The photons of the Cosmic Microwave Background have interacted with matter for the last time 300000 years after the Big Bang, about 15 billion years ago. That interaction was a scattering against free electrons in the hot plasma of the primeval fireball. Such physical process is the well known Thomson scattering.

In 1968 Martin Rees noted for the first time that if there is a quadrupole anisotropy in the incoming primeval radiation, the scattered radiation gains a certain degree of linear polarization: so we expect polarization on the CMB.

There is indeed anisotropy in the primeval radiation, as measured in detail by COBE, BOOMERanG, MAXIMA, DASI, VSA, CBI, ARCHEOPS and other instruments. Major results are expected from the MAP satellite early in 2003, and "final" results will come with the Planck Mission in 2007.

The power spectrum of the anisotropy is consistent with the adiabatic inflationary scenario for the origin and formation of structure in the Universe.

The measurements of CMB anisotropy show that the quadrupole component is small, but non vanishing, so we expect a small degree of linear polarization in the CMB, about 5-10% of the anisotropy. This low level of polarization in the CMB has been recently detected for the first time by the DASI interferometer at 30 GHz.
This measurement is extremely important, because it detects the signature of the physical effect happening at recombination, and represents a compelling confirmation of the entire scenario.

The mission of B2K is to improve the precision of the polarization measurement in sensitivity and in spectral coverage, in order to gain new cosmological information. B2K measures polarization simultaneously at 145, 245 and 345 GHz.

Possible systematic effects for B2K (instrumental and astrophysical) would be orthogonal to the ones expected for DASI and MAP.

A measurement of the power spectrum of CMB polarization in which the "acoustic" features are resolved and Galactic contamination
is excluded will allow us to

• Constrain the cosmological parameters with new information, independent on the one contained in the power spectrum of the anisotropy <TT>. If the polarization field is divided into gradient (E) and curl (B) components, there are three independent spectra measurable: <TE>, <EE>, <BB>. The measurement of <TE> and <EE> allows to break the degeneracy between some of the cosmological parameters determined with <TT> measurements alone.

• Constrain the presence of isocurvature fluctuations mixed to the adiabatic ones: this will be the first insight into the initial conditions of the process of structure formation.

• Start to constrain the presence of tensor fluctuations originated during the inflation process by measuring the <TE> spectrum at multipoles around 50.

We plan to recover B2K and setup a more aggressive experiment (B2K4) with new technology focal plane arrays. This will have the sensitivity to measure the B-modes <BB>, opening a window on the physics of the very early universe, at energies of the order of 10¹⁹ GeV; moreover, high sensitivity coverage of the 100-350 GHz band will produce detailed knowledge of the polarization properties of cirrus dust and other foregrounds.