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