Probes of cosmic inflation: from the CMB to quantum analogues

Abstract

This thesis presents the development of data analysis and numerical simulation methods to probe the physics of inflation, a brief period of exponential expansion thought to have occurred a fraction of a second after the Big Bang. Although inflation provides the initial conditions for standard cosmological evolution and the seeds for structure formation, its underlying mechanism remains poorly understood. This motivates the search for inflationary signatures in the temperature and polarization anisotropies of the cosmic microwave background (CMB). The first part of the thesis focuses on parity-odd B-mode polarization patterns, which are a distinctive imprint of primordial gravitational waves (PGW) on large scales. The Simons Observatory (SO), a new ground-based CMB experiment, is targeting these B-modes with unprecedented sensitivity. Realizing the full potential of this high-precision instrument requires mitigating polarized Galactic foregrounds and secondary B-modes induced by weak gravitational lensing. In Chapter 2, we implement an analysis pipeline relying on multi-frequency observations and a lensing B-mode template to extract the primordial signal with optimal precision. Validated on SO-like simulations, this method removes 65% of the lensing B-modes, yields unbiased estimates of the PGW amplitude (parametrized by the tensor-to-scalar ratio r), and reduces statistical errors by 27–37%. In Chapter 3, we combine data from the Atacama Cosmology Telescope (ACT), the unWISE galaxy catalog, and Planck to reconstruct the actual CMB lensing field. Our combined tracer is 55–85% correlated with the true lensing convergence and the resulting B-mode template achieves a delensing efficiency of 47%, the highest demonstrated on real observations to date. The preliminary correlation of this template with early SO polarization maps at 93 and 145 GHz leads to the first positive detection (at 3.5σ and 4.9σ, respectively) of lensing B-modes with SO. Beyond its five-year nominal mission aiming to constrain r at the level of σ(r) ≲ 0.003, SO will undergo significant upgrades; combined with other next-generation CMB surveys, the full SO configuration is expected to bring uncertainties down to σ(r) ≲ 0.001 in the early 2030s. This will enable us to discriminate between broad classes of models, and will inform our understanding of the fundamental nature of inflation and possible extended scenarios. The second part of the thesis investigates the dynamics of false vacuum decay (FVD), a first-order phase transition at the heart of the eternal inflation paradigm. In Chapter 4, we use semiclassical lattice simulations to model the phenomenon in real time and extract ensemble-averaged bubble profiles at the time of nucleation. Our results are in closer agreement with the classical critical bubble than with the expected Coleman instanton profile, suggesting the presence of large renormalization effects or the identification of a distinct classically-allowed decay channel. In the near future, a quantum analogue experiment based on Bose-Einstein condensates will probe the full dynamics of FVD and contribute to clarifying this open question. We conclude by outlining prospects for combining cosmological and quantum probes of inflation, leveraging the connection between the smallest and largest scales to gain a new perspective on the early Universe.