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The study is lead in France by Profs. Bruno Maffei and Nabila Aghanim. Our Prof. Giorgio Savini is contributing with the design and modelling of the spectrometer and calibration source.

The exceptional discoveries about the evolution and properties of our Universe made by the space missions COBE, WMAP and Planck, as well as the many sub-orbital and ground-based CMB (Cosmic Microwave Background) experiments including Boomerang, Archeops, CBI, VSA, ACT, SPT and BICEP, have opened numerous new areas of research over the past decades. Thirty years after the CMB spectrum was first precisely measured by the COBE/FIRAS instrument, we are now in the unique position to perform measurements of the CMB energy spectrum by several orders of magnitude, allowing transformational steps towards the detection of signals expected within the Lambda-Cold Dark Matter (LCDM) model.

Although COBE demonstrated that the average CMB energy spectrum follows that of a near-perfect black body at a temperature of T = 2.7255 +/- 0.0006 K, theory predicts small deviations from Planck’s law, known as spectral distortions. Measuring these distortions will provide a unique way of tracing the thermal history of our Universe, to gain deeper insight in the theory of inflation, structure formation and particle physics. This prospect makes high precision CMB spectrum measurement one of the priorities in experimental cosmology and astrophysics, as clearly recognized by several space agencies.

To advance the field of research from FIRAS to spectral distortion measurements with a future space mission, unprecedented sensitivity and control of systematics are required. This demands a new instrument concept that must be consolidated and validated in the laboratory, possibly using a precursor.

An international team of researchers has conducted a Phase 0 study with CNES to investigate the possibility of an initial distortion measurement from a stratospheric balloon. BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) is designed as a balloon-borne differential spectrometer, which capitalises on the heritage of FIRAS and directly benefits from earlier studies carried out as part of the PIXIE (NASA), PRISTINE (ESA) and FOSSIL (ESA) mission proposals. BISOU is coordinated by Prof. Bruno Maffei and the IAS Cosmology team and is a concerted effort by several laboratories in France, Italy, Ireland, the UK, the USA and Japan.

In addition to acting as a precursor for developing key technologies and consolidating the measurement concept using a Fourier Transform Spectrometer (FTS), BISOU's scientific objectives include the first detection of the LCDM y-type spectral distortion monopole, the

measurement of the CMB temperature to a greater precision and putting a better constrain on the Cosmic Infrared Background (CIB) total intensity. It will also shed new light on the origin of the ARCADE radio excess as well as rule out early-universe models with enhanced small-scale power or exotic relic particles. These exciting prospects will bring the scientific community together around a cutting-edge project to prepare the next generation of researchers and engineers in anticipation of a future space mission.

The Phase 0 has demonstrated the viability of such a balloon-borne mission and consolidated BISOU's scientific objectives. As a result, the Phase A study will begin in June 2024 to finalise the detailed concept of this stratospheric balloon project, but also allowing for an emerging concept of a future spaceborne CMB spectral distortion observatory.

At the same time, thanks to joint funding from CNES and the ÃŽle de France region DIM-ORIGINES programme, a laboratory cryogenic BreadBoard will be developed at the IAS. This will be used to study the various sub-systems, as well as the instrument as a whole and will also allow for prototyping. This step will be crucial to understand the systematics of such new measurement concept in order to achieve the required high sensitivity.

The IAS is building on its heritage acquired during the Planck-HFI years, but also on more recent R&D programmes, to pave the way towards brand new scientific goals.