Since the dawn of science, there have been a wide variety of naturalistic theories proposed to explain how the universe began. These stand in addition to earlier creationist, religious, and mythological accounts of the origins of the universe and of humanity. Today, the most influential and commonly accepted scientific explanation of how the universe began is referred to as the Big Bang theory, and states that the universe was once an extremely hot, dense point which then explosively expanded into the far-flung firmament we can see in the skies now.
- About the Big Bang -
The theory of the Big Bang rests fundamentally on work by Albert Einstein, Georges Lemaitre, and others, although ironically enough, the term itself belongs to one of its early opponents, Fred Hoyle. It emerged from the observation, in the first half of the twentieth century, that as a general rule distant "nebulae," a term which at the time encompassed galaxies, were heading away from Earth. Logically, if most or all objects were travelling apart from each other now, they must have been far closer together at a distant point in the past. If this was followed to the ultimate conclusion, Lemaitre observed, then at some point all of the matter dispersed in the universe today must have originally been concentrated at a single point, which he called the "primeval atom."
Today, cosmologists believe that this original, high-density, high-temperature point - no longer referred to in Lemaitre's words - was present about 14 billion years ago. In a monumental explosion, all of this mass blew itself into a widely dispersed, expanding cloud in a tiny fraction of a second. At this point, matter consisted of almost equally matched matter and antimatter. However, when matter and antimatter come into contact, they annihilate each other - and the remainder left over, our current matter-dominant universe, owes itself to the fact that there was a very tiny surplus of matter over antimatter. Within several minutes, as the universe cooled, the first atoms began to form. Unfortunately, until we have a better knowledge of quantum theory and gravity, it will be difficult to piece together the full details of what probably happened within the first few fractions of a second.
From there, the speed of changes slowed while the universe continued to expand. Within a few hundred thousand years, the matter was beginning to clump together under the force of its own gravity, eventually leading to the formation of the first, giant generation of stars. This process essentially continues today, as old stars blow themselves apart and the matter released then forms into the next generation of stars. What is less clear is what has happened to the rate at which the universe expands. Current observations suggest that it is actually continuing to accelerate - a fact which violates what we assumed about the force of gravity, and which have led physicists to assume that some other unknown quantities (referred to as "dark matter" and "dark energy" because so far we have not been able to actually see them) are altering the balance. Otherwise, gravity would be sufficient that the universe would eventually slow, stop expanding, and then contract back to its original high-density point. If this were to happen, in theory it could cause another Big Bang, resulting in a cyclically expanding and contracting universe.
- Alternative Theories of How the Universe Began -
Cosmology, or scientific theorization about the origins of the universe, is principally a twentieth-century phenomenon, resting on Albert Einstein's innovations in physics but at least as importantly on Edwin Hubble's proof that our Milky Way galaxy was only one among an extremely large number of such galaxies. Initially, however, Hubble's discovery provoked a large debate between advocates of what became the Big Bang theory, described above, and proponents of the so-called steady state universe, led by Fred Hoyle. Hoyle argued that the universe was static: it had no discernible beginning or end point in time, and overall was essentially no different today than it was fifteen billion years ago, or will be one trillion years from the present.
The steady state theory did not violate any of the basic precepts of Einsteinian relativity. However, it did have great difficulty with other new discoveries about the universe. The realization that the universe was continuing to expand led steady state theorists to adopt the nearly-but-not-quite untenable position that hydrogen and helium were being steadily produced through some unknown process to fuel the expansion. However, the discovery of cosmic microwave background radiation in the 1960s - the glowing remnants of an early explosive period of transition - did in the steady state theory, as it showed that there was at least some sort of explosive process in our distant past.
At least to date, no scientists have come forward with credible alternatives to the Big Bang to explain an empirical record which clearly shows that we live in an expanding universe, with a glowing remnant of microwave radiation coming from all directions which originated in an earlier form. Today, virtually all "challenges" to the dominant Big Bang model actually involve tweaking or modification of certain elements of the Big Bang, rather than challenging the overall picture. Of course, new data and new theories could always emerge in the future to challenge or overturn today's common scientific understanding.