The universe is defined as everything that exists, including all matter and energy. While there are many theories about how it began, the one that has gained credibility among scientists is called the Big Bang. The Big Bang Theory states that the universe began as a huge explosion that occurred somewhere between 10 and 20 billion years ago.
According to the Big Bang Theory, all of the matter and energy in the universe started out compressed into a space no bigger than the nucleus of an atom. Suddenly, a huge explosion occurred that sent everything that makes up the universe out in all directions. For an instant, the universe was an extremely hot ball of fire that began to expand rapidly. Extreme heat from the explosion (10 billion C or 18 billion F) caused the formation of subatomic particles.
Immediately after the explosion, the universe began to expand and cool. Some scientists believe that it expanded from the size of an atomic nucleus, to 610^30 kilometers in a fraction of a second. In less than one second, the expansion of the universe started to slow down. The universe became a cloud of matter and energy that was rapidly cooling and becoming less dense as it expanded. After a few minutes, at temperatures around 1 billion C, hydrogen nuclei began forming. Next, hydrogen nuclei began combining in pairs to form helium nuclei.
Ten thousand years after the explosion, most of the energy in the universe was in the form of electromagnetic radiation of different wavelengths including X rays, radio waves, and ultraviolet radiation. As the universe continued to cool and expand, these waves were changed into a form called cosmic microwave background radiation which can be measured today.
After 300,000 years, the temperature had cooled to around 10,000 C. Lithium atoms began to form at this stage and electrons joined with the atomic nuclei to form the first stable (neutral) atoms. The universe continued as a giant cloud of gas until about 300 million years after the Big Bang. Parts of the gas cloud began to collapse and ignite to form clusters of stars-the first galaxies. The universe has continued to form galaxies since then. These galaxies continue to expand outward from the initial point of the Big Bang.
In the early 1900s, Edwin Hubble began to study the motion of galaxies. He used Cepheid stars to determine the distances of galaxies from Earth. Next, he studied the Doppler shift of each galaxy and found that the farther away a galaxy was, the faster it was moving. By the early 1930s, he had enough evidence to prove that galaxies were moving away from a single point in the universe. This supported two key parts of the Big Bang Theory: the universe is expanding and the universe originated from a single point