The reason the universe did not collapse after the Big Bang is a consequence of the initial conditions and the interplay between different factors, including the distribution of matter and energy, the expansion rate of the universe, and the properties of gravity.
In the early stages of the universe, immediately after the Big Bang, the universe was extremely hot and dense. At this point, the energy content of the universe was dominated by radiation rather than matter. During this phase, the universe was in a state of rapid expansion known as cosmic inflation. This rapid expansion caused the universe to cool down and become less dense.
As the universe continued to expand and cool, matter particles, such as protons and neutrons, started to form and eventually combined to form atoms. This process, known as recombination, occurred about 380,000 years after the Big Bang. Once recombination took place, radiation was able to travel freely through space, and the universe became transparent to light.
At this stage, the matter content of the universe started to dominate over radiation. Over time, the matter began to clump together under the influence of gravity, forming structures like galaxies and clusters of galaxies. However, the expansion of the universe counteracted the gravitational pull of matter, preventing rapid collapse.
The critical factor in preventing the collapse of the universe was the expansion rate driven by the energy content of the universe. The energy content is determined by the matter and various forms of energy, including radiation and, later, dark energy. In the early universe, the expansion rate was high enough to counteract the gravitational attraction and prevent collapse.
It's important to note that dark energy, which is believed to be responsible for the current accelerated expansion of the universe, emerged much later in the universe's history, around 5 billion years ago. Prior to that, the expansion of the universe was primarily influenced by the matter and radiation content. The effects of dark energy became significant as the universe aged and matter became diluted due to the ongoing expansion.
In summary, the expansion rate of the universe, along with the interplay between matter, radiation, and eventually dark energy, played crucial roles in preventing the collapse of the universe after the Big Bang. The balance between gravitational attraction and expansion determined the overall evolution and structure of the universe over time.