We determined the fate of the geometrical destabilization of inflation, reaching the conclusion that this phenomenon does not end inflation, but that the latter continues in an unusual manner. We studied this so-called sidetracked phase of inflation in detail, including the properties of the primordial density fluctuations it generates. From this, new types of inflationary attractors with strongly non-geodesic motion in field space have been identified: they allow to inflate on steep potentials, they offer interesting prospects for embedding inflation in realistic high-energy physics theories, and leave peculiar observational imprints, like unusual, flattened-type, deviations from Gaussianity of the statistics of primordial fluctuations. We showed that the departure of inflation from an exact de Sitter phase, as well as Planck-suppressed derivative operators, play a decisive role in (de)stabilizing the Higgs during inflation. Using methods from nonequilibrium quantum field theory, we derived for the first time a manifestly covariant general theory of multifield stochastic inflation.
We gave a new insight into the question of the non-Gaussianities generated from realistic models of inflation with multiple degrees of freedom by generalizing Maldacena’s computation to such setups, highlighting the observable effects that derive from a curved field space. We developed the cosmological flow framework that enables one to systematically compute tree-level cosmological correlators in all theories of the early-universe, following their time evolution from their origin as quantum zero-point fluctuations throughout the primordial universe with our code CosmoFlow (
https://github.com/deniswerth/CosmoFlow(s’ouvre dans une nouvelle fenêtre)). We extensively studied the physics of the cosmological collider in realistic situations including multiple fields, different sound speeds, features and strong mixing, each time identifying new observational signatures. We used bootstrap techniques to find exact analytical solutions for primordial correlators in theories that break de Sitter boost symmetry and involve the exchange of a massive field. From this, we identified a novel discovery channel to detect new physics during inflation, called the cosmological low-speed collider signal. It is characterised by a distinctive resonance in non-Gaussian correlators that indicates the presence of a heavy particle and that enables one to reconstruct its mass, and we studies in detail its properties, notably at strong mixing and in the presence of parity violation. Using semi-classical methods, we made the first non-perturbative computation of the wavefunction of the universe, showing that even minute oscillations in the inflaton potential give exponentially large effects on rare events in the tail of the distribution.
We suggested a new, top-down motivated and model-independent mechanism to generate primordial black holes that is specific to multifield dynamics and offers interesting observational prospects. We showed how features of the density power spectrum manifest themselves as specific oscillatory patterns in the frequency profile of the scalar-induced stochastic gravitational wave background, offering a new probe of inflation on small scales and motivating new target signals for gravitational wave observatories. More generally, we studied signatures of the dark era of inflation in the stochastic gravitational wave background from particle production, excited states, resonant features, non-standard thermal history after inflation, dark matter isocurvature, primordial non-Gaussianity, and assessed the corresponding potential of discovery with the ESA mission LISA, also highlighting the importance of the phenomenon of infrared rescattering. After having shown the breakdown of perturbation theory in some of these models, we developed the first simulations of inflation with enhanced perturbations resulting from a departure from slow-roll, revealing an inflationary butterfly effect where small-scale quantum physical processes drastically alter the evolution of the entire Universe.
