I like starting my thoughts on the big bang from the evidence first, then working backwards to the conclusions.
The speed of light is constant. It always travells the same speed.
If a fighter jet is flying at the speed of sound, and fires a bullet which is accelerated to a static speed of mach 1, the bullet will initially fly out the barell at mach 2 compared to the ground. If the same gun is on a plane flying mach 2, the bullet will be Mach 3 on barell exit.
If a torch is shone out the front of a spacecraft on the launchpad the lightbeam flys out at the speed of light (C)
Put that spacecraft into a few thousand km/hr escape trajectory, the light is still travelling at C
push the Spacecraft right up to 90% of C, the light still travels at C
Light is a wave, just like when an ambulance (or jet etc) zooms past you, the sound at the front is higher pitched than the sound behind the moving vehicle, this is because the sound waves are travelling at the same speed in front and behind the vehicle, however the waves are effected by being closer together at the front of the moving object and further apart behind.
Redshift is the same effect just with light. Red is behind, Blue is in front. Something going toward you is blueish, something going away is redish.
Generally most objects outside our local group of galaxies are all moving away from us. This either means we are the centre of the universe and the universe is expanding, or it means we are out amongst the "not centre of the universe" and everything is expanding :P. However however which way you look at it, everythings expanding.
If the universe is bigger now than when you started reading this thread, just reverse time for a bit... a bit more... right till you can't go any further... yep if it's getting bigger, that means it once had to be smaller. and if that is the case, it can't be "Forever Infinity Years Old". because if you go back, at some point something which is getting bigger, was once so small it was nothing.
Finally it has been said that the expansion is actually speeding up and not slowing down, meaning that gravity shouldn't be able to reverse the expantion into a contraction (assuming the acceleration is true, and that it keeps on going at it's current rate)
It's like a car driving out of new york toward Cleveland... if it keeps acellerating toward the west, if you assume it will keep doing that, it will burl through Cleveland and head on toward Seattle at high speed. Not slow down and then go in reverse back to NYC. Remembering this is a natural process and not something driven by a random car driver.
http://www.sciencemag.org/cgi/conten...295/5564/2341b