Cosmology

During the time of Hubble's work, V. M. Slipher at the Lowell Observatory had been acquiring spectra of these fuzzy clouds. By breaking the light of astronomical objects into the various "colors" or wavelengths which make up that light, astronomers can determine much about the composition, temperature, and pressure of the material that emitted that light. The familiar spectrum of hydrogen so common to so many astronomical objects did not fall at the expected wavelengths, but appeared shifted to longer wavelengths. We now refer to such a change in wavelength as a redshift. Slipher noted that the fainter galaxies seemed to have larger redshifts and he sent his collection of galactic spectra off to Hubble.

Hubble interpreted the redshift as being caused by the Doppler effect, and thus representing motion away from us. The increasing faintness was related to distance so that in 1929 Hubble turned Slipher's redshift-brightness relation into a velocity-distance relation and the concept of the expanding universe was born. Hubble found that the velocity of distant galaxies increased in direct proportion to their distance. The constant of proportionality is denoted by the symbol H₀ and is known as Hubble's constant.

Although the historical units of Hubble's constant are (km/s/mpc), both kilometers and megaparsecs are lengths so that the actual units are inverse time (i.e. 1/sec). The reciprocal of the constant basically gives a value for the time it took distant galaxies to arrive at their present day positions. It is the same time for all galaxies. Thus, the inverse of Hubble's constant provides an estimate for the age of the expanding Universe called the Hubble age.

Due to its importance, the determination of the correct value of the Hubble constant has been a central pre-occupation of many astronomers from the time of Hubble to the present. Since the gravitational pull of the matter in the universe on itself should tend to slow the expansion, values of Hubble's constant determined by hypothetical astronomers billions of years ago would have yielded a somewhat larger number and hence a somewhat younger age. Therefore the Hubble age is an upper limit to the true age of the Universe which depends on how much matter is in the Universe.

The notion of a Heavens (i.e. universe) that was dynamic and changing was revolutionary for the age. Einstein immediately modified the equations of the General Theory of Relativity which he applied to the universe as a whole to deal with a dynamic universe. Willem de Sitter and quite independently Alexandre Friedmann expanded on Einstein's application of General Relativity to a dynamically expanding universe.

The concept of a universe that is changing in time suggests the idea of predicting the state of the universe at earlier times by simply reversing the present dynamics. This is much like simply running a motion picture backwards to find out how the movie began. A Belgian priest by the name of Georges Lemaitre carried this to its logical conclusion by suggesting that at one time the universe must have been a very congested place with matter so squeezed together that it would have behaved as some sort of primeval atom.