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Name: Jordan
Status: student
Grade: 9-12
Country: United Kingdom
Date: Summer 2013


Question:
After learning Hubble's Law in school, I am confused about the 'way' the universe is expanding. say you took a point in time and measured the distance to a galaxy and its recessional velocity. If you waited a few years and then measured the distance and velocity again, the distance will have increased, does that mean the velocity will have increased as well? or does it stay the same?

Replies:
Jordan,

Certainly this can be confusing because the universe appears to be expanding everywhere. Where does it expand into? In fact, we can look at it as though space itself is expanding. A classic analogy is to use the two dimensional surface of a balloon as analogous to our three dimensions. If we start by drawing dots on the surface of the balloon and begin blowing it up, all the dots start moving away from one another. If we cover the whole balloon uniformly with dots, then there will be no spot on the balloon where we would say the dots are not moving away from each other, and we could relate the distance from one dot to another to a “Hubble constant” just as in the universe. The farther two dots are apart, the faster they are flying away from each other. (Ignore for the moment that you can make some odd observations by the fact that the balloon is round) In this case, “space,” or the surface of the balloon is stretching. As the dots get farther apart, they will appear to speed up if we assume the surface is stretching at a constant rate (given by the Hubble constant). Why? Because there is more space in between to stretch, and the balloon is stretching everywhere at a constant rate. Thus, if you wait long enough, you should be able to measure an increase in velocity. The wait to see this effect is quite long, so astronomers simply measure objects at different distances to see how uniform the expansion is.

Kyle J. Bunch, PhD, PE


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