CosmosClub: Ariel Sánchez (09/07/20)

CosmosClub Ariel Sánchez

Date: July 9th 2020, 10.00 a.m.

Speaker: Ariel Sánchez (MPE Garching/ Max-Planck-Institut für extraterrestrische Physik )

Title:   Let us bury the prehistoric h: arguments against using h1Mpc units in observational cosmology

Room: Zoom Meeting (connection details will be updated soon)


It is common to express cosmological measurements in units of h^-1 Mpc. Here, we review some of the complications that originate from this practice. A crucial problem caused by these units is related to the normalization of the matter power spectrum, which is commonly characterized in terms of the linear-theory rms mass fluctuation in spheres of radius 8h^-1 Mpc, σ8. This parameter does not correctly capture the impact of h on the amplitude of density fluctuations. We show that the use of σ8 has caused critical misconceptions for both the so-called σ8 tension regarding the consistency between low-redshift probes and cosmic microwave background data, and the way in which growth-rate estimates inferred from redshift-space distortions are commonly expressed. We propose to abandon the use of h^1 Mpc units in cosmology and to characterize the amplitude of the matter power spectrum in terms of σ12, defined as the mass fluctuation in spheres of radius 12Mpc, whose value is similar to the standard σ8 for h0.67.


CosmosClub: Erwan Allys (02/07/20)

CosmosClub Erwan Allys

Date: July 2nd 2020, 10.00 a.m.

Speaker: Erwan Allys (ENS Paris / École Normale Supérieure, Laboratoire de Radioastronomie )

Title:   The Wavelet Phase Harmonics, a new interpretable statistical description for analysis and synthesis of the LSS

Room: Zoom Meeting (connection details will be updated soon)


The statistical characterization of non-Gaussian fields is a major problem in current astrophysics, and no method has clearly emerged up to now to do so. In this presentation, I will introduce the Wavelet Phase Harmonics (WPH), a low-dimensional and interpretable set of statistics that efficiently characterizes the couplings between scales in non-linear processes. This description, that has been recently introduced in data science, is inspired from neural networks. Applied to projected matter density field from Quijote N-body Large Scale Structure (LSS) simulations, I will show how the WPH are able to provide better constraints on five cosmological parameters than the joint power spectrum and bispectrum, as well as to produce new realistic statistical syntheses from a maximum-entropy model. These results open the path to the use of a new type of statistical description for non-Gaussian fields in astrophysics.


CosmosClub: Florent Mertens (10/03/20)

Date: March 18th 2020, 10.30am

Speaker: Florent Mertens (LERMA / Kapteyn Astronomical Institute)

Title: The challenges of observing the Epoch of Reionization and Cosmic Dawn

Room: Cassini 


Low-frequency observations of the redshifted 21cm line promise to open a new window onto the first billion years of cosmic history, allowing us to directly study the astrophysical processes occurring during the Epoch of Reionization (EoR) and the Cosmic Dawn (CD). This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under astrophysical foregrounds orders of magnitude brighter and contaminated by numerous instrumental systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are currently underway aiming at statistically detecting the 21-cm brightness temperature fluctuations from the EoR and CD. While no detection is yet in sight, considerable progress has been made recently. In this talk, I will review the many challenges faced by these difficult experiments and I will share the latest development of the LOFAR Epoch of Reionization and NenuFAR Cosmic Dawn key science projects.


CosmosClub: Celine Gouin (20/02/20)

Date: February 20th 2020, 10.00 am

Room: Kepler

Speaker: Céline Gouin (IAS, COSMIX)

Title: Probing the azimuthal environment of galaxies around clusters. From cluster core to cosmic filaments


Galaxy clusters are connected at their peripheries to the large scale structures by cosmic filaments that funnel accreting material.Therefore, the vicinity of galaxy clusters are ideal places to quantify the geometry and topology of the cosmic web.These filamentary structures are studied to investigate both environment-driven galaxy evolution and the growth of massive structures. In this presentation, I probe angular features in the distribution of galaxies around clusters by performing harmonic decompositions in large photometric galaxy catalogues around low-z clusters. In the clusters’ outskirts, filamentary patterns are detected in harmonic space: massive clusters seem to have a larger number of connected filaments than low-mass ones. Our results suggest also a gradient of galaxy activity in filaments around clusters.

CosmosClub: Irène Waldspurger (04/12/19)

Date: December 4rd 2019, 10.30am

Speaker: Irène Waldspurger (CEREMADE,  Université Paris-Dauphine)

Title: Convex and non-convex algorithms for phase retrieval

Room: Cassini 


Phase retrieval problems consist in recovering elements of a complex vector space from the modulus of their scalar product with a fixed family of measurement vectors. Traditional reconstruction algorithms rely on simple local optimization heuristics. Although they can in principle, because of the non-convexity of the problem, get stuck in local optima, they are observed to work well in many situations.

In this talk, we will see which theoretical correctness guarantees one can establish, in a particular setting, for the most well-known such algorithm. We will also present a different family of algorithms, based on so-called convexification techniques, describe its advantages and limitations.



CosmosClub: Miguel Zumalacarregui (06/11/19)

Testing Gravity and Dark Energy with Cosmology and Gravitational Waves

Date: November 06th 2019, 15h30

Speaker: Miguel Zumalacarregui (UC Berkeley & IPhT Saclay)

Title: Testing Gravity and Dark Energy with Cosmology and Gravitational Waves

Room: Cassini


Alternative theories of gravity may provide viable models of cosmic acceleration with the possibility of alleviating shortcomings of the standard paradigm such as discrepant measurement of the Hubble parameter. I will present recent progress in constructing viable, yet extremely predictive theories of gravity and dark energy, extracting their cosmological implications and testing them with data, current and forthcoming. I will also present how most of these theories affect the propagation of gravitational waves. In particular, the speed of gravitational waves provides the most stringent test for a large class of theories, which have been recently ruled out by the GW speed measurement following the neutron star merger GW170817. Other effects on gravitational wave propagation (damping, modified dispersion and oscillations) can be used to test the landscape of gravitational theories.

CosmosClub: Fangchen Feng (10/10/19)

Date: October 10th 2019, 15h00

Speaker: Fangchen Feng (Laboratoire Astroparticule & Cosmologie)

Title: Reconstruction and characterisation of polarisations of a gravitational-wave signal

Room: Kepler


Polarisation properties of gravitational waves carry crucial information about the physics of gravitational sources (binary compact systems of black holes or neutron stars, etc. ) such as precession effects. In practice, the reconstruction of the two polarizations h+(t) and h×(t) is made possible by the use of at least two non-aligned detectors. To this aim, we propose a complete analysis procedure of gravitational-wave signals. Starting from measurements, this procedure estimates the sky position of the source, reconstructs the two components h+(t) and h×(t) and estimates instantaneous Stokes parameters of the wave. This set of non-parametric observables encodes many fine properties of the astrophysical source without close bounds to a specific dynamical model, making them particularly suited to decipher precession effects.

CosmosClub: Catherine Heymans (22/10/19)

Date: October 22nd 2019, 11am

Speaker: Catherine Heymans (ROE, Edinburgh)

Title: low-z-vs-CMB tensions with KiDS and DES and  photo-z calibration

Room: Kepler


Catherine Heymans will give a 15-20min introduction on recent work on low-z-vs-CMB tensions with KiDS and DES ( and some more recent work on photo-z calibration. Then we will have a "bring a plot" session, in which each of us brings the plot we are most excited about and we comment and discuss on it together.

CosmosClub: Sebastian Rojas Gonzalez (12/07/19)

Date: July 12th 2019, 11am

Speaker: Sebastian Rojas Gonzales (KU Leuven)

Title: Gaussian processes for simulation optimization

Room: Kepler


The use of kriging metamodels (also known as gaussian processes) in simulation optimization has become increasingly popular during recent years. The majority of the algorithms so far uses the ordinary (deterministic) kriging approach for constructing the metamodel, assuming that observations have been sampled with infinite precision. This is a major issue when the simulation problem is stochastic: ignoring the noise in the outcomes may lead to inaccurate predictions. In this work, we propose a stochastic multiobjective simulation optimization algorithm that contains two crucial elements: the search phase implements a kriging method that is able to account for the inherent noise in the outputs when constructing the metamodel, and in the identification phase uses a Bayesian multiobjective ranking and selection procedure in view of maximizing the probability of selecting the true non-dominated points by optimally allocating the available computational budget. We evaluate the impact of these elements on the search and identification effectiveness on a set of artificial test problems with varying levels of heteroscedastic noise. Our results show that the characterization of the noise is crucial in improving the prediction efficiency; yet, the allocation procedure appears to lose effectiveness in settings with high noise. This emphasizes the need for further research on multiobjective ranking and selection methods.

CosmosClub: Doogesh Kodi Ramanah (20/06/2019)

Date: June 20th 2019, 11am

Speaker: Doogesh Kodi Ramanah (IAP)

Title: Fast complex dynamics emulators for cosmological inference

Room: Cassini


I will present an overview of our recent work in developing various aspects of Bayesian forward modelling machinery for an optimal exploitation of state-of-the-art galaxy redshift surveys. I will focus on the development of a generative model for mapping dark matter simulations to 3D halo fields using physically motivated neural networks. We employ the Wasserstein distance as a metric to train our halo painting emulator and demonstrate its efficacy in predicting 3D halo distributions using summary statistics such as the power spectrum and bispectrum. I will subsequently briefly review our novel cosmological parameter inference framework that extracts several orders of magnitude more information from the cosmic expansion relative to standard approaches, and a sophisticated likelihood that is robust to unknown foreground contaminations.