Supervisors' Foreword; Abstract; Acknowledgements; Contents; 1 Introduction; References; 2 Baryon Asymmetry in the Early Universe; 2.1 Big-Bang Cosmology; 2.1.1 Dynamics of an Expanding Universe; 2.1.2 Brief Thermal History of the Universe; 2.2 Dynamical Generation of the Baryon Asymmetry; 2.2.1 The Sakharov Conditions; 2.2.2 Baryogenesis: A Call for New Physics; 2.2.3 Relating Baryon and Lepton Asymmetries; References; 3 Baryogenesis via Leptogenesis; 3.1 Neutrino Oscillations and Seesaw Type I; 3.2 Vanilla Leptogenesis; 3.2.1 Boltzmann Equations, Weak and Strong Washout

3.3 Resonant Leptogenesis3.4 Open Challenges in Thermal Leptogenesis; References; 4 Effective Field Theories; 4.1 What Is an EFT?; 4.2 Principles of Construction; 4.2.1 An Example for a Matching Calculation: A Heavy Scalar Particle; 4.3 An EFT Prototype for Heavy Particles: The HQEFT; 4.3.1 The HQEFT Lagrangian; 4.3.2 Concluding Remarks; References; 5 Thermal Field Theory in a Nutshell; 5.1 Why Thermal Field Theory?; 5.2 Green's Functions at Finite Temperature; 5.2.1 Imaginary-Time Formalism; 5.2.2 Real-Time Formalism; 5.3 Comparison Between the ITF and RTF: A Tadpole Computation

5.4 Self-energies and Discontinuities5.4.1 Bosonic Case; 5.5 Particle Production Rates: Right-Handed Neutrinos in a Heat Bath; References; 6 EFT Approach for Right-Handed Neutrinos in a Thermal Bath; 6.1 Non-relativistic Majorana Fermions; 6.2 EFT for Non-relativistic Majorana Neutrinos; 6.2.1 Green's Functions for M ggT; 6.2.2 EFT Lagrangian at Order 1/M3; 6.2.3 Matching the Dimension-Five Operator; 6.3 Thermal Width in the EFT; 6.4 The T/M Expansion; References; 7 CP Asymmetries at Finite Temperature: The Nearly Degenerate Case