When the universe cooled during the process of forming atoms, the immense heat energy transformed into various forms. Here's an overview of what happened to that energy:
Photon Decoupling: In the early stages of the universe, the energy was primarily in the form of extremely high-energy photons (particles of light). As the universe expanded and cooled, a critical event called photon decoupling occurred. This is when the universe became transparent to photons. Prior to decoupling, the universe was filled with a plasma of charged particles (protons, electrons, and photons) that effectively scattered and absorbed photons, making the universe opaque. However, as the universe cooled further, the density and energy of the photons decreased, allowing them to travel more freely.
Cosmic Microwave Background Radiation: The photons that were released during the process of photon decoupling are what we now observe as the cosmic microwave background radiation (CMB). The CMB is a remnant of the early universe and is present throughout the entire cosmos. It is the cooled and redshifted afterglow of the hot, dense state of the universe when atoms began to form. The CMB represents the residual energy from the hot early universe, which has since cooled to an average temperature of about 2.7 Kelvin.
Formation of Atoms: As the universe continued to cool, the energy was used to facilitate the formation of atoms. Initially, the universe was a plasma of free electrons and protons. However, as the temperature dropped below a certain threshold, known as the recombination temperature (around 3,000 Kelvin), the free electrons and protons combined to form neutral atoms, predominantly hydrogen and helium. This process released energy, which contributed to further cooling.
Expansion of the Universe: The expansion of the universe also played a significant role in cooling. As space expanded, the photons within it stretched and their wavelengths increased, causing a redshift. This redshift corresponds to a decrease in temperature. The expansion of the universe continues to this day, further diluting the energy density and causing a further drop in temperature.
In summary, as the universe cooled and atoms formed, the immense heat energy transformed into lower-energy photons (CMB radiation) and facilitated the formation of neutral atoms. The expansion of the universe also contributed to the cooling process by redshifting the photons and reducing their energy.