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]]>Einstein had considered the implications of his new theory for cosmology even before he had finished it. General relativity was a theory of space and time. He depicted that gravity which is the driving force for sculpting the cosmic architecture was reason for the distortion of spacetime’s geometry generated by the presence of mass and energy. He presented an equation to show how the spacetime geometry was determined by the density of mass energy. Since spacetime and mass energy account for basically everything, the entire cosmos works as general relativity’s equation required.

**Newton’s law of gravity** had caused problems in that case. If every mass attracted every other mass, as Newton had explained, then all the matter in the universe will to have just collapsed itself into one big blob. He suggested that the universe was infinite, filled with matter, so that attraction inward was balanced by the attraction of matter farther out. Nobody considered that explanation. For one thing, it required a really precise arrangement: One star out of place, and the balance of attractions disappears and the universe collapses. It also required an infinity of stars, making it impossible to explain why it’s dark at night.

Einstein hoped his theory of gravity would resolve the cosmic counterintuitive conclusions of Newtonian gravity. So in early 1917, less than a year after his completion of his paper on the general theory, he delivered a short paper **‘Cosmological Considerations in the General Theory of Relativity’** to the** Prussian Academy of Sciences** outlining the implications of his theory for cosmology. He noted the problems posed by using Newton’s gravity to describe the universe. Einstein showed that Newton’s gravity would require a finite island of stars sitting in an infinite space. His mathematical challenge was to show that such a finite cosmic spacetime would be static and stable. Matter’s effect on spacetime curvature would therefore be pretty much constant, and the universe’s overall condition would be unchanging. All this made sense to Einstein because he had a limited view of what was actually going on in the cosmos. Like many scientists in those days, he believed the universe was basically just the Milky Way galaxy. All the known stars moved fairly slowly, consistent with his belief in a spherical cosmos with uniformly distributed mass.

General relativity’s math did not work as it suggested the universe would not be stable. Einstein realized that his view of the static spherical universe would succeed if he added a term to his original equation. Einstein’s 1917 paper demonstrated the mathematical effectiveness of lambda also called the ‘cosmological constant’. In another paper, published in 1918, he commented that lambda represented a negative mass density and it played “the role of gravitating negative masses which are distributed all over the interstellar space.” Negative mass would counter the attractive gravity and prevent all the matter in Einstein’s spherical finite universe from collapsing. Though, there is no danger of collapse, because the universe is not static to begin with, but rather is rapidly expanding. After Edwin Hubble established such expansion, Einstein abandoned lambda as unnecessary it was set equal to zero in his equation. Others built on Einstein’s foundation to derive the math needed to make sense of Hubble’s discovery, eventually leading to the modern view of an expanding universe initiated by a Big Bang explosion.

But in the 1990s, astronomers discovered that the universe is not only expanding, it is expanding at an accelerating rate. Such acceleration requires a mysterious driving force, nicknamed ‘dark energy’ exerting negative pressure in space. . As he wrote in 1917 to the Dutch physicist-astronomer **Willem de Sitter:**

*“One day, our actual knowledge of the composition of the fixed-star sky, the apparent motions of fixed stars, and the position of spectral lines as a function of distance, will probably have come far enough for us to be able to decide empirically the question of whether or not lambda vanishes.”*

Einstein might not have been surprised by all of this. He realized that only time would tell whether his lambda would vanish to zero or play a role in the motions of the space-time.

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