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Originally Posted by Surio
And GDwarf, how do we know the universe is expanding. I've heard that many times but I've never heard of the proof.
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MMKB provided a link to information on Redshift, but I think I'll try my hand at explaining it here.
Red (and blue) shift are caused by the Doppler effect. The most common way you'll experience this in every-day life is when listening to trafic.
You've probably noticed that as a car approaches you the sound of it's engine is rather higher-pitched, when it's beside you it sounds close to the engine of your own car, and when it is moving away from you it sounds lower. (EEEEEEEEEEEEWWOOOOOOOOO is the sound effect normally used to describe it.)
This occurs because the sound-waves from an object moving towards you are closer together then when it's moving away from you. This is because the speed of sound is independent of the speed of it's source (So, even though that car is going faster, sounds made by it aren't.)
So, let's say that the car lets out a sound "pulse" once every second. If it was stationary then the pulses would be space equally in every direction, so it'd sound the same no matter what. However, as soon as it starts moving, you get an interesting problem. Since it's moving, say, east, every "pulse" that heads east will travel the same amount of distance that it did when the car was still. However, the car itself is moving, so the actual distance between it and the "pulse" is less by the time it releases the next one. Conversley, all the "pulses" heading west have more distance between them.
This ends up looking something like this: (((([] ) ) ) ) ).
Since our ears determine pitch based on the frequency of sound-waves, if you were to the left of the car it would sound higher-pitched then if you were beside it, and if you were behind it it would sound lower-pitched.
This also holds true if the car is still but you're moving towards/away from it, since to you the sound waves will still seem closer/farther apart.
What does all this have to do with the universe expanding? Well, something similar happens with light. Light shining from/reflecting off of something does the exact same thing. If the object is moving towards you then the light-waves become compressed, if it's moving away they become stretched. When light becomes compressed it moves more towards the blue end of the spectrum, when it's becoming stretched it moves towards the red.
So, when astronomers look at stars, they notice that the light from them is all moved further towards the red end of the spectrum then it should be. The obvious conclusion to make is that every star is moving away from us. Ergo, the universe is expanding.
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What about the comets? They can't last longer than 10 thousand years. If the universe was created billions of years ago, then why are there still comets out there.
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Why could comets only last 10 thousand years? The life-span of a comet depends entirely on how close it gets to the star it orbits. Ones that stay reasonably far away could last longer than the star.