The study of the very early Universe has given birth to a discipline that is broadly referred to as astro-particle physics, a subject that sits on the border of high energy particle physics, nuclear physics and astrophysics. Astro-particle physics has proven to be a wonderful symbiosis of experimental physics and experimental astrophysics in which the early Universe is, in effect, a high energy physics laboratory. Arguably, it was Gamow and his collaborators, in the immediate post World War II decade, who took the first tentative steps in this direction by arguing that the early Universe was the site of synthesis of the chemical elements.

The physics domain of cosmic nucleosynthesis is the first few minutes of the Big Bang. Hayashi (1950) took the first step further back into the past and towards the Big Bang itself with his exploitation of weak interactions to describe what had happened just prior to the period of nucleosynthesis. Since then, particle experiments have pushed back our understanding of the physics of matter to the point where we can now discuss the period before the first micro-seconds of the cosmic origin within the context of known high energy physics.