If the universe is expanding, where is it expanding to? After all, the universe contains everything there is, both matter and empty space. Into what is the universe expanding?
This was the next big question, along with the discrepancy between the age of the earth and the age of the universe, to face the Big Bang proponents. The prevailing theory among Big Bang backers in the late 1920s was that space itself is expanding. Imagine a deflated balloon. Take a marker and cover the balloon with dots. Now inflate the balloon. Each dot remains in the same place on the surface of the balloon but as the size of the balloon increases the dots move apart from each other. The dots represent galaxies and the surface of the balloon is the universe. Just as the balloon expands so does space itself.
Friedmann did not live to see his theory vindicated but Lemaitre was still alive to revel in his new-found acceptance. To recap, both Friedmann and Lemaitre predicted that, at a time in the distant past, everything was squeezed into a tiny point. Lemaitre called this the primeval atom while Friedmann said it wasn’t even that big at the beginning. Friedmann developed his theory by ignoring general relativity while Lemaitre accepted general relativity but rejected the cosmological constant. Either way, Hubble’s observations supported a moment of creation, or, as Lemaitre put it, “a day without a yesterday.”
Einstein had lost interest in cosmology in the 1920s but with Hubble’s discovery his interest was rekindled. In January, 1931, he admitted that he was wrong about a steady state universe and accepted Lemaitre’s theory. He called the cosmological constant, which was necessary to overcome the effect of gravity that would eventually cause the universe to reverse its expansion and collapse on itself, the greatest mistake of his career. Not all scientists were so happy to abandon the cosmological constant, though. The Big Bang proponents found a use for it because by tweaking it they could change the rate of expansion of the universe.
Einstein’s endorsement of Lemaitre’s theory catapulted Lemaitre into celebrity status. Because Lemaitre was both a physicist and a priest he had dual status as a celebrity. Lemaitre was careful to keep religion out of science, saying “A lot of people believe the Bible actually pretends to teach science. This is a good deal like saying that there must be authentic religious dogma in the binomial theorem.” Nevertheless, Big Bang critics jumped on Lemaitre’s religious training by arguing that his theory was nothing more than a pseudo-scientific attempt to justify the Book of Genesis. The “primeval atom,” they argued was simply a prop for a master creator. To bolster their argument against the Big Bang theory they pointed to a defect we’ve already noted: the Big Bang theory predicted a universe that is younger than the Earth. To them, the universe was eternal and unchanging. Yet the eternal universe group knew they couldn’t just throw rocks at the Big Bang Theory. They had some explaining of their own to do, such as how does Hubble’s Law fit into their model? If the universe is static and unchanging how do you explain the red shift of virtually all the galaxies racing away from each other?