LSST Group – IJCLab / IN2P3
The LSST group at IJCLab (Laboratory of the Physics of the Two Infinities, IN2P3/CNRS, Université Paris-Saclay) currently consists of seven members: four researchers, one engineer, one postdoctoral fellow, and one PhD student.
We are actively involved in the scientific program of the Rubin Observatory LSST (Legacy Survey of Space and Time), one of the major projects in observational cosmology for the coming decade.
Understanding Dark Energy
Our main goal is to understand the nature of the dominant component of the universe responsible for the accelerated expansion of cosmic space.
This acceleration, discovered in the late 1990s through the study of Type Ia supernova flux as a function of their redshift, profoundly changed modern cosmology.
The cause of this accelerated expansion is attributed to a mysterious component known as dark energy, not to be confused with dark matter, which mainly governs the formation and dynamics of cosmic structures (galaxies, clusters, filaments).
Dark energy is described by its equation of state relating its pressure P to its energy density ρ:
w = P / ρ
In the reference cosmological model, w = -1 corresponds to a cosmological constant.
More general models allow for a time-evolving parameterization of w such as:
w(a) = w₀ + wₐ(1 – a)
where a is the cosmological scale factor.
Measuring w₀ and wₐ with high precision remains one of the central challenges of modern cosmology.
The Rubin–LSST Survey
To address these questions, we work within the Rubin Observatory Legacy Survey of Space and Time (LSST).
This ten-year survey will map the entire southern sky with an 8.4-meter telescope and a 3.2-gigapixel camera — the largest ever built.
Rubin-LSST will measure the positions, shapes, and fluxes of tens of billions of galaxies up to a redshift of z ~ 3, across six photometric bands (u, g, r, i, z, y) spanning the ultraviolet to the near-infrared.
This unprecedented dataset will constrain cosmological parameters with unmatched precision, improving by roughly an order of magnitude the measurements of (w₀, wₐ) compared to previous surveys (SDSS, DES, HSC, etc.).
Research Activities at IJCLab
The research conducted by the LSST group at IJCLab is organized around three main complementary areas:
instrumental techniques, computational developments, and cosmological analyses.
1. Instrumental and Technical Activities
The scientific exploitation of the Rubin-LSST survey requires extremely accurate photometric calibration and rigorous control of all factors affecting measured fluxes.
Our group contributes to this effort through:
- Real-time monitoring of atmospheric transmission over the entire wavelength range covered by LSST, to correct for color-dependent variations introduced by the Earth’s atmosphere;
- Calibration of the telescope’s transmission, including the characterization of optical filters and the quantum efficiency of CCD detectors, which are essential to ensure consistent photometric measurements across all bands.
2. Computational Activities
The huge data volume produced by Rubin-LSST (several tens of terabytes per night) requires efficient and automated analysis tools.
Our work includes:
- Automatic identification and classification of transient alerts, in particular supernovae, via the FINK alert broker developed by the LSST-France community;
- Development and application of Artificial Intelligence (AI) methods, which are now essential at every stage of data analysis: source detection, classification, redshift estimation, and candidate selection.
3. Cosmological Analyses
The core scientific mission of our group lies in the cosmological interpretation of LSST observations.
We investigate several complementary probes to constrain the nature of dark energy:
- Photometric redshift estimation of galaxies, crucial to reconstruct the three-dimensional structure of the universe;
- Analysis of the flux–distance relation of Type Ia supernovae, a direct tracer of the expansion history of the universe;
- Other cosmological probes such as weak and strong gravitational lensing, and the spatial distribution of galaxies, which together allow us to map dark matter and the large-scale geometry of the cosmos.