Hubble's constant

Edwin Hubble (at right) inferred on the basis of measured red shifts, that galaxies appear to be moving apart with velocities that are proportional to their distance from us.

H is known as Hubble's Constant. The distance to galaxies in deep space is estimated by assuming that all galaxies are, on v average, equally bright, and then using the inverse square law.

Hubble's constant (64 (km/s)/Mpc) has the units of s^-1 if a megaparsec is converted to km. The inverse of Hubble's constant is then an estimate of the age of the universe in seconds.

One Megapasec is approximately 3x10¹⁹ kilometers. Converting the value above, 64 (km/s)/Mpc to (km/s)/km and taking the inverse gives the time for which the apparent expansion has been going on (the age of the universe) in seconds. The time is somewhere between 12.5 and 15.6 billion years.

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