A little more than a century ago, Albert Einstein turned his attention from gravity itself to the fate of the entire universe. In 1917, shortly after developing his general theory of relativity, he began applying his new equations to cosmology. General relativity had transformed our understanding of gravity, so it made sense to ask what those same equations said about the universe on the largest possible scales.
Gravity was the natural place to start. On average, the universe is electrically neutral, which means electromagnetism does not dominate its large-scale behavior. Einstein also knew nothing about the strong and weak nuclear forces (to be fair, nobody did), but those interactions only operate over very short distances.
For cosmology, gravity determines how matter behaves across enormous distances. If you imagine a collection of matter representing "a universe," Einstein's equations can tell you how that collection should evolve. What Einstein found surprised him.
General relativity did not naturally describe a universe that remained unchanged forever. Instead, the equations pointed toward a dynamic cosmos that would either expand or contract. That conclusion conflicted with the prevailing view at the time, which held that the universe was static and had remained essentially the same throughout cosmic history.
Einstein responded by adding a "cosmological constant" to his equations, represented by the Greek letter Lambda. General relativity already allowed such a term. In simple terms, it acts like a gravitational influence built into spacetime itself, one that can exist even in otherwise empty space.
Depending on its value, that effect can produce either attraction or repulsion. Einstein selected a value that counteracted the gravitational pull of matter, allowing the universe to remain stable. An Expanding Universe Changes Everything Within a few years, Edwin Hubble would discover that the universe is expanding.
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