Digital illustration showing the earliest moments in the creation of the Universe. Cosmologists define several 'epochs' in the early Universe, which are: Planck Epoch, Grand Unification Epoch, Inflationary Epoch, Electroweak Epoch, Quark Epoch, Baryogenesis, Big Bang Nucleosynthesis and Recombination. Each epoch sees a drop in temperature and subsequent 'distillation' of successively larger particles, culminating - at the end of nucleosynthesis- in hadrons, such as protons and neutrons, which are the building blocks of atoms. This is followed by recombination - a period during which the background temperature is cool enough to permit atomic nuclei to bond with electrons to produce neutral atoms. By the end of recombination, 380,000 years after the Big Bang, the universe is transparent for the very first time, and consists mostly of hydrogen, with significant quantities of helium and traces of lithium.