The "Big Bang," at least with respect to its theoretical incipience, stems from the Theory of General Relativity, first proposed by Albert Einstein in 1917. But it would not be Einstein who would recognize the idea of a primeval atom, as the theory was originally referred to. In fact, when Monsignor George Lamaitre first proposed the theory in 1925, Einstein was one of the first to take exception with it, thinking that Lamaitre by supposing a beginning of the universe was implying a status of creation, something that Einstein deplored with the greatest of disdain. As it turns out, Lamaitre had a better understanding of Einstein's General Theory of Relativity then did Einstein, and some time later when Einstein realized that Lamaitre got it right, he said so himself.
Astronomers in the 1920's had notice that the light from some stars was shifted towards the red end of the spectrum, but they didn't know why. Resolution of this enigma would be the product of observations made by Edwin Hubble at the 100 inch Mt. Wilson observatory; then the finest optical telescope in the world. Hubble had first discovered that the Milky Way galaxy did not contain the whole universe, and that there were thousands of galaxies beyond it. What was interesting to Hubble, was that the light from almost all of these galaxies was shifted to the red end of the spectrum. In what had to be a "Eureka" moment, Hubble realized that all of these distant, red shifted galaxies were traveling away from our Earthly perspective, and away from a central point in the universe. The universe in fact, just as Lemaitre had predicted, was expanding. It was a profound realization that turned the sciences of physics and astronomy on their side.
Up until 1965, there were two theories, Lemaitre's Primeval Atom and Willem de Sitter's "Steady State" theory, supported by physicist Fred Hoyle among others. It was Hoyle who in lampooning Lemaitre's expanding universe theory, referred to it as the "Big Bang"in 1949, and the label stuck. What didn't stick was the steady state theory of the universe, but I am getting ahead of myself.
Russian born physicist, George Gamow, predicted that if there had been a primeval atom, and an instant when energy had turned into matter, then that event should have left a radiation signature which should be quantifiable, and still measurable. How to go about measuring it was a more difficult proposition. Arno Penzias and Robert Wilson would stumble upon the solution by accident, while working with a satellite communications receiver for the first satellite Tellstar in 1965. On their receiver, they were picking up a faint hiss, microwave noise coming from every direction in space. They had discovered the radiation signature of the Big Bang, proof positive of Lemaitre's primeval atom, and an event which took place 14 billion years ago, when all of the matter in the universe came into being.
In the 1990's, NASA launched a satellite called the COsmic Background Explorer (COBE) to study and better understand this radiation. The mission confirmed what was already known, and provided some new revelations as well. For instance, Gamow had predicted that there would be ripples in the radiation, and sure enough, COBE found some. Today, physicist understand the "Big Bang"in much greater detail, but rather then thinking of it as one event, they have quantified an evolutionary path that the expanding universe has taken.
The first phase of universal expansion is known as the Plank epoch, so named for physicist Max Plank. Beginning with a moment in time, indeed, the very first trillionth of a second, an instant that physicist refer to as the "Singularity," all the laws of nature and physics were suspended and energy was converted into matter, but not matter as we know today. What did exist was Quark Gluon Plasma. This plasma was extremely dense and incredibly hot. Everything we can see today in the universe, all the stars and galaxies and dark matter were contained in a space the size of a baseball, but not for long. Within the first second, the universe expanded to become billions of times bigger then it was.
As the universe expanded it cooled down to a point when Quarks and Gluons started to form baryons, which are protons, neutrons, and electrons. This process, called baryogenesis, lasted for several minutes as the universe continued to expand, and cooled to below a billion degrees Kelvin. In the next phase called nucleosynthisis, baryons began to form the nuclei of hydrogen and helium atoms, and the electromagnetic force came into being. After about 360 thousand years this phase would come to an end, as the universe had expanded and cooled to a point where free electrons were picked up by the hydrogen and helium nuclei. Gravity too then came into play and hydrogen and helium atoms clumped together to form the first massive stars. It was the beginning of the stelliferous age, a period that continues to this day.
What I have presented here, is the briefest of over views, of what the Big Bang entailed and a historical perspective of how we came to know what we do know about it. No, we don't have all the pieces of the big bang puzzle yet. We do have some pretty good models that predict what may have taken place. We have enough of the picture however, to establish that Lemaitre's theory of a primeval atom and expanding universe, which Fred Hoyle coined the "Big Bang," happened, is still going on, and is theory no more.
John Farrell, "The Day Without Yesterday," Thunders Mouth Press, New York, 2005
Walter Isaacson, Einstein," Simon and Schuster, New York, 2007