Today, Thursday 21 April, in a special issue of the journal "Journal of Cosmology and Astroparticle Physics", eight articles signed by the international collaboration QUBIC (Q&U Bolometric Interferometer for Cosmology), which is making a telescope in Argentina for the study of the early universe that will make use of an innovative technique. QUBIC, in fact, will observe and map the properties of the cosmic microwave background, the residual echo of the Big Bang, focusing on the measurement of particular components of the orientation of the oscillation of the microwaves of the cosmic microwave background radiation on the plane of the sky (polarization), called B-modes, indicative of the possible perturbations induced by the gravitational waves generated in the first moments of life of the universe. The project sees Italy as a protagonist thanks to the scientific and technological contributions provided by the INFN (National Institute of Nuclear Physics) and the Universities of Milano Statale, Milano-Bicocca, University of Rome "Tor Vergata" and Sapienza University of Rome. QUBIC will observe the sky from the end of 2022, from a high-altitude desert site (5000 m) in Argentina, near San Antonio de Los Cobres.
After its development and integration in the European laboratories of the universities and research institutions involved in the collaboration, QUBIC arrived in Argentina, in the city of Salta, in July 2021, where the final stages of calibration and laboratory testing are proceeding. The results of these activities, presented in the eight articles published in the 'Journal of Cosmology and Astroparticle Physics', have confirmed the correct functioning of the instrument and of the 'bolometric interferometry', that is the newly developed technique on which QUBIC's observations will be based, which combines the very high sensitivity of detectors cooled to almost absolute zero (-273 °C) and capable of measuring the energy of the radiation of the cosmic background transforming it into heat (bolometers), with the precision of interferometric instruments.
"QUBIC is an original and extremely complex tool: for this reason, it was necessary to publish in advance all the details of its hardware and new methods of exploitation of the collected data. In addition, with these long and exhaustive calibrations we have demonstrated in the laboratory the efficiency of QUBIC as a bolometric interferometer. It is an essential step for the subsequent measures of interest for cosmology and fundamental physics", explains Silvia Masi, professor at Sapienza University of Rome and INFN researcher, who coordinates the Italian participation in the experiment.
Thanks to its extreme sensitivity, which will allow to distinguish the details of each of the 'pixels' in which the celestial map will be divided, QUBIC will be able to discriminate the B-modes from the signals generated by the other sources of the sky, providing direct evidence of the theory of inflation. This is today the reference theory for the description of what would have happened in the first moments of the universe, developed in the 80s to explain some characteristics of the universe, including the 'flatness' and the extreme homogeneity of spacetime.
According to the theory of inflation, the very rapid phase of expansion of the universe immediately after the Big Bang, which lasted less than one hundred thousandth of a billionth of a billionth of a billionth of a second (about 10-32 seconds), would have left a weak background of gravitational waves, which in turn would have produced particular very weak traces, called B-modes, in the polarization of the cosmic microwave background. In practice, the electromagnetic waves of the cosmic background would not oscillate in random directions. Instead, directions that form a swirling pattern in the sky would be slightly preferred.
To the accuracy of the measurements that will be carried out by QUBIC will also contribute the clarity and absence of humidity that distinguish the air of the Alto Chorrillo site where the telescope will be installed, about 5000 meters above sea level, on the La Puna plateau in northern Argentina, near the town of San Antonio de Los Cobres, in the province of Salta.
"QUBIC will be brought to the Alto Chorrillo site within a few months. The first measurements will demonstrate the efficiency of the new method of bolometric interferometry for the first time by observing astronomical sources. The instrument will then be completed by increasing the number of detectors, so that measurements of cosmological interest can be performed within three years. The road is long, and QUBIC presents itself as extremely original and complementary to all the others trying to measure this elusive primordial signal", explains Aniello Mennella, INFN researcher and professor at the University of Milan.
The search for B-modes represents a formidable and central challenge for physicists and astrophysicists. The signal to be measured is so weak that it requires ultrasensitive detectors and high-precision telescopes, even to remove, during data analysis, other polarized signals of local origin that could confuse the measurement. QUBIC's measurements will therefore be contemporary with those of half a dozen other experiments in the world that have the same scientific objective. Unlike the latter, which produce images directly via single-aperture telescopes, QUBIC will be the only instrument to make observations by collecting microwaves from many openings and making them interfere.
"The measure of such a weak signal," says Mario Zannoni, INFN researcher and professor at the University of Milan-Bicocca, "will be considered free from systematic errors only if there are consistent results from very different instruments. Precisely for this reason QUBIC, the only bolometric interferometer, represents an irreplaceable asset in the search for B-modes and in the study of the first moments of the universe". Thanks to the multispectral and self-calibration capabilities, "QUBIC will produce completely original and complementary data to those of the other experiments, offering analysts countless possibilities of cross-checking and therefore an unparalleled robustness of the results", concludes Giancarlo De Gasperis, INFN researcher and professor at the University of Rome "Tor Vergata".
QUBIC is the result of the collaboration of 130 researchers, engineers and technicians in France, Italy, Argentina, Ireland and the United Kingdom. The instrument was integrated in Paris at the APC laboratories in 2018 and calibrated during 2019-2021.
The Italian contribution was fundamental for the development of the instrument, and will continue to be so in the later stages of the experiment. The instrument is housed in a cryostat, designed and built in the laboratories of Sapienza and the Rome Section of the INFN, capable of cooling close to absolute zero not only the detectors but also the entire optical system of the interferometer. The same group has also created the cryo-mechanical system that rotates the optical components inside the cryostat to measure the state of polarization of the radiation. Italian are also other cryogenic components, which work at a temperature below -270 °C, such as the very advanced corrugated antennas that select the photons to be interfered with, made in the laboratories of the Milano University and the INFN Section of Milan, while the optics that focus the photons on the detectors and the shutter system that allows to vary the configuration of the interferometer and self-calibrate are made by the University of Milano Bicocca and INFN Section of Milan-Bicocca. The University of Rome "Tor Vergata" and the INFN Section of Roma2 contribute to the development of the complex data analysis software.