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The cosmic microwave background

One of the fields where Italian research has distinguished itself, and is producing new and extraordinary results right now, is the study of the cosmic microwave background (CMB), the residual, fossil light from the Big Bang. This radiation, coming directly from the primordial Universe (about 380,000 years after the Big Bang), carries with it the imprint of the density perturbations that gave rise to structure, and its power spectrum shows the relative importance of the various size scales. Galaxies, clusters and super-clusters each have their counterparts in the CMB fluctuations that we observe on different scales on the sky, the so-called anisotropies. In fact, an Italian led experiment, Boomerang in 2000, showed with precision, for the first time, the peaks and troughs of this power spectrum, providing extraordinary evidence in support of the theory of formation of galaxies based on cold dark matter, and establishing one of the pillars of the current cosmological model. Following Boomerang, the American WMAP mission extended the Boomerang measurements to the whole sky, and the next step is now represented by the Italian/French experiment Planck, currently in flight. Planck is already producing unique images, and in the coming years extraordinary results are expected thanks to its sensitivity and angular resolution, that allow it to probe even smaller angular scales. An ambitious long term goal of Planck is the direct identification of the so-called "B-mode" spectrum of the polarisation anisotropies, associated with tensor perturbations in the primordial Universe. This measurement would constitute a confirmation of the inflationary model for the primordial Universe. Polarisation maps sufficiently deep for this research would give direct indications of the inflationary energy scale and verifications in very high energy physics, well beyond anything possible with present or future particle accelerators. We recall that the theory of inflation elegantly resolves some basic problems of the Big Bang theory, by postulating a phase of incredibly rapid (exponential) expansion of the primordial Universe. This simple idea would explain two crucial aspects: 1) the essentially flat geometry of the Universe; 2) the extreme isotropy of the Universe between regions that, in the absence of an inflationary phase, should never have been in causal contact. The flatness and isotropy both appear to be confirmed by CMB measurements on all angular scales and by observations of the large scale structure of the Universe. Although this does not represent a direct confirmation of the inflationary scenario, it certainly supports it. The study of the first instances of the Universe, during the inflationary phase, is done at the limits of our present understanding, making cosmology a subject able to shed light on as yet unknown aspects of fundamental physics.


The accurate analysis and interpretation of the CMB requires an extremely precise and reliable separation of the various spurious radiation components produced by our own galaxy or by sources that photons encounter along their path, before arriving. This cleaning of the signal is typically obtained by a combination of observations in contiguous bands (radio and infrared), and the development of accurate astrophysical models of the various sources of diffuse noise. These lines of research have at the same time a specific relevance of their own, given the astrophysical and cosmological importance of the various classes of extragalactic sources which "dirty" the CMB, such as galaxy clusters, responsible for the Sunyaev-Zeldovich effect, or the Galactic sources of diffuse and compact emission.


The high level of signal-to-noise needed to achieve the above mentioned goals, doesn't seem achievable even in the near future, but it is extremely important for the Italian community to take the intermediate steps in this direction so as to develop the necessary technology. Technological progress is promising for both bolometric and coherent detectors, particularly for large focal plane arrays. The validity of these experimental technologies will first be tested from the ground and balloons, that will have a good chance of detecting the B-mode of the polarisation, and then incorporated into space missions, such as B-Pol or CMB-Pol, proposed by ESA and NASA respectively. Both will be able to observe the whole sky with high sensitivity and excellent control of the systematic effects.

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