Motion in the Universe
In this reference frame, an observer is a rest, apart from the expansion of the universe, that is, someone in this reference frame is not moving with respect to distant galaxies.
Think of the universe like the surface of a balloon, with pennies stuck onto the surface. These pennies are the clusters of galaxies. As you blow the balloon up, the pennies get further and further apart - this is the expansion of the universe. (The pennies/galaxies don't get bigger, because they are held together by gravity, or gravitationally bound.) The co-moving frame is the reference frame in which the coordinates expand along with the balloon. This is the closest you can get to having an absolute rest frame in an expanding universe.
So, after you account for the motion of the earth around the sun, the solar system within the galaxy, the galaxy within a cluster of galaxies, etc, eventually all you are left with is the expansion of the universe...and that's about as close as you could get to being absolutely still!
|last modified 12/16/2003 firstname.lastname@example.org|