French roadmap for Cosmic Microwave Background science June 2016 The front cover image illustrates our view of the observable Universe in the millimetre range. The colours indicate the intensity variations, while the lines give the direction of polarization. The inner sphere corresponds to emission from our Galaxy, while the outer sphere corresponds to the Cosmic Microwave Background (CMB) signal, the most remote light we can ever measure. The CMB can be interrogated so as to reveal the physical conditions in the primordial Universe, symbolised here by the white outer shell. The central question mark represents the assessment presented in this document on how the polarized sky will be mined for cosmological content, and whether France will be part of such a quest. The image of the back cover illustrates the intensity part of the observable Universe emissions in the millime- tre range as mapped by the Planck mission facing simulations of the evolution of the large scale distribution of matter. These images under creative commons license were designed and realised with data from ESA and the Planck collaboration by the HFI French outreach group and the Canopée company. ii French roadmap for Cosmic Microwave Background science Executive summary The highly successful Planck cosmic microwave background (CMB) mission has now accurately measured more than a million harmonic modes of the CMB sky with a signal-to-noise greater than one, and has com- pletely fulfilled its principal goal of extracting most of the cosmological information contained in the primary CMB anisotropies in temperature (at least within the ΛCDM class of models). Planck has also exceeded its goals and measured the first 100 000 polarized E-modes, and contributed to the measurement of the tens of fainter B-modes now known. The basic ΛCDM model fits all the data, with parameters known at the per cent level. The deviation from scale invariance expected from inflation has been established beyond doubt, and the knowledge of many key parameters have been improved about a hundred-fold, providing precise constraints on a host of possible extensions to the minimal model (e.g., spatial curvature, neutrino properties, primordial non-Gaussianities or isocurvature modes). Fundamental questions however remain, in particular on early universe physics and cosmology. Nevertheless, the CMB will continue to offer arguably the cleanest experimental window on these, through the millions of additional, but weaker, modes remaining to be measured. Indeed, the precision with which the cosmological model can be determined scales with the inverse of the square root of the number of relevant modes. Among other things, the poorly-known polarization B-modes offer the exciting prospect of a first detection of pri- mordial gravitational waves, which would be a key experimental manifestation of quantum gravity. Such a detection is likely to remain completely out of reach of direct gravitational detectors for the foreseeable future. On the late-time cosmology side, another very exciting endeavour is the mapping of the dark matter distribution through its lensing effect on the CMB at higher redshifts than possible by any other methods. In addition to constraints on the evolution of dark energy, or the locus of Galaxy formation, it will tell us about the neutrino sector of particle physics. Additionally, the CMB constraining power on extensions to the basic ΛCDM model will be enormously increased, potentially discovering the failure or limitation of the now standard ΛCDM model; this will also offer a large increase in the leverage of other astrophysical probes such as Euclid and LSST. It should also be noted that the best constraints on distortions of the mean CMB spectrum are still those determined by COBE-FIRAS some twenty years ago, yet it should now be possible to search for these, which must exist at a level well within reach of today’s technology, and to extract the additional information encoded therein, e.g., on the end of the dark ages or the integrated effect from hot and warm matter. With the completion of the Planck project, the French community is now at a crossroad, with demonstrated expertise in all relevant aspects of CMB science and the ability to continue to contribute to this cosmological quest with future experiments; this expertise indeed includes instrument design, assembly, calibration, opera- tions, processing and scientific analysis in all relevant scientific areas. In that context, it is particularly important to emphasize its strong expertise in promising and rapidly developing technologies in cryogeny and detectors, as well as data processing. However, if not preserved through a long-term program of CMB measurements, these key human and technological assets may well disappear and will be very hard to rebuild. In order to propose a roadmap for future CMB measurements, this report reviews the CMB scientific potential and the obstacles, surveys the current landscape and proposes an analysis of current projects. The main conclusions are the following. Space provides a unique environment for CMB measurements by offering freedom from atmospheric fluc- tuations and the lack of transparency caused by the atmosphere at essential frequencies, access to the full sky resulting in no loss of information particularly on large angular scales, long term measurement stability, and exacting control of systematic errors. Given the necessity of space to provide the benign environment from which measurements will allow the extraction of crucial information that cannot be obtained otherwise, the long term priority for the French community is a strong participation in a CORE-like experiment. Such a CMB polarization mission will be proposed by the European community in October 2016 to ESA’s call for opportu- nity for the M5 slot, as an evolution of the previous proposals in L3 and M4. M5 is a priori targeted for a 2030 iii launch, although it might be launched as early as 2026, if the instruments can be ready in time, perhaps within the context of a collaboration with a non-European agency like NASA and/or JAXA. There are two, potentially earlier, smaller scale CMB projects in space which are known to us, LiteBIRD at JAXA, and PIXIE at NASA. LiteBIRD is less powerful and of narrower scientific scope than CORE, but is still technically quite challenging, especially given the cost cap. It should only be considered for substantial participation in the event that CORE is not selected for a phase A study. PIXIE is proposing an interesting experimental alternative with exciting and unique scientific path-finding capabilities regarding CMB spectral distortions. Since it would involve only a relatively small fraction of the French community, participation was considered as highly commendable as a proposed mission of opportunity. Any space project will give results in more than 10 years from now, even including the LiteBIRD and PIXIE projects which are not firmly selected yet. All new data and progress in this field in the next decade will there- fore come exclusively from the ground and balloons. This exclusive phase will then be followed by a synergy phase between space and ground where the data sets will be complementary in order to cover (optimistically) the full sky at the required frequencies at high angular resolution. To preserve and leverage its expertise, as well as to benefit fully from lessons to be learned in the coming decade, the French community must therefore strongly participate in the suborbital effort. Following the classification of our US colleagues, the CMB ground-based experimental path may be de- scribed in a series of stages, with stage-three currently beginning to deploy on the ground of the order of 10 000 detectors to map portions of the sky. This will be followed by a stage-four targeting of the order of 500 000 detectors to observe from the ground an increasing fraction of the sky. Perhaps optimistically, and depending on funding, those experiments which are part of stage-four may start to operate after around 2020. These plans set the stage for any French effort on the ground in the coming years, which would best be framed within a joint European effort. Of course European ambitions on the ground will take into account the fate of the M5 proposal to scope it, but, in any case, direct participation is vitally needed and activities in that direction need to be vigorously pursued. Shorter term projects have been analysed within this global M5/S4 framework. In particular, we note the scientific promise of the proposed B-SIDE balloon-borne experiment that is com- petitively positioned to reveal crucial information on the detrimental effects of dust polarization and its com- plexities on CMB B-mode experiments, provided it can be flown before 2020. In addition to this niche, B-SIDE also offers a proving ground in real conditions for KIDS detectors, a most promising technology, expertise in which constitutes a significant French asset both for the long term highest priority projects M5 and S4. The ground project QUBIC proposes an innovative way to control low-level polarization systematics. The project has to demonstrate rapidly the validity of its instrumental concept on the sky in order to be a possible stepping stone towards participation in the S4 effort. To achieve this, the current schedule has to be strictly adhered to in order to demonstrate nominal sensitivity and systematics control on the sky by the end of 2018. The QUBIC collaboration and funding bodies should therefore make clear decisions urgently. In any case, the suborbital effort in France and Europe has now to change gears. Indeed, while the CMB future is bright, it may very well turn out to be rather bleak in France barring fast actions from the community and its funding agencies. iv Contents 1 Introduction 1 1.1 Current status...........................................3 1.2 The French community......................................4 2 Scientific potential of CMB measurements5 2.1 The early universe.........................................5 2.2 The spectral distortions......................................9 2.3 Constraining the matter content of the universe........................