Introduction

Abstract

AbstractThe cosmic microwave background (CMB) radiation [1] and the primordial abundances of the light nuclei [2] provide direct evidence for a hot thermal phase in the early universe. While the CMB represents a full-sky picture of the hot early universe close to its minimal temperature, primordial nucleosynthesis allows us to probe the history of the universe up to the first tenth of a second after the big bang. Going further back in time, beyond the generation of the light elements, the theoretical extrapolation becomes increasingly uncertain. Up to temperatures slightly above the electroweak scale, we are still able to make an educated guess about the evolution of the universe based on the established and well-tested physics of the standard model of particle physics. At temperatures around the scale of quantum chromodynamics (QCD), we thus expect the occurrence of a phase transition, in the course of which quarks and gluons become confined into hadrons. Similarly, one presumes a phase transition around the electroweak scale, which causes the Higgs boson, the electroweak gauge bosons as well as all fermions expect for neutrinos to acquire a mass via the Higgs mechanism. If indeed realized in the early universe, the electroweak phase transition would correspond to the cosmological implementation of electroweak symmetry breaking. Meanwhile, in anticipation of new insights from observations and experiments as to the physics beyond the standard model, we are at present merely able to speculate about the nature of the processes taking place at even higher energy scales. While the conclusive identification of a successor to the standard model is still pending, we know for sure that some processes must occur in the very early universe, which cannot be accounted for by the known laws of physics.KeywordsDark MatterHiggs BosonBoltzmann EquationCosmic Microwave BackgroundEarly UniverseThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.