The Observational Signatures of Cosmic Strings
Riccio, Giuseppe (2009) The Observational Signatures of Cosmic Strings. [Tesi di dottorato] (Inedito)
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Cosmic strings were postulated by Kibble in 1976 and, from a theoretical point of view, their existence finds support in modern superstring theories, both in compactification models and in theories with extended additional dimensions. One of the best observational evidences for cosmic strings is the gravitational lensing effects they produce. A first effect is produced by an intervening string along the line of sight which splits in two components (double images) faint background galaxies, thus forming a chain of lensed galaxies along the path of the string. The second optical method is the serendipity discovery through anomalous lensing of extended objects. The huge ratio existing between the string width and length leads to a sort of step function signature on the gravitationally lensed images of background sources. The optical research of cosmic strings signatures suffers from many spurious effects mainly induced by the fact that, in order to be effective, the detection of background galaxies needs to be pushed down to very low flux limits. At these flux levels photometric errors, as well as noise statistics increase the number of spurious detections and, for instance, an application to the Sloan Digital Sky Survey leads to an huge and unrealistic number of candidate pairs. One way to minimize the contamination introduced in the catalogues by the spurious detection, is to increase the contrast by selecting pairs in the 3D space, i.e. by attributing to each galaxy a redshift estimate. At this purpose, a new method for photometric redshifts estimation has been created. The method is based on multiwavelength photometry and on a combination of various data mining techniques developed under the EuroVO and NVO frameworks for data gathering, pre-processing and mining, while relying on the scaling capabilities of the computing grid. This method allowed us to obtain photometric redshifts with an increased accuracy (up to 30%) with respect to the literature. The second fundamental observational evidence for cosmic strings is the signature they are expected to leave in the CMB a signature which may be sought for in the available WMAP data and in the soon to come Planck data. Theory shows that a moving string should produce a step-like discontinuity of low S/N ratio in the CMB, as a consequence of the Doppler shift due to the relative velocity between the string and the observer, thus causing the temperature distribution to deviate from a Gaussian. In the simplifying assumption that the string is a straight discontinuity in space time, we used the S.Co.P.E. computational grid to produce a large number of simulations covering a wide range of values for the velocity of the string, its direction and its distance from the observer. Simulations are produced using a C++ code that generates realistic maps of the CMB temperature distribution in presence of a straight cosmic string. By varying its characteristic parameters, it is possible to explore the signatures left by various types of moving strings. In order to amplify the step-like discontinuity and smooth the noise, maps are then subjected to a “squeezing” procedure. Successively, on the “squeezed” maps, we tested some filters that recognizes high value differences between close pixels. The excellent results of our filter on simulations prompted us to apply it on WMAP 5 years data.
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