Theoretical Foundations of PGWs Printed in the CMB and its Observational   Status
  The anisotropy study cosmic microwave background (CMB) is one of the main observational tools for modern cosmology. However, alongside the study of the thermal fluctuations of the CMB are other equally important information, which is known as the polarization of the CMB. The inflationary model predicts that the CMB is linearly polarized and the physical mechanism of this polarization is studied from the Thompson scattering, the dominant process on the surface of last scattering. There are basically two types of polarization called E and B modes, the first produced by scalar perturbations and the latter by tensor perturbations, such as those due to gravitational waves in the primordial universe. So if we are able to measure these types of polarization will have an entry to the study of the inflationary epoch. This paper presents the main physical mechanisms that support theoretically the polarization of the CMB due to primordial gravitational waves (PGW) and the revision of the main observables, grouped in so-called Stokes parameters (Q, U, I), Which brings us information to achieve the contrast the angular power spectrum produced by the polarization of the CMB, which shows to be in excellent agreement with the $\Lambda$CDM model. 
Alexander Bonilla Rivera
astro-ph.CO