Doctoral student in cosmology (M/F)

Description of the thesis subject

From slitless spectrophotometry to the equation of state of dark energy with the StarDICE and LSST experiments

The objective is the development of slitless spectrophotometry as an asset for the measurement of the equation of state parameter of dark energy. The nature of dark energy is one of the great open questions in cosmology. The current decade will bring measurements of unparalleled precision on 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 cosmological surveys capable of scanning the entire sky every few days in order to collect a maximum of transient astronomical events, such as the explosion of type Ia supernovae. The latter are very high luminosity stellar explosions serving as standard candles for the measurement of distances in the Universe and therefore allow the measurement of the properties of dark energy.

By 2030, the Hubble diagram (distance versus redshift) of type Ia supernovae will be populated by tens of thousands of measurements at high and low redshift, thanks in particular to the LSST and ZTF surveys. However, the precision of measuring the properties of dark energy with these data will be limited by systematic uncertainties in the photometric calibration of telescopes. To counter this limitation, the Parisian laboratories IJClab and LPNHE have joined forces to 1) remeasure flux standard stars with the StarDICE project 2) develop slitless spectrophotometry to measure the atmospheric transmission of observatory sites, in particular with the LSST Auxiliary Telescope (AuxTel).

The subject of the proposed thesis is part of the LEMAITRE project (Latest Extended Mapping of Acceleration with an Independent Trove of Redshifted Explosions) whose goal is to construct a Hubble diagram of supernovae composed of thousands of events that have never been published, and thus to propose a new measurement of the properties of dark energy, independent of previous surveys. It will be composed of supernovae from the ZTF, SNLS and Subaru surveys. At the end of the thesis, it can be completed by the first sample of supernovae from the LSST survey. The thesis will have two parts. The first part will be the demonstration of the capacity of slitless spectrophotometry to measure the transmission of the atmosphere in Chile and at the Haute Provence Observatory. This technical work, which will immerse the student in the heart of a cosmological survey, will lead him or her to go to the observation sites to carry out these calibrations. A major expected result will be the publication of a brand new network of standard stars by StarDICE. The second part will consist of propagating these calibrations to the fluxes of the light curves of the ZTF and LSST supernovae. The Hubble diagram published at the end of the thesis will be the first diagram of cosmological distances calibrated in flux thanks to StarDICE, shedding new light on the expansion of the Universe and the nature of dark energy.

Indicative bibliography:

S. Perlmutter, G. Aldering, G. Goldhaber, RA Knop, et al., Measurements of Ω and Λ from 42 High-Redshift Supernovae, The Astrophysical Journal 517, 565 (1999), doi:10.1086/307221, arXiv:9812133
Neveu et al., Slitless spectrophotometry with forward modeling: principles and application to atmospheric transmission measurement, arXiv:2307.04898, A&A, 684, A21 (2024)
Neveu et al., On the importance of Earth’s atmosphere for SNIa precision cosmology, arXiv:2407.01058

Work context

The Irène Joliot-Curie Physics of the Two Infinities Laboratory is a physics of the two infinities laboratory under the supervision of the CNRS, the University of Paris-Saclay and the University of Paris, born in 2020 from the merger of the five UMR located on the Orsay university campus: the Center for Nuclear and Material Sciences (CSNSM), the Laboratory of Imaging and Modeling in Neurobiology and Cancerology (IMNC), the Institute of Nuclear Physics of Orsay (IPNO), the Linear Accelerator Laboratory (LAL) and the Theoretical Physics Laboratory (LPT).

The laboratory’s research themes are nuclear physics, high energy physics, astroparticles and cosmology, theoretical physics, accelerators and particle detectors as well as technical research and development and associated applications for energy, health and the environment.

The structure has very significant technical capabilities (around 280 engineers and technicians) in all the major fields required to design, develop and implement the experimental devices necessary for its scientific activity: mechanics, electronics, computer science, instrumentation, acceleration techniques and biology techniques. These technical strengths represent a major asset for the design, development and use of the necessary instruments (accelerators and detectors). The presence of research infrastructures and technological platforms gathered on the laboratory site is also a major asset. Finally, around 90 ITAs from the administrative and support services work alongside scientists and engineers.
The thesis will take place in the LSST team of the IJCLab, involved in the construction and calibration of the Rubin observatory since 2012, under the supervision of Jérémy Neveu, lecturer in cosmology. The doctoral student will be involved in the international collaborations LSST, StarDICE and LEMAITRE, in which he/she will be able to regularly present the progress of his/her work. Financial resources are provided for travel to the various observatories and presentations at conferences.

IJCLAB bases its recruitment policy on the promotion of equality, diversity, and inclusion. These core values ​​enable the professional development of staff, who are key contributors to collective success, as well as the development of the laboratory itself.

The position is located in a sector covered by the protection of scientific and technical potential (PPST), and therefore, in accordance with regulations, requires that your arrival be authorized by the competent MESR authority.

Constraints and risks

The thesis will be attached to the PHENIICS doctoral school (ED 576).
Face-to-face supervision, weekly monitoring in the laboratory
. Monitoring by the individual thesis monitoring committee (CSI) for annual registration. Monitoring by the laboratory’s thesis support committee (CAT).
Travel to Chile and the Haute Provence Observatory will be required.