"Hubble, who knew nothing of the work of Friedmann and Lamaître, had read de Sitter, whose sometimes comic absentmindedness masked a supple and inventive mind. Stimulated by one of Einstein's papers on general relativity penned in 1916, de Sitter entered into a fruitful correspondence with its author and soon produced three lengthy papers of his own on the subject. In the third article, published in 1917, he simplified his calculations by assuming that the universe is devoid of matter, a mathematical fiction that he defended on the basis that the real universe is composed mostly of space anyway. He then conjectured that if two stationary objects were introduced into this void, light passing between them with respect to one another would be redshifted. Curiously, the redshift is not due to either the expansion of intergalactic space or the Doppler effect. Instead, it is the effect of the mysterious slowing down of time at great distances. ...the amount of reshift in de Sitter's model was directly proportional to the distance between the emitting and receiving objects, a relationship that had only to be tested by someone with a telescope powerful enough..."