In the first three decades of this century the modern science of cosmology was forged from general relativity theory and new observing methods and instruments, particularly large optical telescopes perched thousands of feet above sea level in California and Arizona. These radical changes in the theoretical and observational tools used by astronomers, physicists and mathematicians accompanied revolutionary changes in cosmology itself. The new cosmology of the early 1930s included two key cognitive features absent from the cosmology of the turn of the century: first, the existence of galaxies outside our own stellar system that are visible in Earth‐based telescopes, and second, that these galaxies evince the expansion of the universe.

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