Fabien Lacasa

Researcher in Cosmology


Current position
I am chercheur associé at the Institut d'Astrophysique Spatiale, in Université Paris-Saclay.

Past light cone
At high redshifts, I learned sciences in Lycée le Corbusier (Poissy, France), then Mathematics and Physics at classes préparatoires du Lycée Hoche (Versailles).
At the dawn of the current millenium, I became an élève de l'École Normale Supérieure (Paris) and studied Mathematics (Master 1, agreg) and Physics (Master 2).
At the start of this decade, I did a PhD in Cosmology at Institut d'Astrophysique Spatiale in Orsay, under the direction of Nabila Aghanim. I was involved in the Planck mission.
I then worked as a postdoc in Cosmology at ICTP-SAIFR in São Paulo Brazil, involved in Dark Energy Survey.
More recently I worked as a postdoc in Cosmology at the University of Geneva, involved in Euclid.
Most recently, I worked as a CNES postdoc in Cosmology at Institut d'Astrophysique Spatiale, involved in the Euclid mission.



My research focusses on the Large Scale Structure of the Universe : the distribution of matter, galaxies and galaxy clusters.
I want to understand how these structures form and are distributed in space, and what they can tell us about the content of the Universe, its history and the laws of Gravity.

Observations and Experimental Collaborations

I study observations both in the microwave : Cosmic Microwave Background (CMB), and in the visible : galaxy surveys.
I first worked with CMB observations from the Planck mission, being a member of the core-team of the High Frequency Instrument. I then worked on galaxy data from the Dark Energy Survey, being a member of the working groups on the Large Scale Structure and Theory and Combined Probes.
I now participate in preparing the Euclid mission, being a member of the Euclid Consortium and involved in several working groups for the study of galaxy clustering, weak lensing, clusters, internal probe combination and combination with the CMB.

In more details

In the CMB field, I have worked on constraining models of the early Universe with primordial non-Gaussianity, star formation history with the Cosmic Infrared Background, and cosmological parameters and gas physics with the thermal Sunyaev-Zel'dovich effect.
In the field of galaxy surveys, I work on constraining our cosmological model with Clusters, Weak Lensing and Galaxy Clustering, defining the methodology to combine these probes.

A line of research that I particularly develop is on the impact of the non-linearity of the Large Scale Structure. It indeed has three important consequences :
  • On small scales, new contributions appear to our standard cosmological observable : the 2-point correlation function / power spectrum.
    I have shown how this contribution has a different sensitivity to cosmology and astrophysics, so it can break parameter degeneracies and improve scientific constraints.
  • Information leaks away from the power spectrum into higher order statistics.
    I worked on the measurement and scientific analysis of a 3-point statistic (bispectrum), both for primordial NG, the CIB and the tSZ effect.
  • Intrinsic variability (cosmic variance) of our measurements is increased, both for single probes and in probe combination.
    I work on the modeling of this extra variance, study its impact and how to mitigate it. I develop an exhaustive modeling of these terms with the halo model, with some works concentrating on the case of super-sample covariance.
In the past, these effects could be more or less ignored because surveys had a large noise level and concentrated on large scales. Now however, dealing with non-linearity is a necessity to push current surveys to their limits, and will be critical for future surveys (Euclid, LSST, CMB-S4, SKA etc).


You can see my publication list on inSPIRE or ADS
All the articles are freely available as preprint on arXiv
My PhD thesis is also available on arXiv here

Computing material

Public codes

I am starting to share some of the codes I write on my github account.
  • IDL-fablib : a collection of personal routines for the IDL programming language.
    The library contains routines for data analysis in spherical harmonics, first written for CMB analysis, but also fast numerical integration routines and some miscellany.
  • PySSC : A Python implementation of the fast Super-Sample Covariance from Lacasa & Grain 2018 arXiv:1809.05437

Data and results

With my articles, I share on my github account the data and Python notebooks that allow to reproduce all the plots and results.

Linux documentation

Here is a collection of raw text files describing various admin procedures on Linux. The procedures were tested on Ubuntu-based distributions. They are presented without warranty, but should prove useful if you try to accomplish the same task. At least they will be useful to me as future reference.


Institut d'Astrophysique Spatiale (IAS)
Bâtiment 121, Centre Universitaire d'Orsay
91405 Orsay