| In 1929 Edwin Hubble discovered that the light of distant galaxies is redshifted. |
| This Doppler shift toward longer wavelengths indicates the source is moving away from us. |
| The familiar acoustical version is the drop in pitch of a siren as an ambulance rushes past us. |
| Hubble deduced the universe is expanding. |
| (This is a consequence of Einstein's original cosmological model, but Einstein first rejected it as philosophically unacceptable.) |
| Indeed, Hubble's law is that the velocity V of recession of galaxies is proportional to their distance R from us. Specifically, |
| V = H⋅R |
| The constant of proportionality is called Hubble's constant; measurements of Hubble's constant are the basis of estimates of the age of the universe. |
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