As soon as the universe started expanding, it cooled down. At 10 to the negative 35th power seconds, the temperature was only 10 to the 27th power degrees Kelvin. At this critical temperature, the universe underwent a phase transition, like the process that happens when an element changes state. The strong nuclear force--which acts at very short distances and holds protons and neutrons together--split off from the other forces. It released a large amount of energy.