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Theme: Early universe and fundamental physics

Current data (e.g., large scale isotropy as revealed by the CMB data, the
peaks in the CMB anisotropy power spectrum) support the idea of an epoch of
inflation at early times during which the universe experienced exponential
expansion. Quantum fluctuations during this period lead to the creation of
the density perturbations that are observed today as the large-scale
structure of the universe and as CMB anisotropies. The same inflation
mechanism should also generate primordial gravity waves of cosmological
size. Their observation would be clear evidence for inflation and provide
a direct measure of the relevant energy scale (most likely associated with
Grand Unification at ~1015-1016 GeV). It would also be of inestimable
value for fundamental physics - a detection of gravity waves arising from
quantum fluctuations of gravity itself, and an observational probe of
physics unattainable in the laboratory. Although these could be detected
by a space gravitational wave detector, LISA will be limited by the gravity
wave foreground from all galactic binary systems and a second generation
LISA will be required. The CMB, on the other hand, offers a unique way to
observe these gravity waves by their unmistakable signature in the
polarization anisotropies on large angular scales, where their amplitude is
the largest.

The cosmic microwave background (CMB) is a founding pillar of our modern
vision of the universe. It has become one of cosmology's most important
quantitative tools. The discovery of the CMB in the 1960s definitively
established the Big Bang as the cornerstone of cosmological models, and the
mapping of the temperature anisotropies since their first detection by
NASA's COBE satellite in 1992 has produced a number of ground-breaking
results: the flatness of space, verification of a scale-invariant
primordial perturbation spectrum, and an independent measure of the
universal baryon content. The first-year results from the WMAP satellite
have ushered in the era of precision CMB science by accurately tracing the
anisotropy power spectrum down to the second Doppler peak. The coherence
of these observations of the CMB with distance determinations by SNIa, the
HST Hubble constant determination and primordial nucleosynthesis inspire
one to speak today of the concordance cosmological model: a spatially flat
universe composed of roughly 5% baryonic matter, 25% weakly interacting
dark matter of unknown composition and 70% of dark energy, a mysterious
form of energy associated with the vacuum. ESA's Planck mission (launch
2007) will complete the study of the temperature anisotropies down to
angular scales of ~5 arcmins (l~2000) by observing the CMB with detectors
essentially limited only by the ability to remove foregrounds.

The next major step in CMB science will be an equivalent study of the
polarization anisotropies and of the compelling science to which they
provide unique access, including primordial gravity waves, the dynamical
importance and nature of dark energy, neutrino mass and the reionization
epoch.

Two types of polarization anisotropy exist in the CMB: E-modes (of even
parity) and B-modes (with odd parity). In total, there are four different
power spectra that describe the CMB temperature (T) and polarization (E and
B) anisotropies: TT, TE, EE and BB. Density perturbations only give rise
to E modes, while gravity waves are a source of B-modes. In other words,
observation of B-modes (BB power spectrum) on large angular scales is the
`smoking gun' that betrays the presence of primordial gravity waves.

In addition to the primordial polarization signals produced at
recombination, gravitational lensing of primordial E-modes by the matter
distribution along the line-of-sight is another source of B-modes on small
angular scales. This ``secondary'' B-mode signal carries important
information on the matter distribution integrated back to early times and,
hence, on the evolution of large-scale structure. Cosmological parameters
influence this evolution, and we may learn, for example, about dark energy
and its equation-of-state in a manner completely independent of SNIa
measurements. The physics is the same as for cosmic shear measurements,
but the CMB polarization mode probes structure over a larger range of
redshifts and is subject to entirely different systematic effects. Such
independent observable manifestations of dark energy are greatly needed.
The secondary signal offers in addition the possibility of measuring
neutrino masses as low as ~0.05 eV, via their influence on the evolution of
dark matter perturbations. Current oscillation data suggest a slightly
higher mass for the most massive neutrino; CMB polarization measurements
may be the best way to actually measure such low masses.

The ground-based experiment DASI was the first to detect CMB polarization
at small angular scales, and WMAP has recently given a first measurement of
the TE power spectrum (although with very significant uncertainties due to
the foregrounds). Planck will easily map the TE mode and give a good
measure of the EE mode. The exciting physics targeted by the BB mode,
however, lies at much lower signal amplitudes, requiring improvements of up
to two orders of magnitude in sensitivity. This leads to the need for a
CMB polarization measurement at large angular scale limited only by
astrophysical limits: our ability to measure, understand and remove
foregrounds i.e. an experiment for polarization equivalent to Planck for
the temperature.
Although much can be accomplished from the ground at small angular scales,
large angular scale measurements require a space-based platform. It is
needed to provide the well-controlled and stable environment necessary to
fully reduce systematic effects, and to cover the broad spectral range
necessary to control the foregrounds; it is unavoidable in order to
eliminate atmospheric effects that otherwise make full-sky observations of
large angular scales at millimeter wavelengths impossible at the targeted
sensitivities. The gravity wave signal is only present on large scales and
hence demands full-sky coverage; large sky coverage is also necessary to
reach the cosmic variance limit on the secondary B-mode signals, needed to
attain the desired sensitivity to neutrino mass.

Two recently developed detector technologies pave the way to a space-based
measurement of CMB polarization B-modes:

1) the development of low noise coherent receivers integrated in single
chips, which can be assembled in large arrays.
2) the development of antenna-coupled ultra-sensitive bolometers,
replicable in large format arrays by means of litographic techniques.
These developments are underway in European laboratories and
industries, in close sinergy with the development of cryogenic
microcalorimeters for high energy astronomy missions.

This means that compact arrays of thousands of detectors, necessary to
reach the required sensitivity for such a mission, can be qualified to be
used in the Cosmic Vision timeframe.

In addition, exquisite control of all sources of spurious signals, such
as foregrounds and systematic effects, is mandatory. The technologies
mentioned above cover the range of frequency - from cm to sub-mm - required
to monitor all the foregrounds potentially contaminating CMB polarization.
The use of redundant, high polarization purity front-end assemblies is the
key to reduce systematics. State of the art devices of this kind are also
developed in Europe. Important spinoffs in communication technology are
foreseen in this application.

Europe has a tradition in the production of the space cryogenic systems
necessary for this mission, and in the production of state-of-the-art
lightweight sub-mm telescopes with wide, polarization-correct focal planes.


In summary, a mission devoted to the theme of Early Universe and
Fundamental Physics, looking for CMB polarization with the highest possible
precision and accuracy would :

- Solve outstanding mysteries in Cosmology and Fundamental Physics.
- Mantain the leading role of the European Cosmology and Fundamental
Physics community.
- Enhance the visibility of top european science in the society, and
trigger interest in science in the young generations.
- Involve and support important high-tech industrial partners in all
Europe, and induce important spinoffs in applied science.
- Fit perfectly the Cosmic-Vision subject and timeframe, as an ideal
continuation of the Planck and Herschel missions.