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BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) is a balloon-borne pathfinder aiming at detecting spectral distortions in the Cosmic Microwave Background (CMB) using a spectrometer.

Thirty years after the CMB spectrum was measured by the FIRAS instrument on the COBE space mission, no further measurements have been made. Although COBE showed that the CMB emission follows that of a practically perfect black body at a temperature of T = 2.7255 +/- 0.0006 K, theory predicts that deviations from the black body spectrum, called spectral distortions, exist.

These spectral distortions [1] are created by processes that inject energy, causing matter and photons to leave thermal equilibrium after thermalization becomes ineffective, at a redshift of less than z=2x10^6. There are two types of distortion, both resulting from an exchange of energy between electrons and photons through the Compton effect. Compton scattering is very inefficient for a redshift greater than 5x10^4, giving rise to a so-called y-type distortion, tracing the thermal history during recombination and re-ionisation. The mu-type distortion, on the other hand, occurs at a redshift greater than 5x10^4 when Compton scattering is very efficient, and cannot be generated after the recombination epoch. Measuring these distortions is therefore a unique way of tracking the thermal history of our Universe.

Several dedicated space mission concepts have been proposed or co-proposed to NASA or ESA by our team (PIXIE [2], PRISTINE, FOSSIL).

BISOU, coordinated by the IAS Cosmology team, is an international collaboration involving several laboratories in France, Italy, Ireland, the UK, the USA and Japan. As well as acting as a pathfinder for a potential future space mission, developing certain key technologies and consolidating the measurement concept using a Fourier Transform Spectrometer (FTS), BISOU's scientific objectives are to carry out an initial detection of y-type spectral distortions, as well as obtaining greater precision in measuring the temperature of the CMB and the total intensity of the Cosmic Infrared Background (CIB).

A CNES Phase 0 study has been completed successfully in spring 2023, and a Phase A is due to start in March 2024.

Moreover, a co-funding from Ile de France région (DIM-ORIGINES) and CNES has recently been awarded to develop a cryogenic breadboard of the instrument concept, BISOU-BBM.

The BISOU project has been presented at several conferences:

  • 16th Marcel Grossmann meeting (2021), BISOU: a balloon project to measure the CMB spectral distortions
  • 25th ESA symposium on European Rocket and Balloon programmes and related research, Biarritz, 2022
  • SPIE Astronomical Telescopes + Instrumentation 2022, Montreal. BISOU: a balloon project for spectral observations of the early universe, B. Maffei et al, Proceedings of the SPIE, Volume 12190, id. 121900A 12 pp. (2022)
  • SPIE Astronomical Telescopes + Instrumentation 2022, Montreal. Optical analysis of BISOU: a balloon project to measure the CMB spectral distortions, N. Trappe et al, Proceedings of the SPIE, Volume 12190, id. 121901Q 6 pp. (2022)


  1. New horizons in cosmology with spectral distortions of the cosmic microwave background, J. Chluba et al, Experimental Astronomy, Volume 51, Issue 3, p.1515-1554 (2021)
  2. The Primordial Inflation Explorer (PIXIE), A. Kogut et al, Proceedings of the SPIE, Volume 9904, id. 99040W 23 pp. (2016)


Contacts at IASBruno Maffei and Nabila Aghanim


Project status: 
Phase 0 study
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