The Big Bang is a pretty common term, even among non-scientists. Though, I have to agree with Calvin that it should be called the “Horrendous Space Klablooie“.
But it wasn’t any of those things. It wasn’t big, it wasn’t a bang, it wasn’t an explosion, it wasn’t gases burning or reacting in any way. Anyone who describes the “Big Bang” using these concepts or anything like them has no idea what they are talking about.*
What are we actually talking about when we talk about the Big Bang?
As of 2013, the age of the universe is understood to be 13.798 billion years (+- 37 million years). This age has been determined by three unrelated methods which all result in the same age. As new data comes in (for example, from the Plank spacecraft), this age may be refined somewhat, but it’s not going to change significantly.
The early universe (and by this I mean, before the Big Bang), was most likely a highly energetic and homogenous system. It was expanding and cooling (and extremely dense and hot things do). Within 0.00000000000000000000000000000000000001 (10-37) seconds of the beginning of the universe, something interesting happened.
Cosmologists call it a phase transition and it happened pretty quickly. Here’s an example of a phase transition that’s visible in real time.
In the video, we had very, very pure water (tap water won’t work) that was still liquid at 10°F. The water molecules are moving slowly and they (anthropomorphically speaking) haven’t learned how to be ice yet. A sharp tap or drop in a small piece of ice will start a chain reaction of water molecules attaching to each other in the proper way to become solid.
Something kind of similar happened to the universe. Except instead of turning solid or whatever, it began a period of super expansion. It increased in volume exponentially.
Indeed, some scientists (see Lawrence Krauss “A Universe from Nothing“) think that two really interesting things happened during this expansion. The first is that the expansion wasn’t uniform. Like the water to ice transition in the video, parts of the universe began expanding and then other parts started as well. Pretty soon the whole thing was expanding, but it wasn’t in a uniform sphere (or whatever higher dimensional form is appropriate). It would have been more like a lumpy sponge or something.
The second really interesting thing is that the expansion exceeded the speed of light. Now remember, we’re not talking about an explosion shock wave propagating through space or anything like that. This is space itself increasing in size. Yes, at one point in it’s history, the universe was smaller than you are. But that didn’t last very long at all.
This faster than light expansion explains why the universe is 13 billion years old, yet 93 billion light years in diameter.
This suggests another very interesting thing. We can’t see the entire universe. Some parts of the universe expanded so rapidly that light emitted in that part will never reach the Earth. Those parts of the universe exist, but nothing that happens there will ever be observed in any way by us. For all practical purposes, those parts of the universe are gone.
We know the expansion happened. It’s still happening. Except for a small number of galaxies that are gravitationally bound to the Milky Way, all other galaxies are running away from us. Some of the more distant galaxies are receding from us at 93% of the speed of light… that’s 279,000 kilometers per second (roughly).
Indeed, at some point, billions of years from now, we won’t even be able to see these other galaxies. In fact, we** won’t be able to see anything except the few galaxies around us that are actually getting closer.
Anyway, that’s what the Big Bang was. Not a bang, boom, or pop. The universe just*** got bigger. Nothing exploded or burned or anything else.
* As a side note, I spent an amusing few weeks asking a creationist what, exactly, exploded after he claimed that explosions cause disorder, not the order required for the universe. Shockingly, he never answered.
** “We” will be long dead, even our sun will be dead by the time this happens.
*** “Just” in the same way that Ebola is “just” a disease or a 50 megaton fusion bomb is “just” a bomb.