The Perfect Theory by Pedro G. Ferreira Page A

balanced state: fixed and static, exactly as Einstein believed it should be.
    Shying away from the conclusion that the universe was evolving immensely complicated Einstein’s own theory. As he himself would later admit,“The introduction of such a constant implies a considerable renunciation of the logical simplicity of the theory.” By adding the constant, he told a friend he had “committed something in the theory of gravitation that threatens to get me interned in a lunatic asylum.” But it did the job.
    In the crescendo that led up to the discovery of relativity, Einstein would often write and discuss his work with Willem de Sitter, a Dutch astronomer at Leiden University, in Holland. Living in a neutral country during the First World War, de Sitter had been instrumental in relaying information about Einstein’s theory to England, where Eddington had studied his work in detail; de Sitter was the quiet man who had played a pivotal role in the lead-up to the 1919 eclipse expedition.
    A mathematician by training, de Sitter was well equipped to tackle the Einstein field equations. The moment he received a draft of Einstein’s paper describing a static universe born out of the field equations mangled with the cosmological constant, de Sitter realized that Einstein’s solution was not the only possibility. In fact, he pointed out, it was possible to construct a universe containing nothing but the cosmological constant. He proposed a realistic model of a universe that could contain stars, galaxies, and other matter, but in such small quantities that they would have no effect on spacetime and would be unable to balance out the cosmological constant. As a result, the geometry of de Sitter’s universe would be completely determined by Einstein’s fix, the cosmological constant.
    Both Einstein’s and de Sitter’s universes were static and unevolving, exactly as Einstein’s prejudices had led him to believe. Yet de Sitter’s universe had a strange property that de Sitter himself noted in his papers. De Sitter had built his universe so that spacetime was static, just as Einstein had before him. The universe’s geometry, such as how curved space was at each point, would remain unchanged over time. But if you now scattered a few stars and galaxies in de Sitter’s universe—a reasonable thought exercise given that our own universe seems to be full of such things—they would all start to move in a concerted manner, drifting away from the universe’s center. Even though the
geometry
in de Sitter’s universe was completely static and stayed the same for all time, objects within his universe wouldn’t stay still.
    A few weeks after receiving Einstein’s paper describing his static universe, de Sitter had already written up his own solution and sent it back to Einstein. While Einstein recognized that de Sitter’s model was mathematically valid, he was not impressed and he hated the idea of a universe completely empty of the planets and stars that we can see in the night sky. For Einstein, all that stuff was essential and was what made us have a sense that we were moving or turning. Only relative to the firmament of stars could we say if we were accelerating, slowing down, or spinning. They gave us a reference for applying all the laws of physics. Without all that stuff, Einstein’s intuition failed him. He wrote back to Paul Ehrenfest expressing his irritation at this world devoid of matter.“To admit such possibilities,” he wrote, “seems senseless.” Despite Einstein’s grumbling, within just a few years of its creation, general relativity had spawned two static models of the universe that were very different at their core.
    Â 
    While Einstein was working on his general theory of relativity, Alexander Friedmann was bombing Austria. As a pilot for the Russian army, Friedmann had volunteered in 1914, serving first in an air