How did temperature and density change as the early universe expanded?

According to the big-bang model, the universe emerged from a highly compressed, primordial state characterized by extreme temperature and density. As the universe expanded, both of these values decreased significantly.1

In the earliest stages, the universe was a hot, dense "primordial fireball" where temperatures exceeded one trillion Kelvin and energy was dominated by radiation. As the expansion proceeded, the universe cooled, allowing different physical processes to dominate in sequence. For instance, after about three minutes, the temperature dropped to the one-billion-Kelvin range, enabling nuclear reactions that formed deuterium and helium.2

The density of particles and photons also decreased as the universe expanded, with the number density of both scaling by a factor of $(1+z{)}^3$ relative to the redshift $z$. Eventually, after several hundred thousand years, the temperature dropped sufficiently for electrons to bind to nuclei to form atoms, a process known as recombination. This cooling allowed radiation to travel freely through space, which we observe today as the cosmic microwave background.3214