Status of Cosmic Microwave Background Observations for the Search of Primordial Gravitational Waves

The cosmic microwave background (CMB) is one of the most powerful tools for cosmology. Its polarization could have imprinted the sign of an inflationary background of gravitational waves, which is supposed to have originated at (Formula presented.) seconds after the Big Bang. Detecting this background is extremely difficult because of the weakness of the signal (if any) left on the CMB polarization and because of the need to control the systematic effects. Additionally, the presence of astrophysical foregrounds, the possibility of leakage from curl-free to curl-like components, including gravitational lensing, and the instrumental noise and systematics, require sensitive detectors and smart systematic effect control. We discuss the experimental efforts spent in this field, highlighting the key observational difference and the choice that could lead, in the near future, to the detection of the curl component of the CMB polarization, a clear sign of the inflationary expansion.