Théorie

QFT in nearly dS space-times and, more generally, in FRW backgrounds allows us to describe correlations at the end of inflation. However, how to extract fundamental physics out of them is still a challange: we do not even know how fundamental pillars such as causality and unitarity of time evolution constrain them. In this talk I will report on a recent program that aims to construct quantum mechanical observables in cosmology directly from first principles without making any reference to time evolution.

The open question of whether a black hole can become tidally deformed by an external gravitational field has profound implications for fundamental physics, astrophysics and gravitational-wave astronomy. Love tensors characterize the tidal deformability of compact objects such as astrophysical (Kerr) black holes under an external static tidal field. We prove that all Love tensors vanish identically for a Kerr black hole in the nonspinning limit or for an axisymmetric tidal perturbation. In contrast to this result, we show that Love tensors are generically nonzero for a spinning black hole.

At first glance our understanding of the universe seems to be solely anchored in classical gravity. Indeed, General Relativity (GR) is a powerful tool that provides a successful geometric description of the cosmos. However, if one scratches beneath the surface, the universe becomes a fascinating playground for thermal and quantum phenomena.

Antideuteron and antihelium nuclei have been proposed as promising detection channels for dark matter because of the low astrophysical backgrounds expected. After a brief review of the current experimental situation, I discuss some of the various flavors of the coalescence model used to describe the formation of light (anti-) nuclei.

I will discuss the scattering of two compact objects interacting via gravity, using the so-called world-line Effective Field Theory approach in the post-Minkowskian expansion (i.e. expanding in the Newton's constant G but not in the velocities). In particular, I will focus on the computation of classical observables such as the total emitted momentum. This is obtained by phase-space integration of the graviton momentum weighted by the modulo squared of the radiation amplitude.

The possibility of observing a stochastic gravitational wave background originating from a cosmological first-order phase transition elicits interest in studying the transitions. Currently, a limiting factor in accurately determining the gravitational wave spectrum from an underlying microphysical model is the calculation of the nucleation rate. I will discuss recent work in which we have proposed a new effective field theory (EFT) framework for determining the thermal nucleation rate in high-temperature QFTs.

A first-order phase transition in the early universe would have given rise to a stochastic gravitational wave background which may be observable today. In this talk, I will focus on the crucial problem of making reliable predictions of the thermodynamics of such phase transitions in the face of infrared Bose enhancements at high temperature. Such enhancements lead to large theoretical uncertainties in perturbation theory at low orders. I will unravel the structure of the perturbative expansion in this context, and of the misalignment between loop and coupling expansions.

The existence of magnetic fields is ubiquitous on astrophysical (e.g., planets and stars) as well as on cosmological scales (galaxies and galaxy clusters). Low-frequency radio observations are revealing an increasing number of diffuse radio sources in galaxy clusters visible through their synchrotron emission.

A space-based laser interferometer, pioneered by NASA's LISA concept and now a ESA cornerstone mission, will enable direct detection of gravitational waves at lower frequencies than LIGO, without being limited by seismic noise. Perhaps the most intriguing source for LISA is the stochastic gravitational wave background produced by turbulent plasma motions in an early-universe, particularly at the electroweak energy scale.

In the first half of this talk, I will discuss how binary systems can be used as dynamical detectors of gravitational waves (GWs). Since the passage of GWs through a binary perturbs the trajectories of the two bodies, we can infer the presence of a GW signal by searching for changes in the binary's orbital parameters. In the presence of a stochastic GW background (SGWB) these changes accumulate over time, causing the binary orbit to execute a random walk through parameter space.