGUs). For a first, small scale test of the TGU concept, a 40 period prototype high gradient superconductive TGU was built at KIT and will be tested with beam at the ARES-linac in the new accelerator test facility SINBAD (Short Innovative Bunches and Accelerators at Desy) at DESY. The proposed tests are summarized in this paper
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