We present a centennial review of the history of the term known as thecosmological constant. First introduced to the general theory of relativity byEinstein in 1917 in order to describe a universe that was assumed to be static,the term fell from favour in the wake of the discovery of the expandinguniverse, only to make a dramatic return in recent times. We considerhistorical and philosophical aspects of the cosmological constant over fourmain epochs: (i) the use of the term in static cosmologies (both Newtonian andrelativistic); (ii) the marginalization of the term following the discovery ofcosmic expansion; (iii) the use of the term to address specific cosmic puzzlessuch as the timespan of expansion, the formation of galaxies and the redshiftsof the quasars; (iv) the re-emergence of the term in today's Lamda-CDMcosmology. We find that the cosmological constant was never truly banished fromtheoretical models of the universe, but was sidelined by astronomers forreasons of convenience. We also find that the return of the term to theforefront of modern cosmology did not occur as an abrupt paradigm shift due toone particular set of observations, but as the result of a number of empiricaladvances such as the measurement of present cosmic expansion using the HubbleSpace Telescope, the measurement of past expansion using type SN 1a supernovaeas standard candles, and the measurement of perturbations in the cosmicmicrowave background by balloon and satellite. We give a brief overview ofcontemporary interpretations of the physics underlying the cosmic constant andconclude with a synopsis of the famous cosmological constant problem.
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