Cosmic Polarization

This book collects papers on the polarization properties of the cosmic background radiation and related topics, including the Galactic diffuse foregrounds. After a search lasting about three decades, the first years of this century have finally seen the detection of the linearly polarized component of this radiation. This achievement was obtained in the context of an exceptional progress in our knowledge of the background radiation due to ground-based interferometry, balloon flights and, of course, the WMAP satellite. While polarization data at small angular scales appeared to basically confirm the standard cosmological scenario emerging from temperature anisotropy data, WMAP covering also the large scales carried the unexpected result of a strong anisotropy-polarization cross-correlation, generally interpreted as the imprint of an early reionization of the cosmic medium. This discovery, which has recently been confirmed by the second data release, did not simply add a new cosmological parameter to the rich list available today: Rather, it opened a new exciting scenario for research, showing that background polarimetry can allow us to investigate the cosmological Middle Ages at redshifts beyond the present-time possibilities of optical astronomy. However, the application of sophisticated analysis techniques to WMAP's output also revealed several features which seem to survive in the updated maps from three years of flight, but are not consistent with the standard scenario. Although their statistical significance is still questioned and the role of systematics and foregrounds might be not yet understood to such a level of accuracy, some peculiarities might be of cosmological, or at least extragalactic origin. A really unexpected result from the first data release was a strong North-South asymmetry in the evaluated optical depth of cosmic reionization, and it is not yet clear whether the problem persists in the second release. This cannot be regarded as a small correction and, if confirmed, it might even imply a strong violation of the cosmological principle. Although caution is required, we should not forget that some strange results came in these years from astronomy, in particular by large-scale correlations of distant QSOs, and there seems to be some spatial correlation with CMB data. The above considerations, although they are not sufficient to expound a clear picture of the present status of the field, in my opinion show how exciting the present time is for cosmology and, in particular, for people working on CMB polarization. Thanks to the contributions of many authors, this book is intended to review the present status and to illustrate the prospects for the near future. It should not be simply intended as a collection of known results. It emphasizes the present technological status and plans for the near future, so that the reader can appreciate the real chances for further advancements. A significant space is devoted to instrumental problems and to the knowledge of the foregrounds, in particular the synchrotron emission. Cosmological implications are considered especially in connection with the Middle Ages, because of the unique role of the background polarization there. Perhaps open problems are emphasized even more than safe achievements, for which satisfactory discussions can also be easily found elsewhere; the solutions of such problems are deferred to the future, but probably a not so far future.