|MadSci Network: Astronomy|
Let's take your second question first, and look at it from the point of view of observers here in the Milky Way, our own galaxy. Galaxies which are further away from us appear to be moving away from us at higher speeds than galaxies nearby which are moving slowly relative to us. The cause of this motion is the expansion of the Universe.
To demonstrate this, take a wide rubber band and mark three spots on it: one in the middle, one on the left side and one on the right side, so you can see all three marks. Now, stretch the rubber band so that the left and right move away from the middle.
The left side moves away from the middle at a certain speed. This is like a car on the highway which starts next to you but is going faster than you are so it pulls away.
The right side moves away from the middle at the same speed but in the opposite direction.
Now if you think about both right and left, they move away from each other because they are moving in opposite directions. So, from the point of view of the right side, the left is moving away with twice the speed as from the middle. This is like being in your car and passing someone who is driving the opposite direction with the same speed as you - if you think of your car as "stationary" relative to you, the other car looks like it is moving past you at twice that speed.
So, galaxies further away appear to be moving away from us at faster than nearby ones.
Now for your first question: why are the galaxies speeding up (accelerating)? This is a fairly recent result, as for a long time it seemed like the expansion of the universe was constant. Something must be making the expansion accelerate, but we don't know what that is yet. Cosmologists (people who work on this and similar problems) have called it "dark energy" but we really don't know what it might be. In fact, there is still a lot of debate as to whether or not the universe is in fact accelerating, or if we just haven't taken into account some other factor (since everything we observe is by seeing light, anything that changes properties---such as wavelength---of light could affect our conclusions).
[Be careful about over-interpreting the rubberband analogy, though. As one pulls the rubberband, the size of the dots will increase as well. That is not happening to our Galaxy or other galaxies. An even better analogy, though perhaps more difficult to carry out, would be to glue small objects to the rubberband. As the rubberband is stretched, they will separate but not change size. Moderator]
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