Cosmologia

Introduction

The status of a given cosmological model can be broadly measured by its capability to provide answers to two distinct aspects: the origin and evolution of primordial perturbations towards large scale structure distribution (LSS) in the Universe and the nature and dynamics of the homogeneous background. Cosmic microwave background radiation (CMB) carries a wealth of cosmological information to address both aspects, which can be extracted from ts black body spectrum, temperature anisotropies and polarization.

CMB spectrum comes from an epoch when the universe was in thermodynamic equilibrium, before the recombination and decoupling eras, about 300.000 years after the Big Bang. CMB anisotropies are the imprint of primordial density fluctuations in the radiation field and were generated at the end of decoupling ( z ∼ 1100). Polarization has primordial (before decoupling) and secondary (after decoupling) causes, from space-time distortion caused by primordial gravitational waves to Thomson scattering during the photon-baryon coupling to secondary scattering at reionization times.

Spectrum measurements demand sensitivities ∼ 1 mK to allow the investigation of possible spectral distortions. The level of CMB anisotropies and of their polarized fraction are ∼ tens of μK for (ΔΤ/Τ)_total and ≲ 1μK for (ΔΤ/Τ)_pol. The detection of such small signals can be significantly hampered by foreground contamination from the Galaxy and extragalactic radio sources. In particular, the synchrotron component and its high degree of polarization represent a great challenge to measurement at frequencies below 15 GHz.

The CMB power spectrum can be broadly divided in 3 regions, associated with angular scales on the sky. Angular scales larger than about 2° on the sky, corresponding to power spectrum l-scales from l=2 to l=90, probe regions beyond the "causal horizon" ( θ ∼ 2°) at the decoupling time. Scales between 2° and ∼ 5' explore the physics of baryon-photon coupling, which is usually treated as a coupled harmonic oscillator. This coupling causes the imprint of acoustic oscillations in the ionized plasma, seen as acoustic peaks in the CMB power spectrum. Both scales probe a still linear universe and the prediction of CMB physics in these scales are very well confirmed by a large number of experiments since the first detection of anisotropies by the COBE satellite [20]. The smallest scales (below ∼ 5') start to probe the Universe when it is about to enter a non-linear regime and the CMB data becomes dominated by secondary, non-primordial fluctuations. In these scales the major source of information come from the large scale galaxy surveys, such as the SDSS, 2dF and 2MASS.