Redshifting Away

Recall from Chapter 22, "A Galaxy of Galaxies," our discussion of Hubble's Law and Hubble's constant. We know from observing the redshift in the spectral lines of galaxies that all the galaxies in the universe are flying away from us, and that the galaxies farthest away are moving the fastest. And an observer in any galaxy would see exactly what we do.

Armed with Hubble's Law and Hubble's constant, it is quite easy to calculate how long it has taken any given galaxy to reach its present distance from us. In a universe that is expanding at a constant rate, we simply divide the distance by the velocity. The answer for all the galaxies is 15 billion years.

So if we were able to rewind the expanding universe to see how long it would take for all the galaxies that are flying apart to come together, the answer would be about 15 billion years.

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Telescopes Mastery

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