A Brief Overview of the Big Bang Theory of the Universe

To see the role of neutrinos in the universe in the past, present, and future, we, we need to have an overview of the basic features in the evolution of the universe. The universe is now about 13 billion years old, starting from the moment of the “Big Bang.” It went through many stages in its evolution that eventually created all the known features that we see today, such as galaxies, stars, and various elements in different abundances. So how did it all happen? There have been many people thinking about these issues for the past century, starting with Einstein, De Sitter, Lemaitre, Hubble, Friedmann, Robertson, and Walker, just to name a few. The universe provides a setting that is unlikely to be achievable in the laboratory, but the ideas that went into our understanding of the universe have been inspired by many phenomena that have been observed in the laboratory. For example, the change of water to steam or ice to water is a common phenomenon known as phase transition in physics, and similar phenomena are expected to have occurred in the early stages of the universe.