The explosive beginning of our universe, the Big Bang marks the earliest time we can probe with current physical theory. Theory has to guide our understanding of the first fraction of a second, since we can’t recreate the extremely high temperatures that existed during the earliest history of the universe in any earthly laboratory. What theory tells us is that from an initial state in which matter and radiation are both in an extremely hot and dense form, the universe expands and the matter cools. At that time, it is believed that all four of the fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—were unified.

       The evolution of the earliest universe is not well understood because it is not clear exactly what laws were at work. However, it is known that by the end of the first second of time, the building blocks of matter had formed. By the end of the first three minutes, helium and other light nuclei (like deuterium) had formed but for a long time, temperatures remained too high for the formation of most atoms. At around one million years following the Big Bang, nuclei and electrons were at low enough temperatures to coalesce to form atoms. But the universe didn’t start to look like it does today until small perturbations in the matter distribution were able to condense to form the stars and galaxies we know today.