Séminaire

More fields are different: Stochastic view of multi-field inflationary scenario

High-energy physics often motivates multi-field inflationary scenarios where stochastic effects play a crucial role. Peculiar to multi-field models, the noise-induced centrifugal force results in a longer duration of inflation depending on the number of fields, even when the stochastic noises themselves are small. We show that, in such small-noise regimes, the number of fields generically discriminates whether inflation successfully terminates or lasts forever. Our results indicate that inflation with an extremely large number of fields may fail to realise our observable Universe.

General relativistic magnetohydrodynamics simulations of accretion disks: from simple radiative cooling to a Boltzmann treatment of radiation beyond the moment formalism

I will present cuHARM, a general relativistic radiation magnetohydrodynamic solver optimized to exploit exascale computing facilities. After describing the core numerical strategies enabling efficient calculation for multi-node and multi-GPU architectures, I will discuss how radiative cooling changes the dynamics of magnetically arrested accretion disks. In the second part of the talk, I will detail how radiation and its feedback on the dynamics are modeled. In cuHARM, the specific intensity is discretized in space and momentum, and is evolved through the solution of the radiative transfer equation via the discrete ordinate method. This approach eliminates the need for a closure relation and enables to resolve the anisotropy of the specific intensity. Finally, I will present the first results obtained for a radiative accretion disk operating at 0.1 times the Eddington luminosity.

Can Kerr-NUT-(A)dS be charged in higher dimensions?

Almost four decades have passed since the generalization of vacuum Kerr solutions to higher dimensions in the form of Myers-Perry black holes, yet an exact solution generalizing their charged extension (Kerr-Newman) to higher dimensions remains unknown in Einstein-Maxwell theory. Likewise, an exact solution for charged multi-NUT spacetimes in higher dimensions is still missing. In the first part of my talk, considering the Kerr-Schild class in Einstein-Maxwell theory, I will discuss a "No-Go" result related to the charging of Myers-Perry black holes that we obtained in arXiv:2309.02900. In the second part, I will present a slightly stronger generalization of this "No-Go" result in the context of generalized Kerr-Schild spacetimes, which also addresses the charging of multi-NUT solutions. Time permitting, I will further discuss some of the different classes of solutions identified in our analyses.

The Present and Future of CMB Polarization Observations

One of the big goals of today’s CMB experiments is to look for the faint signature of primordial gravitational waves in the B-mode polarization. Finding it would give us a unique window into the physics of the very early Universe and the Inflationary epoch.

Probing ancient cosmic messengers with paleodetectors

Natural minerals, through their ability to record particle interactions over geological timescales, are emerging as a novel tool for astroparticle physics. The paleo-detector concept leverages the nanometer to micrometer-scale damage tracks left by nuclear recoils to search for rare events like dark matter and neutrinos. Our work takes a unique approach by treating the tracks from cosmic ray muons, typically a background for other paleo-detector studies, as the primary signal for probing the high-energy astrophysical history of our galaxy.

Higgs inflation and the Palatini formulation of General Relativity

The Standard Model Higgs can be the inflaton, providing an attractively economical scenario. The core idea is simple, but there are complications both on the side of quantum corrections and gravity. I will discuss possibilities and problems due to such complications. In particular I will cover how the predictions of Higgs inflation depend on what is the correct formulation of general relativity: metric, Palatini or something else.

Searches for High-Energy Neutrinos from the Cygnus Bubble, Galactic Plane, and AGNs with Multi-Messenger Observations

High-energy neutrino and γ-ray emission are expected from the Galactic plane, produced by interactions between cosmic rays and the interstellar medium or radiation fields. Recent LHAASO observations have detected diffuse γ-rays from the Galactic plane and an ultrahigh-energy γ-ray bubble (Cygnus Bubble) up to PeV energies, suggesting the possibility of neutrino emission. Using 7 years of publicly available IceCube track data with the full detector, we conducted searches for neutrino signals correlated with LHAASO diffuse Galactic γ-ray emission and Cygnus Bubble using a template method.

Modeling particle transport and acceleration with stochastic differential equations

Understanding the complex transport of particles in turbulent plasmas is of great relevance in various fields. In astrophysics, the diffusive transport of high-energy particles is often described in an ensemble-averaged way, employing a transport equation that describes the time evolution of the particles distribution function in space and momentum. The standard transport equation can also be re-written into a set of stochastic differential equation.

Beyond the Limits of Quintessence

Current cosmological data seem to point beyond the limits of quintessence for the behavior of dark energy accelerating the universe. This requires challenging physics such as modified gravity, interactions, or altered vacuum, in analogy with how non-Gaussianity in inflation requires physics beyond standard dynamics. We explore some possibilities.

Séminaire