Speed of light not constant = young universe

[LostLocke]

You think of light as a constant. It is not.

To the observer, it is. That's where the 'constant' comes from, not the actual speed being constant.

If I'm on a train going 50mph, and I throw a ball at 50mph, how fast is the ball going?
If you answered 100mph or 50mph or 0mph, you are correct.
The relative speed of the ball will vary depending on whether you are on the ground or on the train, and what direction I throw the ball (if you are on the ground).

With light, there's only one correct answer. It makes no difference how fast the train is going, how fast I throw the light, what direction I throw it, if you're on the ground or on the train, etc. Everyone will see the light moving at the same speed.