By the late 1920s the large-scale structure of the universe had been clarified: distributed throughout distant space are countless white nebulae, galaxies like our own Milky Way galaxy, each consisting of hundreds of millions of stars. The pivotal event in the history of modern cosmology was Hubble's discovery, in 1929, that these nebulae exhibit red shifts that vary in a systematic way with distance, the size of the red shift of a galaxy being proportional to its distance from us. The findings of Hubble and other astronomers in the period from 1912 to 1929 were a result of technological advances in instrumentation and improved observing conditions afforded by mountaintop observatories. In one of history's great coincidences the work of the astronomers occurred at just the same time as Albert Einstein's (1879—1955) seminal mathematical investigations in the general theory of relativity. Einstein's theory laid the groundwork for a systematic mathematical approach to cosmology and led to the formation of a vigorous group of researchers in the new field of relativistic cosmology. These researchers were ready and able to interpret the sensational discoveries in nebular astronomy as they came along. The relativists succeeded in making the concept of an expanding universe central to all modern thinking about the universe.

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