The number of particles in the universe is not constant, and it has changed significantly since the time of the Big Bang. Understanding the evolution of the number of particles requires a brief overview of the early stages of the universe.
During the initial moments after the Big Bang, the universe was extremely hot and dense. At this stage, the energy in the universe was predominantly in the form of high-energy particles and radiation. As the universe expanded and cooled down, particle interactions and various processes took place, leading to the creation and annihilation of particles.
One significant event in the early universe is known as "cosmic inflation," a rapid expansion phase that occurred shortly after the Big Bang. During this inflationary period, the universe expanded exponentially, and the number of particles remained roughly constant, though the total volume of space increased dramatically.
After the inflationary period, the universe continued to expand and cool, undergoing different phases known as radiation domination and matter domination. During these phases, the universe went through particle-antiparticle annihilations, which reduced the number of particles. However, at the same time, particle creation processes such as particle decays, particle collisions, and pair production also took place, increasing the number of particles.
As the universe continued to evolve, it entered the era of nucleosynthesis, when the temperature dropped enough for protons and neutrons to form atomic nuclei. This process produced a variety of light elements, such as hydrogen, helium, and lithium, while some heavier elements were formed later in stars through nuclear fusion.
It's important to note that the precise details of the particle content and number density depend on various cosmological models and the specific physics governing the early universe. However, in general, the number of particles in the universe has changed significantly over time due to particle creation, annihilation, and the formation of new particles during different phases of cosmic evolution.
In summary, the number of particles in the universe has not remained constant since the Big Bang. It has undergone significant changes as the universe expanded, cooled, and went through various physical processes, such as particle creation, annihilation, and the formation of atomic nuclei.