Louis Legrand

About me

I am a postdoctoral researcher in cosmology based in Cambridge, working at the Department of Applied Mathematics and Theoretical Physics (DAMTP) and the Kavli Institute for Cosmology, supported by a Swiss Postdoc Mobility fellowship. Prior to this, I was at the ICTP South American Institute for Fundamental Research in São Paulo, and at the University of Geneva in the cosmology group.

My research is driven by the quest to understand the content and evolution of the Universe, the value of the mass of the neutrinos and the nature of the accelerated expansion of the Universe. To do this, I study the large-scale distribution of matter using a variety of cosmological probes.

I currently focus on the gravitational lensing of the Cosmic Microwave Background (CMB), which encodes an integrated map of the matter distribution from us to the last scattering surface. I develop optimal estimators and analysis tools for reconstructing this lensing signal in upcoming CMB experiments, with the goal of extracting precise cosmological information.

I am also actively involved in combining CMB lensing with other tracers of large-scale structure, such as galaxy clustering and weak lensing from the upcoming Euclid mission.

Main Publications

You can find a complete list of my publications here. I highlighted some of them below.

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