It has been shown in the previous chapter that the amplification of tensor fluctuations, and the corresponding formation of a cosmic background of relic gravitational waves, represent important sources of information on the dynamics of the inflationary Universe. This chapter is devoted to another important aspect of the inflationary kinematics, related to effects which have already been observed [1, 2], and whose measurements are becoming more and more accurate [3, 4, 5].

We are referring to the inflationary amplification of the scalar part of the metric fluctuations, and to the observed temperature anisotropies ΔT/T of the Cosmic Microwave Background (CMB) of electromagnetic radiation. Tensor perturbations can also contribute, in principle, to such an anisotropy. If the tensor spectrum is growing, however, its contribution turns out to be largely suppressed at the (very small) frequency scales relevant to the observed anisotropy. If the spectrum is flat or decreasing, on the contrary, then an observable tensor contribution is possibly allowed at frequencies near to the present Hubble scale; however, the tensor contribution tends rapidly to become negligible with respect to the scalar one as one considers higher-frequency modes, i.e. higher multipoles in the spherical-harmonic expansion of the temperature anisotropy (see e.g. [6]).