The Big Bang |
Imagine that you are a wildlife biologist walking through
the woods with a colleague. You spy a set of footprints in the
soft dirt that look exactly like wolf prints you have identified
many times before. And there, as the prints pass under a low bush,
is a tuft of hair, which appears to be wolf hair. But now, just
beyond you is a small cave-like hole under a large boulder where
several small wolf pups stare out at you and whimper. Suddenly a
chill goes up your spine as you hear a deep menacing growl behind
you. You stand petrified with terror.
At this point you would very reasonably conclude that a large adult wolf was behind you and that you were in danger of losing life or limb.1 But since your fear-induced paralysis prevents you from turning around, you cannot be absolutely certain that it is not your partner trying to spook you with fake wolf sounds. But your partner is not prone to practical jokes and chances are it's really a wolf. If you can't turn around to look, all you can do is make your best educated guess and hope to survive.
This is a bit like our situation with the Big Bang. People complain that it is "only" a theory, and they are right. But like the idea of a wolf breathing down your neck in our hypothetical situation, there is some pretty strong circumstantial evidence.
The Big Bang didn't leave footprints, but it did impress on the Universe an unmistakable outward expansion that we can observe today. Some have tried to explain this expansion in terms other than the Big Bang, tend to be generally unconvincing.
If you came upon a pool table with the arrangement of three colored balls and one cue ball as shown, you wouldn't immediately know what the original arrangement was before the shot. (For that matter, if the balls were not still moving, you wouldn't even know for sure that there had been a shot at all. Someone could have just arranged them in that manner.) But what if someone had filmed or videotaped the shot? You could play it backwards, frame by frame, to tell exactly how the balls had moved. That's just what scientists can do with the present Universe. What we can observe is that every distant galaxy appears to me moving away from us as if there had been a huge outward expansion at some point in the past. We can see the arrangements and motions of the distant galaxies, and using mathematics and the lawas of physics rather than a recording, we can work backward in time to find the original arrangement -- the Big Bang! (Click on the image to see the original arrangement of balls.)
There is no tuft of Big Bang "hair," but there is strong evidence in the form of the "Cosmic Microwave Background," (also known as the "Cosmic Background Radiation") considered to be radiation from the cooling embers of the Big Bang fireball. Discovered by accident, the Cosmic Microwave Background admirably fits the theoretical prediction of how the Universe should have cooled off from a hot Big Bang in the distant past. It's a bit like hearing the rumble of distant thunder -- the storm is far away, but you know it is there.
And while there are no pups at the cave entrance, there are galaxies, stars and ... us. When you tally up the atoms of the Universe, we find that roughly 75 percent are hydrogen and nearly 25 percent are helium, in strong agreement with what the Big Bang theory predicted (before the cosmic census of atoms was taken). There is a bit of "wiggle room" left in the precise percentages to account for the heavier elements we find in the Universe today, which were produced afterwards in stars and supernovae, and which are essential for life as we know it.
No one was there to record the Big Bang for posterity, but clear lines of evidence remain. And we cannot ever reproduce the Big Bang, but we can produce conditions similar in the lab on a small scale to test the theory. In the future, more powerful particle colliders may allow us to peer ever closer into the Big Bang. But even now, the Big Bang is one of the most thoroughly documented theories in the history of science.
So as in the old adage, "If it looks like a duck, and walks like a duck, and sounds like a duck -- it's probably a duck."
1Do not consider that this is in any way condones or perpetuates the age-old and irrational fear and hatred of wolves. Research has shown that wolves are not overly agreesive or dangerous to humans, but will -- quite rightly -- defend their pups and den. As with most things in nature, leave them alone and they will leave you alone. They have a right to be here.
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