The Big Bang theory nicely explains the expansion of the universe (discovered by Hubble in the 1920's). Obviously, that the universe is expanding suggests that it was a good deal smaller in the past. Likewise, the Big Bang theory nicely explains the cosmic microwave background radiation (detected by Penzias and Wilson in the 1960's, and predicted by Wilkinson and Dicke before that). This pervasive, pretty uniform, low-temperature radiation suggests that the universe was considerably hotter in the distant past. The rival "steady state" model of the universe has difficulty explaining these observations. However, the "Big Bang theory" is no longer a single theory. Various alternative Big Bang models have been developed (such as the "inflationary" Big Bang theory) to account for additional facts that have been discovered (such as the value of omega, the ratio of the universe's total kinetic energy of expansion to its gravitational potential self-energy, which tends to slow down the expansion). At the...

I, too, have heard it said that the phrase "before the Big Bang" is meaningless. One analogy I have heard drawn is between the phrase "before the Big Bang" and the phrase "more northerly than 90 degrees north latitude". Just as the latter phrase refers to no real location on Earth, so the former phrase is supposed to refer to no real location in time. According to cosmology's current picture of the Big Bang (as I understand it), the analogy is apt. (Of course, that doesn't rule out the possibility of further scientific developments resulting in corrections to the theory of the Big Bang.) It may seem unsatisfying to you that a scientific theory could just rule out as "meaningless" a notion that seems pretheoretically to be perfectly sensible. Intuitively, it seems like the question "What happened before the Big Bang?" ought to have an ordinary answer, rather than a cop-out answer like "There is no such time." However, the history of science is full of examples of questions that were once thought to...