The name "Big Bang" gives one an idea that the
universe's start is
somehow an uncontrolled
explosion, like a
grenade. In fact, to arrive
at the
flat universe that we live in, its
energy density needs to be
extremely fine-tuned: 1 part in 10^120, at least.
(While it is mathematically comprehendable,
it is very hard to actually grasp the sheer
largeness of 10^120.) If the value were
0.0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 00000001%
smaller or larger, we would not be in a flat universe. This is significant to
us, because a non-flat universe would crunch soon after the big bang,
or it would not contain any
galaxies or
stars. These two things have nasty consequences for life.
This fine-tuning, along with many other examples, has led to a consensus
among physicists: The initial conditions of our universe really are astounding.
Unfortunately, there isn't any better name than "big bang" that is as short, because nothing in human
experience carries anything even close to the proper amount of precision.
Evidences for the Big Bang:
- The cosmic background radiation. It fits, in all manner,
exactly what the Big Bang model would predict. There used
to be some aspects in the radiation that appeared to conflict with the Big Bang, but when analysed further there only was more evidence.
- Universal expansion. In astronomy there is a law called
"Hubble's Law", which basically says that far away objects
are usually moving away from us. It has been verified time
and time again, and it serves as a good evidence for the
Big Bang. (If we extrapolate backwards in time we find all
the visible matter coming from a small point)
- The abundance of light elements - not all have been
turned into heavier ones as we would happen in an old
universe
(We now have the ability to explain the big bang right up to t = 0, using
string theory. Unfortunately, it is a very difficult theory to test. But it
is a start - we are no longer needing to rely on wild speculations.)