Description du sujet de thèse

From slitless spectrophotometry to the dark energy equation of state with the StarDICE and LSST experiments
The objective is to develop slitless spectrophotometry as a tool for measuring the dark energy equation-of-state parameter. The nature of dark energy is one of the great open questions in cosmology. This decade will bring unprecedented precision in measuring the expansion of the Universe, thanks to new wide-field cosmological surveys (Euclid, DESI, LSST, ZTF). LSST (Chile) and ZTF (USA) are two wide-field surveys capable of scanning the entire sky every few days, collecting numerous transient astronomical events, such as Type Ia supernova explosions. These are highly luminous stellar explosions that serve as standard candles for distance measurements in the Universe, enabling constraints on the properties of dark energy.
By 2030, the Hubble diagram (distance vs. redshift) for Type Ia supernovae will include tens of thousands of measurements at both high and low redshifts, notably from LSST and ZTF. However, the precision in measuring dark energy properties using these data will be limited by systematic uncertainties in photometric calibration. To overcome this limitation, the IJCLab and LPNHE laboratories in the Paris region have partnered to:
1. remeasure flux standard stars with the StarDICE project, and
2. develop slitless spectrophotometry to measure atmospheric transmission at observatory sites, notably with the LSST Auxiliary Telescope (AuxTel).
The proposed PhD project is part of the LEMAITRE project (Latest Extended Mapping of Acceleration with an Independent Trove of Redshifted Explosions), which aims to construct a Hubble diagram using thousands of unpublished supernovae events, offering a new, independent measurement of dark energy properties. The sample will consist of supernovae from the ZTF, SNLS, and Subaru surveys, and by the end of the PhD, it may include the first LSST supernovae sample.
The thesis has two components:
• The first part involves demonstrating the capability of slitless spectrophotometry to measure atmospheric transmission in Chile and at the Observatoire de Haute Provence. This technical work, placing the candidate at the heart of a cosmological survey, includes travel to observatory sites to carry out calibrations. A major expected outcome is the publication of a new network of flux standard stars via StarDICE.
• The second part involves applying these calibrations to the fluxes in the supernova light curves from ZTF and LSST. The Hubble diagram published at the end of the PhD will be the first flux-calibrated cosmological distance diagram using StarDICE, offering new insights into the expansion of the Universe and the nature of dark energy.
Suggested Bibliography
• Perlmutter et al., Measurements of Ω and Λ from 42 High-Redshift Supernovae, ApJ 517, 565 (1999), doi:10.1086/307221, arXiv:9812133
• Neveu et al., Slitless spectrophotometry with forward modelling: principles and application to atmospheric transmission measurement, A&A 684, A21 (2024), arXiv:2307.04898
• Neveu et al., On the importance of Earth's atmosphere for SNIa precision cosmology, arXiv:2407.01058

Contexte de travail

The Irène Joliot-Curie Laboratory of Physics of the Two Infinities (IJCLab) is a joint research unit under the supervision of the CNRS, Université Paris-Saclay, and Université de Paris. Created in 2020 from the merger of five laboratories on the Orsay campus, IJCLab conducts research in nuclear physics, high-energy physics, astroparticles and cosmology, theoretical physics, accelerators, particle detectors, and related technical developments and applications in energy, health, and the environment.
With around 280 engineers and technicians, the lab has strong technical capabilities in key domains such as mechanics, electronics, computing, instrumentation, and accelerator and biological techniques—crucial for developing the experimental devices required for scientific activities. Additionally, approximately 90 administrative and support staff contribute to the laboratory's success.
The PhD project will take place within the LSST team at IJCLab, which has been involved in the construction and calibration of the Rubin Observatory since 2012. Supervision will be provided by Jérémy Neveu, lecturer in cosmology. The candidate will participate in the LSST, StarDICE, and LEMAITRE international collaborations and will present their progress regularly. Funding is available for travel to observatories and for presenting results at conferences.
IJCLab promotes equality, diversity, and inclusion as essential values for professional and institutional development.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

• Affiliation with the PHENIICS Doctoral School (ED 576)
• In-person supervision, with weekly lab meetings
• Additional monitoring via thesis individual follow-up committee (CSI) and laboratory support committee (CAT)
• Travel to Chile and the Observatoire de Haute Provence is expected

Informations complémentaires

Expected Skills:
• Master's degree in fundamental physics or astrophysics
• Proficiency in Python and interest in data analysis
• Strong writing skills and ability to develop and manage a scientific project
• Teamwork ability
• Autonomy, organizational skills, and reporting ability
• Language skills: French and English
Application Requirements:
• Detailed CV
• At least two references (contactable individuals)
• One-page motivation letter
• One-page summary of Master's thesis
• Transcript of grades for Master 1 and/or Master 2 or engineering school
This position is located in a ZRR (Restricted Area Zone); hiring is conditional upon obtaining favorable clearance from the HFSD. The stated start date is therefore indicative and may be postponed.