In 1917 Albert Einstein began to explore the cosmological implications of his recently published general theory of relativiy. General relativity is a theory of gravitation, the only force acting between stars and galaxies, so it stood to reason that it should have something to say about the structure of the universe.
Einstein wrote his equations, gave them a nudge and watched them soar. To his astonishment, they revealed that under general relativity the universe could not be static, but must be either expanding or contracting. There was, at the time, no observational evidence for this, and Einstein was forced to conclude, much to his chagrin, that there must be something wrong with the theory. He did not discard it. Instead, he modified the equations adding an artificial term containing what he called the “cosmological constant.” The cosmological constant, he thought, would hold the universe in check.
He was wrong. The Russian mathematician Aleksandr Friedmann found that Einstein had made an algebraic error, upon correction of which the universe happily took wing again. Einstein was puzzled. The equations of general relativity were simple and elegant. They had the kind of mathematical beauty in which the insightful physicist discerns physical thruth even before the equations are tested experimentally. But the astronomers he consulted told him that the stars wander more or less randomly through space, showing no concerted motion. Nature, it appeared to Einstein, had spoken, and against nature´s last word no physicist in his right mind --least of all Einstein-- ought to rise.
At the time many astronomers still believed that the stars in the Milky Way galaxy were more or less the whole universe. The spiral nebulae had not yet been recognized as galaxies in their own right. Many scientists thought they were solar systems in the process of formation, so when the astronomer Vesto Slipher of Lowell Observatory discovered that several spiral nebulae seemed to be receding from the earth at speeds much greater than the typical velocities of stars, nobody knew what to make of his data. He had in fact found the first observational indication that the universe is expanding.
But Slipher did not know that his spiral nebulae were faraway galaxies. Only after Edwin Hubble discovered Cepheid variable stars in the spiral nebulae were they identified as such. Moreover, the presence of Cepheid variables in the spirals allowed astronomers to determine their distances. In 1929 Hubble plotted the distances of 25 galaxies against their velocities of recession from the earth as measured by the “redshift” in their spectra. If the velocities were random, if the observation that most spirals seemed to be receding from the earth were just a coincidence, the graph would show a swarm of points scattered every which way. Instead, Hubble found a straight line.
Hubble was no theorist, and he was completely innocent of general relativity. He was wary of this “redshift-distance relation,” as he cautiously called it, and did not draw conlcusions from his discovery. But his graph was a message in the handwriting of the powers that be. To all who had eyes it read: “Behold, the universe is expanding.”
Einstein later called the cosmological constant his "biggest blunder". However, watch this for later developments in the fate of this strange antigravity force: