Final answer:
The first event after the Big Bang was the formation of protons. Next, these protons and neutrons combined to form deuterium and eventually helium in a process known as nucleosynthesis.
Step-by-step explanation:
When the universe was about 3 minutes old, it had cooled down to about 900 million K, allowing protons and neutrons to combine. This process is known as nucleosynthesis. The sequence started with protons and neutrons forming first. They combined to form deuterium nuclei, which are made of one proton and one neutron. From here, a series of reactions occurred; deuterium nuclei captured neutrons to form tritium and captured protons to form helium-3. Eventually, these products underwent further reactions to produce helium-4. Since the ratio of protons to neutrons was about 7:1, nearly all the neutrons were used to form helium nuclei. This nucleosynthesis phase of the early universe produced not only helium but also trace amounts of deuterium, tritium, helium-3, and a little lithium. It is critical to understand that the formation of neutral hydrogen atoms came later, once the universe cooled enough for electrons to join with nuclei. In response to the student's question, the event that happened first after the Big Bang was the formation of protons.