Category: Events

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

Abstract

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.

Date: October 10th 2019, 15h00

Speaker: Fangchen Feng (Laboratoire Astroparticule & Cosmologie)

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

Room: Kepler

Abstract

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.

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

Abstract

Catherine Heymans will give a 15-20min introduction on recent work on low-z-vs-CMB tensions with KiDS and DES (https://arxiv.org/abs/1906.09262) 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.

Date: July 12th 2019, 11am

Speaker: Sebastian Rojas Gonzales (KU Leuven)

Title: Gaussian processes for simulation optimization

Room: Kepler

Abstract

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.

Date: June 20th 2019, 11am

Speaker: Doogesh Kodi Ramanah (IAP)

Title: Fast complex dynamics emulators for cosmological inference

Room: Cassini

Abstract

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.

Date: June 13th 2019, 10:30am (/!\ unusual time)

Speaker: Isabella Carucci (CosmoStat)

Title: 21cm intensity mapping: using cosmic neutral hydrogen as tracer of large scale structure

Room: Kepler

Abstract

This is an opportunity to introduce my past work to the group. I’ll start by briefly illustrating what we mean by intensity mapping (IM) and why to use the 21cm radiation coming from neutral hydrogen. I’ll then move to what its observational status is and how we can model this signal. In particular, I’ll show how IM will be a remarkable test both for dark energy and dark matter models, presenting forecasts for the bounds that the SKA telescope will be able to uniquely set. On the other hand, this signal is literally buried under foregrounds (about 4 orders of magnitude more intense). In this spirit, I’ll conclude by sketching what our plans are to use and optimise Cosmostat-developed tools for tackling the foreground and instrumental systematics problems of IM.