Inflation

During the accelerated expansion phase which characterizes inflation, generation and amplification of quantum fluctuations are unavoidable. During this epoch, quantum fluctuations leave the Hubble horizon, reentering in later times to seed matter fluctuations responsible for the structure formation process. These matter overdensities couple to the radiation field to generate the well-known CMB temperature anisotropies.

In today's scenario of precision cosmology, experimental efforts are mainly devoted to measurements of CMB polarization, as it will allow to break the degeneracy of fundamental cosmological parameters. Furthermore, detection of its B-mode will provide a strong evidence for the theory of inflation, since one of its three core predictions is the existence of a relic background of nearly scale-invariant gravitational waves with generation of B-mode anisotropies in the CMB (see, e.g., [16]). The other two predictions for testing the inflationary framework have already been nearly confirmed: a spatially flat Universe ( ) and a nearly-scale invariant, nearly power-law spectrum of Gaussian adiabatic, density perturbations (see, e.g., [3]).

CMB temperature anisotropies have been the focus of attention during the 90's. The 2003 WMAP results set a new standard for anisotropies measurements and their amplitude and distribution properties now seem to be well understood [2]. This fact shifted the main focus of experimental cosmology to CMB polarization. Recently, there were reports of TE (Temperature E-mode) anti-correlation polarization detections by the experiments DASI [13], CBI [18], ACBAR [14] and the WMAP satellite [12]. These results are a distinctive signature of super-horizon adiabatic fluctuations [21] caused by inflation and rule out models that require a dynamical process to generate spatial variations in the energy density field [17]. On the other hand, measurements of the so-called CMB curl-mode (B-mode) polarization can help quantify the influence of gravitational waves on the production of structure in the very early universe and can be taken as another inflation signature. B-mode polarized fluctuations are tensor fluctuations generated by gravity-induced space-time distortions (see, e.g., [9]. Detection of B-modes will require a combination of large sky coverage, extreme sensitivity and very good control of systematic errors, demanding a substantial effort to be accomplished. The lack of a reasonable prediction for the level of the B-mode signal, coupled to our ignorance about systematic issues, including foregrounds and CMB confusion, turns this into a very attractive topic to set path for the upcoming space missions devoted to cosmology.