"Shortly after Einstein published his original memoir on cosmology in 1917, de Sitter constructed an alternative static world-model, which satisfied the same laws of world-gravitation. In this model, unlike Einstein's, space-time has an intrinsic structure of its own, independent of the presence of matter. ...there is ...no matter nor radiation. Nebulae... must therefore be considered as 'test particles,' having no influence on the model as a whole. ...whereas a test particle in Einstein's universe will remain at rest if it has no intitial motion, a similar particle... in de Sitter's world will immediately acquire an ever-increasing velocity of recession from the observer. ...in de Sitter's model space-time is 'hyperbolic'. There is no absolute time, and each observer will perceive a horizon at which time will appear to stand still... This phenomenon... is only apparent, like a rainbow. At any point on the (relative) horizon the time-flux experienced by an observer there will be the same as at the original observer. Thus in de Sitter's world there will be an apparent slowing-down of distant atomic vibrations, if these keep standard time. Consequently the radiation from a distant nebula will appear to be shifted toward the red, due to an increase in wave length corresponding to the decrease in vibrational frequency. This effect... will be supplemented by the Doppler effect, due to the relative recession of the nebula regarded as a test particle."