Einstein introduced the cosmological constant into his general theory of relativity in 1917 to account for a static universe. At that time, the prevailing view was that the universe was static and unchanging. However, Einstein's original equations without the cosmological constant predicted a universe that was either expanding or contracting. To match the prevailing view, he introduced the cosmological constant as a term in his equations to provide a repulsive force that counteracted gravity, effectively creating a static universe.
However, in 1929, Edwin Hubble's observational evidence showed that the universe was, in fact, expanding. This discovery contradicted Einstein's static universe assumption and made the cosmological constant unnecessary for explaining the observations. Einstein famously referred to his introduction of the cosmological constant as his "greatest blunder."
In light of the expanding universe, Einstein decided to remove the cosmological constant from his equations, as it was no longer needed to achieve a static universe. He revised his theory to accommodate the expanding nature of the universe without the need for an additional term. Removing the cosmological constant allowed his theory to be consistent with the observational evidence and the expanding universe model.
Decades later, in the late 1990s, astronomers discovered that the expansion of the universe was actually accelerating, rather than slowing down as expected due to gravitational attraction. This unexpected acceleration led to renewed interest in the cosmological constant. It was proposed as a possible explanation for the accelerated expansion, with the cosmological constant representing a form of dark energy permeating space and driving the accelerated expansion.
Thus, while Einstein initially introduced the cosmological constant to achieve a static universe and later removed it, its significance and potential role in cosmology have been revisited and reevaluated over time due to new observational discoveries and theoretical developments.