Séminaire

Nonlinear Quasi-Normal Modes: Uniform Approximation

Nonlinear effects in black hole perturbation theory may be important for describing a black hole ringdown, as suggested by recent works. The influence of these nonlinear effects on quasi-normal modes of black holes is still poorly understood. I will show that a generalization of WKB method, the so-called technique of uniform approximations, can be used to accurately compute 1) nonlinear amplitudes at large distances in terms of the linear ones, 2) linear (and nonlinear) quasi-normal mode frequencies, 3) the wavefunction for both linear and nonlinear modes.

The formation and evolution of dark matter microhalos

Dark matter in the Universe can be considered as a collisionless self-gravitating fluid obeying the Vlasov-Poisson equations. In the standard picture of cosmic structure formation, the first dark matter objects to form are expected to be microhalos of roughly Earth mass and solar system size. These halos can subsequently merge to form larger dark matter halos such as that of our Galaxy. In practice, resolving dark matter dynamics relies on a N-body approach, but with the advent of exaflopic computers it now becomes possible to solve directly Vlasov dynamics in six-dimensional phase-space.

Multifield inflation (with or) without models

Since the discovery of the Higgs boson 12 years ago, terrestrial particle collider experiments have encountered a slowdown in the quest for physics beyond the Standard Model. Arguably, the upcoming decade promises a paradigm shift towards exploring high-energy physics through cosmological observations. With an abundance of data, independent datasets, and improved systematics, a new era of exploration unfolds. Theorists now strive to attain the precision level demanded by upcoming experiments.

Inflationary and Post-Inflationary Scalar Dark Matter Production

Abstract: Dark matter is one of the great mysteries of modern physics. In addition to its precise nature, its production mechanism remains unknown. In this talk I will discuss the possibility of producing scalar dark matter candidates during and after cosmic inflation. By describing the transition from an inflationary epoch to a late-time cosmology, I will describe how the dynamics of the universe can affect the production of dark matter and leave an imprint on cosmology. I will discuss the associated constraints, phenomenological consequences, and possible further developments.

Black Holes in Lorentz-Violating Gravity

I will discuss black holes in the context of Einstein–aether and khronometric gravity — two closely related alternative theories of gravity that allow violations of local Lorentz invariance. Since these theories admit faster-than-light propagation, metric horizons are generically permeable and it is not clear whether proper black holes can exist; surprisingly, in some cases they do, thanks to the appearance of a new kind of “universal” horizon. I will review past and recent results on the topic, with a particular emphasis on the difficulty of finding rotating solutions.

An eikonal approach to gravitational scattering and waveforms

The classical limit of scattering amplitudes offers a convenient strategy to calculate gravitational-wave observables for binary processes in the post-Minkowskian (PM) regime, in which the two objects are far apart and interact weakly. In this talk I will discuss how the eikonal exponentiation offers a simple and conceptually transparent framework to exploit this connection and calculate key gravitational observables from amplitudes: the deflection angle for two-body encounters, energy and angular momentum losses, as well as the emitted gravitational waveform itself.

Probing the deep string spectrum

As is well known, the string spectrum comprises infinitely many states that can collectively be visualized along Regge trajectories of increasing mass and spin. Its massless and lightest levels, as well as certain higher spins including the leading Regge trajectory, have been the focus of past studies. In principle, access to any state is possible, but the traditional methodology is non-covariant and does not immediately lead to irreducible representations of the Wigner little group. In this talk, we will discuss a new and covariant technology of constructing the string spectrum.

Lattice simulations of inflation

Developing non-perturbative methods might be crucial for understanding inflation and its predictions. In this talk, I will present a nonlinear study of the inflationary epoch of the Universe based on numerical lattice simulations. I will first focus on a model known as axion inflation, where the inflaton is coupled to a gauge field via Chern-Simons interaction. As a second example, I will consider a single-field model with a resonant feature in the potential. I will show that, in both cases, nonlinear effects have important consequences for the inflationary dynamics.
Séminaire