In the intervening years the cosmological constant came in and out of vogue as new observational results repeatedly seemed to require it, but then were explained in other ways. When Hubble observationally discovered the expansion of the universe Einstein finally abandoned the cosmological constant completely (Einstein 1931). These results could be considered a prediction that the universe must be expanding or contracting, a remarkable implication of general relativity that was later borne out by observation.
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The attempt proved futile, as shortly thereafter de Sitter (1917) demonstrated an empty universe solution to Einstein's equations (allowing inertia relative to space empty of matter) and Friedmann (1922) derived solutions to Einstein's equations that corresponded to an expanding universe. "The term is necessary only for the purpose of making possible a quasi-static distribution of matter, as required by the fact of the small velocities of the stars" (Einstein 1917).Īt the time, observations of our universe were limited primarily to stars in our own galaxy, so there was indeed observational evidence justifying the assumption that the universe was static.Įinstein's goal was to obtain a Universe that satisfied Mach's principle of the relativity of inertia (for a historical discussion see Pais 1982, Sect 15e), and construct a cosmology that was finite, yet stable against gravitational collapse. The cosmological constant first appeared in a 1917 paper by Einstein entitled "Cosmological Considerations in the General Theory of Relativity" (Einstein 1917), in which he motivates its introduction into the field theory of general relativity by the need to stabilize the universe against the attractive effect of gravity:
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Figure 1: The cosmological constant was originally introduced by Einstein in 1917 as a repulsive force required to keep the Universe in static equilibrium.