Within a second or two after the Big Bang, protons and neutrons start to combine into atomic nuclei (hydrogen, helium, and traces of lithium, beryllium, and boron). Roughly 300,000 years later, the Universe became cold enough for electrons to combine with atomic nuclei and form atoms. At the same time, the Universe became more transparent, enabling photons to travel more freely.
SSP scientists study the interactions between photons and matter. When atoms form during recombination, we learn about the conditions in the early Universe before the formation of stars and galaxies.