Monday, June 1, 2009

Hubble Proved the Universe is Expanding

Edwin Hubble, after whom the newly repaired space telescope is named, used Henrietta Leavitt’s discovery to leap beyond the reaches of our galaxy. Hubble observed (before there were any space telescopes) that galaxies throughout the universe are moving away from Earth at velocities that are proportional to their distances from Earth; this is called Hubble’s Law.

We should not interpret Hubble's Law to mean that Earth is the center of the universe, nor that each galaxy is flying through space with a velocity that just happens to obey a certain formula. It doesn’t seem reasonable that each of the hundred billion galaxies just happens to have the right velocity. There must be some mechanism, some profound principle of nature, that choreographs this cosmic dance. The choreographer is space itself. What is really happening is that space is expanding and carrying with it everything it contains. As the space between galaxies expands, they move apart. Every location in the universe can lay equal claim to being the center of expansion – the universe expands in the same way as seen from every location.

Consider a simpler example. Imagine a strip of elastic tape with coins attached to it. If we stretch the tape, the coins will move apart. Now think of each coin as being a galaxy. Astronomers in each galaxy will observe all other galaxies moving away from them. They will observe galaxies that are twice as far away moving twice as fast, because there is twice as much tape stretching between them. They will discover Hubble’s Law. It isn’t really the galaxies (coins) that are moving, it’s the space (tape) between them that is expanding. In this example of a stretching tape, the expansion is in only one dimension. But, in our universe, all three dimensions of space are expanding at the same time and at the same rate.

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