Question

what needed to happen to the early universe after the big bang before atoms could start to come together?

Explanation:

After the Big Bang, the early universe was extremely hot and dense with high - energy radiation and elementary particles. For atoms to form, the universe needed to cool down. As the universe expanded, it cooled. When the temperature dropped enough (to about 3000 K), electrons could combine with atomic nuclei (protons and neutrons) to form neutral atoms. Before this cooling and the subsequent recombination (the process of electrons and nuclei combining to form atoms), the high - energy photons would scatter off the free electrons, preventing atoms from forming. Also, the universe needed to expand to reduce the density of particles and radiation, allowing the forces (like electromagnetic force for atom formation) to act effectively over distances to bring protons, neutrons, and electrons together to form atoms.

Answer:

The early universe after the Big Bang needed to cool down (via expansion) and have its particle density reduce. When the temperature dropped sufficiently (to ~3000 K) and the density of free electrons/particles decreased, electrons could combine with atomic nuclei (protons, neutrons) in a process called recombination to form neutral atoms. Before this, the hot, dense, high - energy environment (with free electrons scattering photons) prevented atom formation.