F2: From the Big Bang to the future Universe

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  • Cosmology is the study of the Universe in its largest dimensions and of its evolution. In the successful big bang model, the early Universe is hot and dense and cools down as it expands. The question of the status of the original singularity (the so-called big bang) from which space emerged is a central one. It may be studied through the gravitational waves that are produced immediately after the big bang, for example during an explosive phase of expansion, known as inflation, that follows immediately the big bang.
    The study of the expansion of the Universe has recently shown an unexpected acceleration in the more recent stages of the evolution, which is attributed to a new form of energy, known as dark energy. Understanding the nature of dark energy and the fate of the Universe is another fundamental question.


    Although these scientific questions are specific to the understanding of the early Universe and its subsequent evolution, the methods used have far reaching applications. For example, in the context of the Space Campus, teams from IPGP and APC realized that they are using similar methods to analyze seismic data on the Moon surface and fluctuations in the Cosmic Microwave Background (CMB): in both cases fluctuations on a sphere. Also, cosmology requires processing larger and larger amounts of data. For example, the 10 year observations by the LSST telescope (performing large sky surveys to understand the properties of dark energy) will require a database of 60 PetaBytes of raw data. Processing such a vast amount of data is a challenge which will position the field in a very central place for the treatment of massive amounts of data.

    We have identified a certain number of axes:

    1. Support of the Paris Centre for Cosmological Physics

    All the fundamental questions listed above are addressed by the Paris Centre for Cosmological Physics (PCCP: http://www.pariscosmo.fr). We stressed earlier that the goals of PCCP are very similar to those advertised by the LabEx, although the PCCP is more focussed thematically but has a larger laboratory base. The LabEx supports the Centre by providing one postdoc position (PCCP fellow) every year, as well as financing the visits of scientists with a high international visibility through a special UnivEarthS-PCCP programme.

    2. B-mode polarization of the Cosmic Microwave Background

    The contribution of the Labex has been crucial to structure team for preparing the forthcoming years in these fields with an emphasis to consolidate a major contributor of the French community to three CMB projects : QUBIC (as the vector of a new instrumental concept – the bolometric interferometry), LiteBIRD ( the only CMB-B polarisation Space project supported by an space agency) and POLARBEAR/Simons Observatory (for which we are building the main French participation to the US CMB-S4 program).


    The measurement of B polarization modes of the Cosmic Microwave Background (CMB) may provide a direct probe of primordial gravitational waves produced during the inflationary epoch. Measuring precisely the polarization of the CMB is thus the next exciting frontier. Its characterization was improved by the Planck satellite mission (launched May 14, 2009). The weakness of the B-mode signal requires the development of highly sensitive experiment with an exquisite control of systematic errors. Most of the experiments or projects dedicated to the quest are based on the well known direct imaging technology. While imagers measure maps of the CMB, interferometers directly measure Fourier components of the Stokes parameters and thus are expected to be less sensitive to systematic effects. Unfortunately, the classical heterodyne interferometry concept may have reached its limits in term of scale and sensitivity. However, Bolometric Interferometry could combine the advantages of interferometry in terms of systematic effects handling and those of direct detectors in terms of sensitivity.

    Although many experiments are already proposed in the USA (ground based and balloon borned), only one project has emerged in Europe, the QUBIC program supported by a French-Italian-USA-UK-Irish collaboration (http://www.qubic.org). The APC CMB team including its experimental laboratory is leading this research effort with particular interest in:

    • Conception and design of the QUBIC instrument
    • Data analysis and simulations
    • Development of the detection chain:
      • Bolometer arrays based on the Transition Edge Sensors (TES) technology and multiplexed readout.
      • Kinetic Inductance Detectors (KIDs), a new path towards large detector arrays: this new detection technique uses the variation of kinetic inductance of a superconductor when it absorbs a photon flux. Their advantages are the following: (i) they are relatively simple to fabricate, (ii) the readout electronics is inherently multiplexed allowing for a large number of detectors (of the order of 1000 or more) to be readout with a single wire and (iii) the intrinsic sensitivity could theoretically be very high. We propose to couple these new detectors with the current developments made for TESs. Within 2 years, a demonstrator of some 100s KIDs will be realized, fully compatible with the QUBIC requirements. The number of detectors will be further improved to reach some thousands of KIDs within 10 years.
    • Realization of receiver horns based on the platelets technology.


    3. Understanding the nature of Dark Energy

    The other sub-work package has a different timeline: it concerns the analysis of data of experiments searching for the nature of dark energy. They are based on large scale surveys which require to store and analyse massive amounts of data. The François Arago Centre, together with the IN2P3 computing centre in Lyons, plays a significant role in this challenging task. The work is to identify what will be the exact need for data storage and processing (2011-2014) and then to participate in setting up an international centre for dark energy, as already planned in the US by the LSST collaboration (2015-2020).

    Progress in this field is expected both on the theoretical and observational sides.
    On the theoretical side, alternate models of dark energy are examined along with possible large distance modifications of gravity. Both directions should be followed, given that dark energy is currently only known through its gravitational effects. Hence, observations leading to infer its existence can also be explained instead by changes in the gravitational laws at cosmological distances. There are plenty of models which replace a simple cosmological constant by a new more or less exotic dark content of the Universe, but no fully consistent model of large distance modification of gravity is known.

    The APC and LUTh theory goups are involved in both directions, with a special expertise on the study of large distance modifications of gravity at APC. New proposals along this line have been made by APC theorists and are under examination.

    From an observational point of view, understanding dark energy requires more accurate characterization of the properties of this dark energy, and tests of standard general relativity. Two complementary families of measurements should be pursued, since they allow to differentiate modified gravity from sensu stricto dark energy: measurements of the expansion rate of the Universe and its evolution, and measurement of the growth of structures, whose rate is slowed down by dark energy.

    For each of these measurements, several complementary techniques should also be used : the exquisite required precision of these observations requires to carefully control degeneracies and systematic effects, which will affect different probes in different ways.
    APC is involved in several short and long term observational projects using several dark energy characterization techniques, through its wide field astronomy group. It is also participating in the Planck project (see above) and will be able to use CMB data in correlation with other wide field surveys.

    On the longer term, APC is focusing on cosmic magnification, another way to exploit gravitational lensing than the more common shear measurement — this technique is very well adapted to the depth of future surveys. The preparation of future analyses is currently done on SDSS data, and APC plans to apply this technique to LSST and Euclid data. LSST (Large Synoptic Survey Telescope) is a telescope with a 8.4 m diameter main mirror, which should get its first scientific images in 2019. It is located on top of Cerro Pachón in Chile, which already houses the Gemini South telescope. The camera has a mosaic of 200 4k x 4k CCDs, totaling 3.2 billion pixels, with a field of view of ten square degrees.

    This project led by Anthony Tyson from the University of California, Davis has been ranked by the Astronomy and Astrophysics Decadal Survey “New Worlds and New Horizons in Astronomy and Astrophysics,” as its top priority for the next large ground-based astronomical facility. Euclid, a space based project, is proposed by a European consortium (led by Alexandre Refregier from CEA/IRFU/SAp) and was selected by ESA in October 2011 (see the Euclid ESA page). Its launch is planned for 2021.

    Both experiments should be in operation around 2019-2021. APC is already strongly involved in their preparation — camera control software and photometric calibration for LSST, and most importantly, data processing for both LSST and Euclid ground segment. Euclid is a space project that relies on ground-based data to fully exploit its science data: LSST data will be an important asset in Euclid’s science exploitation, and APC will be in charge of merging LSST and Euclid data. Since LSST data will represent tens of petabytes, the data processing will heavily rely on computing resources and staff at CC-IN2P3 and François Arago Centre (FACe).



    Position Name Laboratory Grade, employer
    WP leader Giraud-Héraud, Yannick APC DR – CNRS
    WP co-leader Piat, Michel APC Professor – Université Paris Diderot
    WP co-leader Hamilton, Jean-Christophe APC DR – CNRS
    WP co-leader Aubourg, Eric APC CEA
    WP co-leader Langlois, David APC DR – CNRS
    WP member Bartlett, James G. APC Professor – Université Paris Diderot
    WP member Ganga, Ken APC DR – CNRS
    WP member Bucher, Martin APC DR – CNRS
    WP member Stompor, Radek APC DR – CNRS
    WP member Patanchon, Guillaume APC MCF – Université Paris Diderot
    WP member Errard, Josquin APC CR – CNRS
    WP member Grandsire, Laurent APC IR1 – CNRS
    WP member Tartari, Andrea APC/PCCP PCCP fellow
    WP member Cavet, Cécile APC/FACe IR – CNRS
    WP member Smoot, George APC/PCCP Professor – Université Paris Diderot/USPC
    WP member Hazra, Dhiraj Kumar APC/PCCP PCCP fellow
    WP member Prêle, Damien APC IR2 – CNRS
    WP member Voisin, Fabrice APC IR2 -CNRS
    WP member Chapron, Claude APC IR1 – CNRS
    WP member Bleurvacq, Nathan APC IE – CNRS
    WP member Decourcelle, Tanguy APC CDD-IE – CNES
    WP member Ascaso, Begoña APC Marie Curie fellow
    WP member Traini, Alessandro APC PhD – Sorbonne Paris Cité – ED560
    WP member Beck, Dominic APC PhD – Sorbonne Paris Cité – ED560
    WP member Doux, Cyrille APC PhD – Sorbonne Paris Cité – ED560
    WP member Hoang Duc Thuong APC PhD – Sorbonne Paris Cité – ED 560

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    QUBIC, as a bolometric interferometer, is anticipated to offer an unprecedented level of control of systematics thanks to the possibility of performing self-calibration. However, this clear advantage comes at the cost of a more complicated data analysis than with classical imagers: the synthesized beam is very structured (multiply peaked, frequency dependent with non-Gaussian beam-shapes) so that the map-making process needs to be based on specific algorithms. This important issues was solved in the past three years by Pierre Chanial, thanks to the funding from the Labex Univ’Earths. He has developed a pipeline using High-Performance Parallel Computing to enable the implementation of a realistic simulator of QUBIC Time-Ordered-Data using multiple convolutions of the input sky and time-ordered-data, accounting for realistic instrumental configurations (including 1/f noise).

    In 2017, the main contribution of the labex WP F2 to QUBIC has been targeted, with the arrival of Maria Salatino, in the project, the development of scripts for data acquisition and analysis of the TESes arrays and the improvement of the laboratory testbed. She elaborated a set of scripts to automatically execute the data analysis and charaterize the TES arrays.

    Last point we would like to mention is that, in 2017, as APC is the PI laboratory of QUBIC, four collaboration meetings have been organized at Paris Diderot University : April 20-21, June 22, September 21 and November 28.


    POLARBEAR/Simons Observatory


    In 2017 the POLARBEAR collaboration published new constraints on the CMB B-mode power spectrum on small angular scales. These are based on two observational campaigns and update the previous constraints published in 2014. The new result amounts to nearly 4\sigma detection of the amplitude of the lensing-generated signal. The latest paper was co-coordinated by a former APC student, J. Peloton, and the results are based on the analysis performed with two independent data analysis pipelines, one of which was developed at APC. We also started work on the analysis of data from the third campaign.

    Travel support provided by Labex for the POLARBEAR effort covered a participation of two APC researchers (Radek Stompor, Josquin Errard) and a PhD student (Dominic Beck) in an annual collaboration meeting. This enabled us to extensively present the work done at APC in the year between the meetings, participate in strategic discussions defining the project’s future and start new hands-on collaborations with other members of the collaboration. The funding was key in ensuring our continuing presence in and impact on the project and in establishing Dominic Beck as a full member of the collaboration.



    LiteBIRD is a Japan-led, satellite mission focused on detecting primordial B-mode signal and verifying the inflationary paradigm. The mission is undergoing a phase-A study in Japan and has successfully concluded such a study in the US. Researchers from APC have been involved in LiteBIRD since 2015 either as full-team members or external collaborators. They work on addressing two key issues of the satellite design: the choice of frequency bands and their sensitivities and their impact on the satellite performance, and impact and mitigation of some selected instrumental systematics. In both these areas we play coordinating and leading roles. In 2017 we initiated an establishment of the LiteBIRD-France collaboration, what resulted in a preparation and a submission in Sept 2017 of a mission-of-opportunity proposal to CNES. The LiteBIRD-France collaboration currently involves 35+ researchers from 9 institutes in France and APC is one of the coordinators of this on-going effort. We also are a driving force behind the European level effort, which is now in the process of organization, aiming at preparation of a proposal to ESA.

    The labex contribution to this kind of project is decisive to build the laboratory contribution, it was key in helping us to co-coordinate the effort aiming at defining a European level contribution to LiteBIRD.



    The next generation ground-based CMB experiment, CMB-Stage 4 (CMB-S4), is a massive undertaking to deploy of order 500,000 detectors on the sky to increase sensitivity by at least an order of magnitude.  Using a suite of small and large aperture telescopes distributed between the South Pole and the Atacama desert in Chile, the primary science goals are to search for primordial gravity waves, through the polarized B-mode CMB anisotropy signal, and to constrain the number of light particle species produced in the early universe, by measuring the effective number of degrees-of-freedom.

    APC researchers are actively involved in the development of CMB-S4 and have also participated on the CDT. Thanks to the labex, in 2017, Ken Ganga and James G. Bartlett participated to the two CMB-S4 annual meetings.


    Joint analyses of cosmological probes

    In 2017, we just opened this new field of research despite what we were planning to do. However we can already emphasize a participation to the LSST DESC meeting at Stony Brook University (NY) from July 10 to July 15, 2017.



  • – Takakura, S. and the POLARBEAR collaboration, Performance of a continuously rotating half-wave plate on the POLARBEAR telescope, Journal of Cosmology and Astroparticle Physics, Issue 05, article id. 008 (2017).

    – The POLARBEAR collaboration, A Measurement of the Cosmic Microwave Background B-Mode Polarization Power Spectrum at Sub-Degree Scales from 2 years of POLARBEAR Data, accepted for publication in the Astrophysical Journal, (2017)

    – Poletti, D. and the POLARBEAR collaboration, Making maps of cosmic microwave background polarization for B-mode studies: the POLARBEAR example., Astronomy & Astrophysics, Volume 600, id.A60, (2017)

    – Matsumura, T. and the LiteBIRD collaboration, LiteBIRD: Mission Overview and Focal Plane Layout., Journal of Low Temperature Physics, Volume 184, Issue 3-4, pp. 824-831, (2016)

    Suzuki, A. and the POLARBEAR collaboration, The Polarbear-2 and the Simons Array Experiments. Journal of Low Temperature Physics, Volume 184, Issue 3-4, pp. 805-810, (2016)

    Cyrille Doux et al. Cosmological constraints from a joint analysis of cosmic microwave background and large-scale structure. arXiv.org 1706, arXiv:1706.04583 (2017))

    – Duc Thuong Hoang et al. – Bandpass mismatch error for satellite CMB experiments I: Estimating the spurious signal (https://arxiv.org/abs/1706.09486)