  Did the universe have a beginning or does it exist forever, i.e. is it
eternal at least in relation to the past? This fundamental question was a main
topic in ancient philosophy of nature and the Middle Ages. Philosophically it
was more or less banished then by Immanuel Kant's Critique of Pure Reason. But
it used to have and still has its revival in modern physical cosmology both in
the controversy between the big bang and steady state models some decades ago
and in the contemporary attempts to explain the big bang within a quantum
cosmological framework. This paper has two main goals: First a conceptual
clarification and distinction of different notions of "big bang" and "universe"
is suggested, as well as a multiverse taxonomy and a classification of initial
and eternal cosmologies. Second, and with the help of this analysis, it is
shown how a conceptual and perhaps physical solution of the temporal aspect of
Immanuel Kant's "first antinomy of pure reason" is possible, i.e. how our
universe in some respect could have both a beginning and an eternal existence.
Therefore, paradoxically, there might have been a time before time or a
beginning of time in time. - Keywords: cosmology, big bang, big crunch,
universe, multiverse, time, general relativity, quantum cosmology, loop quantum
cosmology, string cosmology, world models, quantum vacuum, cosmic inflation,
anthropic principle, closed timelike loops, Abhay Ashtekar, Hans-Joachim Blome,
Martin Bojowald, George F. R. Ellis, Maurizio Gasperini, John Richard Gott III,
Alan Guth, Jim Hartle, Stephen Hawking, Mark Israelit, Claus Kiefer, Li-Xin Li,
Andrei Linde, Wolfgang Priester, Eckhard Rebhan, Paul Steinhardt, Neil Turok,
Gabriele Veneziano, Alexander Vilenkin, H. Dieter Zeh.
