The cosmic microwave background anisotropies:: Open problems

We present a review of some interesting theoretical and observational aspects of the Cosmic Microwave Background anisotropies. The standard inflationary model presents a simple scenario within which the homogeneity, isotropy and flatness of the universe appear as natural outcomes and, in addition, fluctuations in the energy density are originated during the inflationary phase. These seminal density fluctuations give rise to fluctuations in the temperature of the Cosmic Microwave Background (CMB) at the decoupling surface. Afterward, the CMB photons propagate almost freely, with slight gravitational interactions with the evolving gravitational field present in the large scale structure (LSS) of the matter distribution and a low scattering rate with free electrons after the universe becomes reionized by the first stars and QSOs. These secondary effects slightly change the shape of the intensity and polarization angular power spectra of the radiation, the so called C-l. The C contain very valuable information on the parameters characterizing the background model of the universe and those parametrising the power spectra of both matter density perturbations and gravitational waves. The extraction of this richness of information from the C-l is complicated by the superposition of the radiation coming from other Galactic and extragalactic emissions at microwave frequencies. In spite of this, in the last few years data from sensitive experiments have allowed a good determination of the shape of the C-l, providing for the first time a model of the universe very close to spatially flat. In particular the WMAP first year data, together with other CMB data at higher resolution and other cosmological data sets, have made possible to determine the cosmological parameters with a precision of a few percent. The most striking aspect of the derived model of the universe is the unknown nature of most of its energy contents. This and other open problems in cosmology represent exciting challenges for the CMB community. The future ESA Planck mission will undoubtely shed some light on these remaining questions.