This fourth edition of BArner's The Early Universe is practically a new book, not just an updated version. In particular, to meet the wishes of many readers, it is now organized so as to make it more useful as a textbook. Problem sections are also added. In the center are the connections between particle physics and cosmology: the standard model, some basic implications of quantum field theory, and the questions of structure formation. A special feature of the book is the comparison of theoretical predictions with observations, separating "facts from fiction." Special emphasis is given to the observed anisotropies of the cosmic microwave background and the consequences drawn for cosmology and for the structure formation models. Nuclear and particle physicists and astrophysicists, researchers and teachers as well as graduate students will welcome this new edition of a classic text and reference. In this corrected reprint first year results from WMAP have been added to take account of recent developments.