My research field is cosmology, in particular weak gravitational lensing. I work on observational data from large galaxy surveys, to constrain cosmological models and to infer information about dark matter and dark energy. To learn more about this topic, have a look at my (technical) review here, or check the video abstract on the companion web page.
I have been using cosmic shear, the distortion of galaxy images by the large-scale structure in the Universe, to measure cosmological parameters. Within the CFHTLenS collaboration (2008 - 2014), I have contributed to constraining dark matter, dark energy and modified gravity. Since then, I have been involved in the preparation of the ESA space mission Euclid, which is expected to improve those constraints by orders of magnitude.
To efficiently explore the high-dimensional parameter space for cosmological inference, I have been working on sampling methods, such as Population Monte Carlo (PMC), and Approximate Bayesian Computation (ABC).
Weak cosmological lensing
The main observable for cosmological weak lensing is cosmic shear, the distortion of galaxies by the large-scale structure in the Universe. For results from CFHTLenS see the published papers. I have been working on second- and higher-order statistics in real space, and shear tomography. For projects at CosmoStat within Euclid see this page.
One particularly interesting weak-lensing observable are peak counts. This higher-order statistic is sensitive to the non-Gaussian aspects of the large-scale structure. Together with Chieh-An Lin, I have developed a new model prediction approach for peak counts. We have compared this model to N-body simulations, explored its stochastic nature for strategies to constrain cosmological parameters, and looked at different filtering techniques.
Dark matter, galaxies, and galaxy clusters
We still have to learn a lot about the the relation between galaxies and dark matter, and how galaxies form and evolve in dark-matter structures. This relation can be studied by combining weak gravitational lensing and galaxy clustering statistics. This can help us to understand when and in which galaxies residing in dark-matter halos stars are formed efficiently.
Sampling the likelihood
I have been developing and implementing the sampling method Population MonteCarlo (PMC) which is an efficient and massively parallelizable method to sample from an arbitrary posterior distribution. PMC readily provides an estimate of the Bayesian evidence. Recently, I have looked into the likelihood-free technique Approximate Bayesian Computation (ABC) and other likelihood estimation methods, together Chieh-An Lin, and, more recently with Emille Ishida and Jessi Cisewski.
Various master student internship offers under my and colleague's supervision are offered for 2019: Topics include optimal transport and deep learning for weak-lensing PSF modelling, weak-lensing shape measurement, statistical redshift estimation for gravitational wave events. The latter topic (cosmology with gravitational waves) can potentially be continued as PhD thesis.
Please check the CosmoStat job page for offers.
Collaborations and projects
Euclid is a large upcoming ESA space mission with the goal to map galaxies and dark matter in the Universe, and to constrain the nature of dark energy. I am deputy lead of the weak-lensing science working group (WLSWG), and further work in shear and science-level (Level 3) data processing and validation.
The Canada-France Imaging Survey, a 5,000 deg2 survey in the Northern hemisphere. The proposal was accepted in 2016, observations have started in early 2017. Lensing-related science with CFIS will be galaxy-galaxy lensing to study properties of dark matter halos, filaments, satellite galaxy stripping, and tests of General Relativity.
The Dark-Energy Spectroscopic Instrument is a next-generation galaxy clustering (BAO, RSD) observatory, with the aim to measure dark energy properties. DESI will be ideal for cross-correlation studies with lensing background galaxies from Euclid, LSST, and CFIS.
XXL (The ultimate XMM extragalactic survey) is an extragalactic survey of 2 x 25 square degrees, with very deep X-ray coverage from XXM. Deep multi-wavelength data is available over the XXL fields (e.g. CFHTLS, BCS), and follow-up observations are underway and planned.
CFHTLS (Canada-France Hawaii Telescope Legacy Survey) is a deep and wide imaging survey of 170 square degrees in five optical bands, observed between 2003 and 2009 with the wide-field camera MegaCam on the 3.6m telescope CFHT.
CFHTLenS (Canada-France Hawaii Telescope Lensing Survey) is the weak-lensing collaboration using data from the CFHT legacy survey.
- Camelus, a model for weak-lensing peak counts. The codes comes with statistical analysis tools for cosmological parameter constraints. In particular, it exploits the stochastical nature of the model predictions using Approximate Bayesian Computation (ABC). [Main author is Chieh-An Lin.]
- CosmoPMC, Cosmology sampling with Population Monte Carlo, an adaptive importance-sampling method to explore high-dimensional posteriors to estimate parameter constraints and the Bayesian evidence. CosmoPMC comes with various cosmology modules such as weak lensing, galaxy clustering, SNIa, BAO distance priors and CMB (via CAMB/WMAP software). The cosmology module is based on nicaea.
The most recent version is 1.2 (December 2012). The user's guide is on the archive (v3).
- athena, a tree code to calculate second-order correlation functions, including shear-shear ("cosmic shear"), shear-position ("galaxy-galaxy lensing") and position-position (spatial angular correlation).
The most recent version is 1.7 (March 2014).
- nicaea: NumerIcal Cosmology And lEnsing cAlculations. It involves basic cosmology functions (distances, growth factor), dark-matter power spectrum and lensing observables (power spectrum, second-order shear statistics). The non-linear power spectrum are the Peacock&Dodds (1996) and Smith, Peacock et al (2003) fitting formulae. The code is easy to use and fast enough to be used for Monte-Carlo sampling.
The most recent version is 2.5 (October 2014).
- C-program to calculate the filter function and E-/B-mode shear function described in arXiv:0907.0795.
2015. Review on cosmological constraints from cosmic shear. In French: Irfu.
2015. First published papers from the XXL survey: ESA.
2012. Astronomers reach new frontiers in dark matter. ROE.
2010. Independent evidence of the accelerated expansion of the Universe, with weak lensing in COSMOS: In French: CNRS.
2008. The largest structures ever measured thanks to weak lensing in CFHTLS: CFHT. In French: IAP.
Video abstract of my cosmic shear review, see this page.
Lecture Notes and Talks
- Weak-lensing lectures at École Polytechnique, from 13/11/2018, and 14/11/2017.
- Weak-lensing lecture notes from the DarkMod workshop, at IPhT Saclay/IAS Orsay, 09/2017.
- Weak-lensing lecture notes from the most recent Euclid summer school in cosmology, 2018 in Roscoff. Parts I, II, and exercise sets can be downloaded from this page. Previous versions are from 2017 (Fréjus), and 2016 (Narbonne; only part I).
- Review talk on cosmic shear, from 2010: From ellipticities to cosmological parameters from the DUEL meeting at Ringberg.
- Lecture notes "Bayesian statistics and model selection" from a lecture I held at Shanghai Normal University in August 2009.
- Download the weak lensing lecture I've given at the IPM Cosmology School 2007 in Tehran.
[ Lecture (15MB) | Part 1 (750kB) (WL by the LSS) | Part 2 (6.7MB) (WL & cosmology) | Part3 (8.6MB) (Observing WL)]
Class, the Cosmic Linear Anisotropy Solving System, by Julien Lesgourgues et al.
What might happen at The end of the Universe?
The 2004 Saas Fee lensing lecture notes, that were published in the book:Kochanek, C.S., Schneider, P., Wambsganss, J.: Gravitational Lensing: Strong, Weak & Micro. G. Mey- lan, P. Jetzer & P. North (eds.), Springer-Verlag: Berlin, p.273
Download Part I (Introduction) and Part III (Weak gravitational lensing), by Peter Schneider.
Check out my piano music: videos and music scores available