In 1915 Albert Einstein published his theory of general relativity which showed that cosmic space expands. However, that the universe was not static or eternal was met with condemnation by Einstein, and soon after he added a “cosmological constant” to his equations. The cosmological constant was a hypothetical force which compensated for the cosmic expansion and made the universe static once more.
It was not too long after that two other scientists Alexander Friedmann and Georges Lemaître built on Einstein’s theory of general relativity in the 1920s that it was found that the universe was indeed expanding. However, the pair argued that if the universe is expanding over time, as Einstein’s equations indicated, then at some point in the finite past all the matter in the universe must have been packed closely together into an infinitesimally small and infinitely dense point. Thus, the conclusion was to be what is now known as the “Big Bang theory.” This theory proposed that the universe had a beginning in time which was contrary to the static model of the universe which had been previously held.
Subsequently, in 1929, an American astronomer by the name of Edwin Hubble came to the conclusion that the galaxies are rapidly receding away from us. It has been said to be analogous to that of an inflating balloon. For instance, if one were to mark a spot anywhere on the balloon to represent our galaxy, and then mark other spots to represent other galaxies, and then inflate the balloon, all the other spots will move away from the first spot. Essentially, on the comic scale of things, that was what Hubble discovered. His observations affirmed that we live in a universe that is expanding.
Soon afterwards Einstein, after having met with Hubble and Lemaître, admitted that his cosmological constant was “the biggest blunder” of his life. The implications of this whole discovery were hugely significant and it brought with it a set of new questions. If the universe had a beginning then one wouldn’t be unwarranted to wonder if it will have an end. Scientists have proposed different hypotheses to answer such a question.