The topic of the CVIII session of the Ecole de Physique des Houches, held in July 2017, was Effective Field Theory in Particle Physics and Cosmology. Effective Field Theory (EFT) is a general method for describing quantum systems with multiple length scales in a tractable fashion. It allows to perform precise calculations in established models (such as the Standard Models of particle physics and cosmology), as well as to concisely parametrise possible effects from physics beyond the Standard Models. The goal of this school was to offer a broad introduction to the foundations and modern applications of Effective Field Theory in many of its incarnations. This is all the more important as there are preciously few textbooks covering the subject, none of them in a complete way. In this book, the lecturers present the concepts in a pedagogical way so that readers can adapt some of the latest developments to their own problems. The chapters cover almost all the lectures given at the school and will serve as an introduction to the topic and as a reference manual to students and researchers.

This book is written from the viewpoint that a deep connection exists between cosmology and particle physics. It presents the results and ideas on both the homogeneous and isotropic Universe at the hot stage of its evolution and in later stages. The main chapters describe in a systematic and pedagogical way established facts and concepts on the early and the present Universe. The comprehensive treatment, hence, serves as a modern introduction to this rapidly developing field of science. To help in reading the chapters without having to constantly consult other texts, essential materials from General Relativity and the theory of elementary particles are collected in the appendices. Various hypotheses dealing with unsolved problems of cosmology, and often alternative to each other, are discussed at a more advanced level. These concern dark matter, dark energy, matter-antimatter asymmetry, etc.Particle physics and cosmology underwent rapid development between the first and the second editions of this book. In the second edition, many chapters and sections have been revised, and numerical values of particle physics and cosmological parameters have been updated.

The three neutrinos are ghostly elementary particles that exist all across the Universe. Though every second billions of them fly through us, they are extremely hard to detect. We used to think they had no mass, but recently discovered that in fact they have a tiny mass. The quest for the neutrino mass scale and mass ordering (specifying how the three masses are distributed) is an extremely exciting one, and will open the door towards new physics operating at energy scales we can only ever dream of reaching on Earth. This thesis explores the use of measurements of the Cosmic Microwave Background (the oldest light reaching us, a snapshot of the infant Universe) and maps of millions of galaxies to go after the neutrino mass scale and mass ordering. Neutrinos might teach us something about the mysterious dark energy powering the accelerated expansion of the Universe, or about cosmic inflation, which seeded the initial conditions for the Universe. Though extremely baffling, neutrinos are also an exceptionally exciting area of research, and cosmological observations promise to reveal a great deal about these elusive particles in the coming years.