Big bang was a cosmic explosion that scientists think started the expansion of the universe . The big bang theory is the most widely held scientific theory of the universe’s origin. According to the theory, the big bang occurred about 13.8 billion years ago. At that time, the universe was much hotter and denser than it is today. As the universe expanded, it grew cooler and less dense.
The laws of physics do not provide a clear picture of how matter and energy behaved under the extreme heat and pressure at the instant of the big bang. Immediately afterward, much of the universe’s matter consisted of quarks (small particles that can combine to form protons and neutrons), electrons, and other elementary particles (see Subatomic particle ). Within a few seconds, protons and neutrons formed. Some protons and neutrons came together to form light nuclei, but the heat prevented the formation of atoms. After that, atoms existed, but matter did not condense into galaxies for several hundred million years.
Evidence for the big bang theory comes from measurements of helium and an isotope (form) of hydrogen called deuterium. In the late 1940’s and early 1950’s, the Russian-born physicist George Gamow and the American physicists Ralph A. Alpher and Robert Herman calculated the nuclear reactions that would have occurred during the big bang. Their work indicated that about 25 percent of the normal matter now present in stars and galaxies should be helium. It also showed that there should be about 1 deuterium nucleus for every 30,000 hydrogen nuclei. Later measurements closely matched their results.
Gamow, Alpher, and Herman also concluded that the big bang would have produced radiation that cooled as the universe expanded. In 1965, the American physicists Arno Penzias and Robert W. Wilson detected this radiation, called the cosmic microwave background (CMB) radiation . Its temperature is about 2.7 Celsius degrees above absolute zero (0 K or -273.15 °C), close to Gamow, Alpher, and Herman’s predictions.
Several questions remain about the big bang theory. Scientists wonder why the CMB radiation appears quite smooth—that is, its temperature is nearly the same in all directions. They also ponder how the universe’s clumpy structure of galaxies and voids could arise from such smoothness. To help explain these mysteries, physicists have developed a theory called inflation theory. According to inflation theory, the universe expanded at an accelerated pace for the first fraction of a second.
Other questions come from the existence of dark matter, an invisible substance that makes up most of the matter in the universe, and dark energy, a mysterious form of energy that appears to be making the universe expand more rapidly. Astrophysicists are working to determine the nature of dark matter and dark energy and the role they played in the big bang.
