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
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.
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
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.