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BISOU (Balloon Interferometer for Spectral Observations of the Primordial Universe) is a pioneering project aimed at detecting spectral distortions in the cosmic microwave background (CMB) using a spectrometer carried aboard a stratospheric balloon.

The Role of the IAS

BISOU, coordinated by the IAS Cosmology Team, is an international collaboration involving several laboratories in France, Italy, Ireland, the United Kingdom, the United States of America, and Japan. In addition to serving as a pathfinder for a potential future space mission, developing certain key technologies, and refining the measurement concept using a Fourier transform spectrometer (FTS), BISOU’s scientific objectives are to make the first detection of y-type spectral distortions , as well as to achieve greater precision in measuring the temperature of the CMB and the total intensity of the cosmic infrared background (CIB).

The project is currently in Phase A of the study with CNES and is expected to be completed in the spring of 2027.

In addition, co-funding from the Île-de-France region (DIM-ORIGINES) and CNES has been awarded for the development of a prototype of this instrument, BISOU-BBM.

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In development

  • Spring 2023: Successful completion of Phase 0 (feasibility study) with CNES.
  • March 2024: Start of Phase A (detailed design).
  • 2021–2022: Presentations at international conferences (ESA, SPIE, Marcel Grossmann).
  • Feasibility Study: Approved in 2023 by CNES.
  • Prototype development: BISOU-BBM, co-funded by CNES and the Île-de-France Region (DIM-ORIGINES).
  • Detailed Design: Launch in March 2024 (Phase A).
  • Scientific Presentations: International conferences to promote the project.

FTS Spectrometer: Measures spectral distortions in the CMB.
BISOU-BBM: Prototype for validating technologies prior to flight.

Contacts

Advisors (scientific and technical)

For more information:

Bisou Moves to Phase A

More than thirty years after the COBE space mission’s FIRAS instrument measured the CMB spectrum, no other measurements have been taken. Although COBE showed that the CMB emission behaves like that of a nearly perfect black body at a temperature of T = 2.7255 ± 0.0006 K, theory predicts that deviations from the black-body spectrum—known as spectral distortions—exist.

These spectral distortions [1] are created by energy-injecting processes that cause matter and photons to depart from thermal equilibrium after thermalization becomes ineffective, at a redshift less than z = 2×10^6. There are two types of distortions, both resulting from energy exchange between electrons and photons via the Compton effect. Compton scattering is highly inefficient at redshifts greater than 5×10^4, giving rise to a distortion known as the y-type distortion, which traces the thermal history during recombination and reionization. Mu-type distortion, on the other hand, occurs at redshifts greater than 5×10^4 when Compton scattering is highly efficient, and cannot be generated after the recombination epoch. Consequently, measuring these distortions provides a unique way to trace the thermal history of our Universe.

Our team has proposed—or co-proposed with NASA or ESA—several concepts for dedicated space missions (PIXIE [2], PRISTINE, FOSSIL).

The BISOU project was presented at several conferences:

  • 16th Marcel Grossmann Meeting (2021), BISOU: A Balloon Project to Measure CMB Spectral Distortions
  • 25th ESA Symposium on European Rocket and Balloon Programs 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 CMB Spectral Distortions, N. Trappe et al., Proceedings of the SPIE, Volume 12190, ID 121901Q, 6 pp. (2022)

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

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