Today eight articles prepared by the QUBIC collaboration will appear on a special issue of the “Journal of Cosmology and Astroparticle Physics”. They describe in detail the current configuration of the experiment and the results obtained from the calibration tests. QUBIC is an experiment conceived to explore the physics of the new-born universe, an infinitesimal fraction of a second after the Big Bang. It will achieve its goals by observing the cosmic microwave background, the first light in the universe, in particular by measuring one of its polarized components, the so-called “B-modes”. QUBIC is a totally new type of telescope based on the concept of “bolometric interferometry”. It will observe the sky from the end of 2022 from its observing site located in Argentina at 5000 m, close to San Antonio de Los Cobres. Following its development and the integration in Europe the instrument arrived at Salta, in Argentina, in July 2021. The tests have confirmed that the instrument is operating as expected

Scientific Context

Despite the impressive progress in cosmology, that is to say in our understanding of the Universe, made over the last few decades several profound questions remain: What is dark matter? What is dark energy? What happened in the first moments of the Universe? QUBIC tries to answer this last question.
The theory of cosmological inflation was proposed in the 1980s to explain the flatness and extreme homogeneity of space-time. More interesting still, inflation provides a physical mechanism for producing primordial density fluctuations in the universe, the seeds that later gave rise to large-scale structure (galaxy clusters, galaxies, stellar structures). Inflation is a period during which the primeval Universe underwent an extremely rapid expansion that lasted for less than one thousandth of a billionth of a billionth of a billionth of a second (10^-35s). It was during this rapid expansion phase that density fluctuations formed, which would later give birth to structures in the universe (stars, galaxies, galaxy clusters).
Even though all observations to date are compatible with the theory of inflation, we still do not have direct proof that it actually occurred. It is precisely this proof that QUBIC is searching for. If Inflation took place, calculations show that it should have left tiny traces in the form of primordial gravitational waves, which would themselves leave their imprint on the cosmic microwave background in the form of a certain type of polarisation of the radiation, called B-modes, that no other primordial mechanism could produce. If primordial B-modes are detected it will be direct proof of the phase of inflation, a major result in cosmology with profound consequences for particle physics.  The study of the B-mode pattern will enable the study of fundamental physics at energies that will be impossible to achieve in particle accelerators for centuries to come.
The search for the B-modes presents a considerable challenge for astrophysicists. The expected signal is extremely weak and its detection requires ultra-sensitive detectors and an exceptionally precise telescope. The signal is also affected by the presence of non-primordial B-modes (especially those produced by dust in our own galaxy) which must be removed.  The topic of B-mode polarisation is at the heart of modern cosmology and the Grüber prize in Cosmology was this year awarded to three physicists for their theoretical work on this question, one of whom, Matías Zaldarriaga is Argentinian and is a close collaborator with QUBIC.

QUBIC

QUBIC is an instrument that was specifically designed to detect the B-mode polarization and is the result of a collaboration between 130 researchers and engineers in France, Italy, Argentina, the United Kingdom and Ireland. In particular, the instrument has been integrated in France at the APC laboratories (Paris) during 2018, and tested in 2019-2021.
The QUBIC concept is completely novel and it is the first ‘bolometric interferometer’, an instrument that combines the extreme sensitivity of bolometric detectors, cooled to -273°C, with the precision of interferometry. In addition, QUBIC offers the possibility of spectro-imaging, that is the simultaneous measurement of the colour of each pixel in an image, the only way of subtracting out the non-primordial B-modes.  QUBIC will be installed on the plateau de La Puna in the north of Argentina, at an altitude of 5000 m and near the village of San Antonio de los Cobres in Salta Province.  The instrument arrived at Salta in July 2021 and is undergoing tests in the integration hall that was built for QUBIC on the CNEA Regional Noroeste site. The tests have confirmed that the instrument arrived safely from France and is operating as expected. The infrastructure on the site is being completed (after a series of delays due to the pandemic) and the installation of the instrument on the observing site is planned for later in 2022.

International context

QUBIC is in competition with a half dozen other instruments researching the primordial B-modes: BICEP/KECK, CLASS, SPIDER from the United States, Ali-CPT in China and the Japanese satellite project (with a significant European contribution) LiteBIRD (planned for 2033). All are variations on the classical telescope concept and don’t offer what QUBIC can in terms of the measurement purity and spectral imaging that one gets from interferometry.  The sensitivities of the instruments are comparable and, of course, the final B-mode discovery will have to be confirmed independently by various groups in order for it to make an impact on the scientific community and beyond.

Further reading

 
QUBIC Web Page: http://qubic.in2p3.fr/
 
Articles for the General Public:

• Le Monde 28 April 2021
• Sciences & Avenir September 2021
• Newsletter of CNRS Rio de Janeiro - August 2021

 
Scientific Articles:

• 8 articles for a forthcoming special edition of JCAP (Journal of Cosmology and Astroparticle Physics)

Contacts

 
Jean-Christophe Hamilton, Spokesperson and Principal Investigator of QUBIC,
Research Director at APC (Paris) - CNRS-IN2P3 - hamiltonapc.in2p3.fr - phone: +33 06 88 01 06 75
 
QUBIC Editorial Board:
 

• Elia Stefano Battistelli (Chair): Physics department – Sapienza University of Rome and INFN Rome section – elia.battistelliroma1.infn.it – phone: +39 06 49914462
• Jean-Christophe Hamilton: Université de Paris, CNRS, Astroparticule et Cosmologie, Paris, France
• Creidhe O'Sullivan: National University of Ireland, Maynooth, Ireland
• Claudia Scoccola: Universidad Nacional de La Plata and CONICET, Argentina