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2021-05-13 |
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The Cosmic Microwave Background radiation is one of the most important observable to probe the early stages of our Universe. The photons that constitue this radiation started to propagate in the free space 380.000 years after the Big Bang. Before this era, the photon were trapped in the primordial plasma by continuos Thomson scattering. When the temperature and the density of the Universe were low enough, the firsts phothons started to propagate in the free space. This photons consitute the CMB radiation that it is actually the first picture of our Universe when it was only 380.000. The wavelenght of this photons has been strached by the expansion of the Universe, and now, we can observe this radiation as a black-body emission at the temperature of 2.7255K, that peaks on the microwave frequencies.
The CMB radiation is not flat but it presents intensity and polarization anisotropies, and their distributions on the sky is not completely random. They represents the first quantum iterations on the very early stages of the Universe. This quantum fluctuations had been enlarged from quantum to cosmological scales by a superluminal expansion due to the broken simmetry in the inflation scalar field. For this reasons, the CMB radiation represents a unique "laboratory" to study the quantum fluctuations on cosmological size. The signal of the anisotropies' correaltions in temperature and in particular in polarization is very faint and the measure represents a very hard challange for the scientific community. The major limitation in the observations is represented by the control of the environmental and instrumental systematics drifts/errors/spurious-contributes, and one of them is represented by the atmosphere. The water vapor and di-oxygen molecules give to the atmosphere a dispersive behavour. The real and imaginary parts of the dieletric functions are linked to the the atmospheric refractive index and to the absorption coefficient, that it means that the trubulent strucuteres of the atmosphere introduce spurious correlations in the pointings directions and in the signal. Fotunately, the atmospheric emission is largely unpolarized, so its spurious contibute in the polarization measurements is largely mitigated, but a significant component of the total power radiations can be converted in polarized signal due to the instrumental (total power) -> polarization leakage due to small imperfection of the OMTs, Polarizers, adn more in general, by some imperfections that affect the whole radiometic acquisition chain.
Only cosmological signal CMB + Dust + Synchrotron emissions
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<img src="/img/mappe_5.png" />
<figcaption>The previous map with atmospheric spurious signal - 1 day of observations - 43GHz</figcaption>
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The pictures presented below, show the effects of the atmosphere on the total intesity measurements. The turbulent atmospheric structures are seen by the telescope introducing correlation in time and cross-correlation between the detectors. Moreover, the atmospheric structures are not fixed in the sky. The wind blows them rigidly along the line of sight of the telescope. This effects is characterized by a \(1/f\) like noise but with absolutely non trivial slope and knee frequency. The high variability and non stationarity of the atmospheric noise hold the destriper algorithm back affecting in particular the correlation at large angular scales where the \(\Lambda CDM\) model predicts the BB reionization bump.