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Dr. Edwin Hubble: Study of Extragalactic Nebulae, 1939

The Hubble Constant

The Hubble constant is one of the most critical numbers to the science of cosmology because it is necessary for estimating the size and age of the universe. The long-sought number is in constant debate, and indicates the rate at which the universe is expanding, from the point of the big bang. The Hubble Constant is represented by the mathematical equation, HO = v/d, where v is the galaxy's radial outward velocity, d is the galaxy's distance from Earth, and HO is the current value of the Hubble Constant. Determining a true value for the Hubble constant is quite complex, as astronomers need two measurements. For the first, observations must reveal the galaxy's redshift, indicating the radial velocity. The second is more difficult to determine, and is the galaxy's precise distance from the Earth.

The value of the Hubble Constant first obtained by Edwin Hubble was approximately 500 km/sec/Mpc, and has since been dramatically revised. The values currently proposed by different experimental and theoretical groups range from 50 to 100 km/sec/Mpc.


Essay, from Edwin Hubble, "The Motion of the Stellar System among the Nebulae" and "The Motion of the Galactic System among the Nebulae," undated (889k) | Page 2 (816k)


Remarks, Presenting Edwin Hubble as recipient of Franklin Medal during ceremony, undated (599k) | Page 2 (238k)