Evidence that the universe is expanding comes from the light of distant galaxies. When astronomers split this light up into a spectrum of colors and look for the telltale spectral lines that reveal the presence of gases in stellar atmospheres (see p.60), they find that the lines are in the wrong place—they are shifted toward longer wavelengths and appear redder than expected. The best explanation for this "red shift" is that it is a "Doppler effect" (see below), caused by the motion of the source galaxy away from our own position.
Different galaxies show different amounts of red shift. In the 1920s, Edwin Hubble (see p.25) compared the distances of galaxies with the red shifts in their light, and found that the two were related—the farther away a galaxy is, the greater its red shift, and therefore the faster it is moving away from us. The only explanation for this is that the entire universe is expanding—the wavefront of emitted radiation_
wavefront spreads farther away an object lies, the larger effect the general expansion has, and the faster it is carried away from us. The expansion of the universe is, however, a large-scale phenomenon— in smaller regions of space, it can be modified or even overcome by the gravitational attraction of galaxies.
The precise measure of the rate at which the speed of a galaxy's retreat increases with increased distance is known as the Hubble constant. If the rate of expansion has remained constant throughout time, then this will also determine the age of the universe, since the expansion can be tracked back to the Big Bang. The best measurement of the Hubble constant so far, made by the Hubble Space Telescope in the 1990s, puts the age of the universe at 13.7 billion years.
Red-shifted spectrum line
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