What fundamental change occurred in the universe approximately one minute after the Big Bang?

Approximately one minute after the Big Bang, the universe underwent significant cooling, which led to a lower temperature environment conducive to the formation of the first atomic nuclei in a process known as nucleosynthesis. During this brief period, the temperature dropped to a level where protons and neutrons could combine to form light elements like hydrogen, helium, and trace amounts of lithium. This cooling was essential for the universe to evolve from its hot, dense state to one that could allow for the formation of matter as we understand it today.

The rapid expansion of the universe at that time also contributed to the cooling effect. As the universe expanded, the density and temperature decreased, which played a crucial role in shaping the cosmos. Therefore, the statement about the significant decrease in the temperature of the universe accurately reflects one of the fundamental changes in the early stages of cosmic history, marking a critical transition from a hot, dense plasma to a state where matter could begin to form.