Measurements from the COBE/FIRAS satellite in the 1990s demonstrated that the Cosmic Microwave Background (CMB) frequency spectrum closely follows a nearly perfect blackbody distribution, with deviations constrained to a few parts in 10⁵. They were a crucial probe for the establishment of the hot big bang model as our standard cosmological model, and were awarded the Nobel prize in physics in 2006. However, within the standard cosmological model distortions of the CMB spectrum caused by energy releasing processes throughout cosmic history are allowed and expected by the formation of starts and large scale structure. Detecting these distortions would open a new observational window into the thermal and energetic history of the Universe, probing epochs and physical processes beyond the reach of traditional CMB anisotropy or large-scale structure measurements.

Recently, a space mission proposal aimed at measuring these spectral distortions—FOSSIL—was submitted in response to the ESA M8 call and has been pre-selected for the second stage of evaluation. If successful, FOSSIL is planned for launch in 2041, bridging the observational gap since COBE/FIRAS. In parallel, the BISOU balloon-borne experiment (currently in CNES Phase A) is designed as a pathfinder mission to conduct critical tests of instrumental performance and foreground separation techniques, preparing for a future space mission and potentially already delivering new exciting results on the physics of the early universe and structure formation. 

The primary goals of this PhD project will be to quantify, optimise, and forecast the scientific performance of next-generation CMB spectral distortion instruments, focusing on BISOU and FOSSIL. This will include rigorous accounting for instrumental systematics and the complexity of contaminating foreground astrophysical emissions.

Building on ongoing efforts to develop an initial instrument model for the BISOU and FOSSIL Fourier Transform Spectrometers (FTS) to measure spectral distortions, this thesis aims at extending the parametric model to fully simulate the measurement process—from the simulated sky through the interferogram to the final cosmological signal recorded by the instrument. This will involve several key activities, including:

- Enhancing the sky model to enable realistic simulations of foreground separation challenges and mock measurements of the monopole  of the spectral distortion signals.

- Implementing more realistic modelling of instrument subsystems, incorporating laboratory measurements where possible, and characterising major systematic effects.

- Designing a first data analysis pipeline based on realistic simulated data to evaluate the impact of systematics and foreground contamination in the  spectral distortion maps recovery, and to assess the feasibility of the BISOU and FOSSIL science objectives.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - nabila.aghanim@ias.u-psud.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time !