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Scientific Report 2007-2009
Astronomy & Astrophysics
A14. Cosmic Microwave Background Polarization Measurements
The origin of primordial tiny fluctuations, about a
perfectly homogeneous and isotropic universe, lies at
the heart of both modern cosmology and high-energy
physics. Inflationary theory offers today the most satisfying
explanation for the origin of these fluctuations:
within 10 −35 s of the Big Bang, during a short phase of
superluminal expansion of space, quantum fluctuations
are stretched to cosmological scales.
Cosmic Microwave Background (CMB) measurements
have shown that the basic predictions of this extraordinary
theory are correct: the universe is almost spatially
flat, and has a nearly Gaussian, scale-invariant spectrum
of primordial adiabatic perturbations.
Another prediction, i.e. the production, during inflation,
of a stochastic background of gravitational waves,
can also be tested using precision measurements of the
rotational component (B-mode) of the linear polarization
field of the CMB. In fact these photons are last
scattered by free electrons at recombination. In Thomson
scattered radiation, linear polarization results if scattered
radiation has a quadrupole anisotropy. At recombination,
both scalar (density) perturbations and tensor
(gravitational waves) perturbations produce quadrupole
anisotropy, with different parity properties.
The difficulty of these measurements lies in the tiny
amplitude of the polarized component (about 1 ppm of
the CMB for the non-rotational component or E-mode,
and even 10 ppb or less for the rotational component or
B-mode).
Figure 1: Launch of the BOOMERanG-03 balloon-borne polarimeter
from the McMurdo base in Antarctica. Our group
has produced the telescope and the cryogenic system of the
instrument, and coordinated the project, in cooperation with
the Caltech group, since the very beginning.
Our group has developed technologies and methods to
measure CMB polarization since long time ago, starting
in the 70s with the pioneering efforts of Francesco
Melchiorri.
We have recently measured the E-modes of CMB
polarization with the BOOMERanG-B03 balloon-borne
polarimeter, a follow-up of the extremely successful
BOOMERanG-B98 balloon mission, which detected for
the first time acoustic oscillations in the primeval plasma
and measured the density parameter Ω o to be close to 1.
To improve over that measurement, we have developed
cryogenic polarization modulators, based on rotating
waveplates [1,2] (see fig.2). These systems have
been tested in the field in the framework of the BRAIN-
QUBIC experiment, funded by PNRA: this is a bolometric
interferometer devoted to sensitive CMB polarization
surveys from the French-Italian Concordia Base,
in Antarctica (Dome-C) [3].
In addition, we are developing large arrays of KID
detectors (see below).
Figure 2: The cryogenic polarization modulator developed
in our laboratory is able to rotate a waveplate in the focal
plane of a polarimeter, with 0.01 o repeatability, and with
negligible heat load on the 2K stage of the cryogenic system.
These developments are absolutely necessary in view
of a future space-borne mission devoted to precision measurements
of CMB polarization. In this framework our
group has been the coordinator of the B-Pol proposal, in
the framework of the ESA call Cosmic Vision 2015-2025
(see http://www.b-pol.org , and [4]).
Meanwhile we are now coordinating LSPE (Large
Scale Polarization Explorer): a stratospheric balloon
mission, funded by the Italian Space Agency. The payload
consists of two instruments, covering the frequency
bands around 40, 70, 140, 220 GHz, with angular
resolution of the order of one degree. Using large
throughput bolometers, the high frequency instrument
reaches sensitivities of ∼ 35 µK/ √ Hz per detector,
with an array of about 100 detectors. The instrument
will be flown during the arctic winter in a 15 days flight
from Svalbard Islands, where our group has setup the
Nobile-Amundsen launch facility in collaboration with
ISTAR, ASI and ARR, and launched the first 800000
m 3 balloon on July 1 st , 2009 .
References
1. L. Pagano et al., Phys. Rev. D80, 043522 (2009)
2. M. Salatino, et al., Mem. S.A.It., 79, 905, (2008)
3. G. Polenta, et al., New Astron. Reviews, 51, 256, (2007)
4. P. de Bernardis et al., Exp. Astronomy, 23, 5, (2009)
Authors
P. de Bernardis, M. De Petris, E. Battistelli, S. Masi, A,
Melchiorri, F. Nati, L. Nati, L. Pagano, F. Piacentini, M.
Salatino, A. Schillaci
Sapienza Università di Roma 161 Dipartimento di Fisica