Origin of the Universe - Theism vs. Atheism
In general, theists attribute the origin of the universe to some sort of transcendent, intelligent Designer. Atheists envision a natural, undirected process by which universes spring into existence spontaneously. Prior to the 20th century most atheists believed the universe was eternal. This changed however as discoveries throughout the 20th Century rendered that view untenable. Einstein’s theory of gravity (which has been thoroughly validated by extensive experimental confirmation) and Hubble’s astronomical observations preclude an eternal universe. We now know beyond a reasonable doubt that the universe began at some point in the finite past.
Now we understand that there are only two legitimate options for the origin of the universe:
(1) Someone made the universe (Intelligent Design), or
(2) The universe made itself (Random Chance).
The third option, the universe has always been here, is no longer a feasible alternative -- it contradicts empirical science. No other scientifically plausible theories for the origin of the universe have ever been proposed.
The implications of various 20th century discoveries have put atheists in an awkward position. Logic now requires that they identify an uncontrolled mechanism by which the universe could have initiated, designed, created and developed itself without an Intelligent Director. Otherwise, intellectual honesty requires the necessity of a Creator God.
Origin of the Universe - The Big Bang Theory
So began the effort to propose an atheistic mechanism for the origin of the universe. Enter the Big Bang Theory and Darwinian Evolution. The original Big Bang Theory seeks to explain the sudden appearance of everything from nothing, while Darwinian Evolution seeks to explain the origin of complex life forms from their supposed simpler ancestors. The premise of the Big Bang is that the entire universe was compacted into a teeny tiny little ball, which, after randomly coming into existence for no apparent reason in the first place, exploded into all space, time, matter and energy in an instant. Yes, that's the theory. No Ph.D. required.
Origin of the Universe - The Inflation Universe Theories
The Big Bang Theory provided an atheistic explanation for the origin of the universe, but its obvious simplicity was subject to multiple attacks. As a result, the original theory is no longer the dominant scientific explanation for the atheistic origin of the universe. While the original Big Bang Theory is now "dead," from its ashes have emerged the various Inflationary Universe Theories (IUTs). Starting with Alan Guth in the late 1990's (The Inflationary Universe: The Quest for a New Theory of Cosmic Origins), the scientific community has now proposed roughly 50 different IUT variants. Scientists hope that one of the current IUTs will sire an accurate reconstruction of the birth of our universe, though it is universally acknowledged that all of the current IUTs have their problems. It seems the only way to get realistic calculations to match an IUT model is to make assumptions that are poorly justified.