Louis Legrand

About me

I am a Postdoc in cosmology working in Cambridge, at the DAMTP and Kavli institute, under a Swiss Postdoc Mobility fellowship. Previously I was at the ICTP South American Institute of Theoretical Physics, in the State University of Saõ Paulo, and at the University of Geneva in the cosmology group.

My interest is in understanding the invisible mass called dark matter and the unknown fluid called dark energy. For this I use several probes to measure the distribution of matter in the Universe. I currently focus on the weak gravitational lensing of the Cosmic Microwave Background (CMB), which gives the integral of the mass distribution up to the last scattering surface. My work is to develop new tools to reconstruct this lensing field for the next generation of CMB surveys, and get optimal constraints on cosmological parameters. I am also working in the combination of the CMB lensing field with other cosmological probes, in particular the clustering and weak lensing of galaxies of the upcoming Euclid survey.

You can find a complete list of my works here. I highlight some of them below.

Main Publications

Robust and efficient CMB lensing power spectrum from polarization surveys
L. Legrand and J. Carron
In this article we show that the optimal lensing power spectrum that we developed is not biased towards sources of anisotropies, such as the mask or the noise. This goes a step further towards the use of this optimal estimator for future CMB surveys.

Lensing power spectrum of the Cosmic Microwave Background with deep polarization experiments
L. Legrand and J. Carron
We develop an efficient and unbiased likelihood pipeline to estimate CMB lensing power spectrum using an optimal lensing estimator. We show this optimal lensing spectrum increases the signal to noise ratio by 80% compared to the traditional quadratic estimator.

Euclid preparation: XV. Forecasting cosmological constraints for the Euclid and CMB joint analysis
Euclid collaboration: S. Ilic et al., including L. Legrand
We forecast the constraints gained by combining CMB probes (primordial CMB and CMB lensing) to the main Euclid probes. We show that combining them allows to break systematic uncertainties and is especially useful to constrain extensions of the LCDM model.

High resolution tomography for galaxy spectroscopic surveys with angular redshift fluctuations
L. Legrand, Carlos Hernández-Monteagudo, Marian Douspis, Nabila Aghanim, Raúl E. Angulo
We show how a new cosmological statistics can allow us to use the information about the peculiar velovity of galaxies in a standard analysis. This sensitivity to peculiar velocity helps in breaking degeneracies between cosmological parameters and puts tigther contraints on the dark sector.

The COSMOS-UltraVISTA stellar-to-halo mass relationship: new insights on galaxy formation efficiency out to z∼5
L. Legrand, H.J. McCracken, I. Davidzon, O. Ilbert, J. Coupon, N. Aghanim, M. Douspis, P. L. Capak, O. Le Fèvre, B. Milvang-Jensen
We trace the evolution of the stellar mass to halo mass ratio of galaxies up to z=5 thanks to exquisite mass and photometric redshift in the COSMOS-UltraVISTA field. We found that the peak of this ratio increases with redshift, pointing to a scenario where cold gas inflows become progressively more important in driving star-formation at very high redshifts.

Tutorials

Jupyter notebook tutorial

here the slides

Curriculum vitae

here