Let's journey back in time. Not so very far, really. Say to the year 4004 B.C.E. A Saturday evening, around 6 PM. The evening before the autumn equinox. A pleasant evening to be sure. The trees in bloom and covered with fruit. A balmy breeze. Maybe a first sniff of winter in the air. Folks looking forward to a nice Sunday on the morrow.
Except there were only two folks.
And at five minutes to six o'clock there were no trees or breezes.
At five minutes to six o'clock there was no five minutes to six o'clock. No time. No Earth. No Sun, Moon or stars.
At five minutes to six o'clock there was nothing.
So deduced James Ussher, 17th-century Anglican bishop, through a painstaking and scholarly study of the Bible.
The universe had a beginning. At a particular moment in time.
Nothing intrinsically absurd about this idea. Contemporary scientific cosmology suggests something similar, although with a rather longer pedigree (13.7 billion years) and not quite such a degree of precision.
But there is nothing intrinsically absurd about other ideas, either -- that the universe is eternal, or cyclical, or that there are multiple universes, or that the universe is branching and fragmenting into an infinity of universes all the time.
Ussher's idea of a unique beginning was, of course, theologically based: Christ's redeeming act was a unique, pivotal event in a story that has a beginning and an end. Ussher never looked at a rock or pondered geological evidence of any kind.
The big bang theory, on the other hand, was forced on initially reluctant physicists by observations of the universe -- the recession of the galaxies, the abundance of the elements, quasars, and the cosmic background radiation. Will the theory stand the test of time? Time will tell.
For the time being, many folks grab onto the big bang as confirmation of the Christian story, like Ussher letting theology be their guide. With, I would argue, as little reason and as much risk.