When Einstein proposed the first cosmological model of the modern era in 1917, he had no reason to suppose that the Universe had a beginning. Everything indicated that the Universe was static and infinitely old, without an "origin" event. (A few redshift measurements made by Vesto Slipher in the United States were inconclusive and probably didn't make it to Europe, anyway.) Everything also indicated that the Milky Way was all there was out there. Other nebulae, seen with telescopes, were considered to be part of our galaxy. Beyond the Milky Way there was just dark emptiness, stretching itself through endless space.
In a little over a decade, everything changed.
To the horror of most scientists, by the early 1930s observations suggested that the cosmos was apparently endowed with a history which, at least qualitatively, was uncomfortably close to the biblical "Let there be light!"
In a succession of remarkable observations, thanks to a new powerful 100-inch telescope atop Mount Wilson and to impeccable methodology and diligence, the American Edwin Hubble and his assistant Thomas Humason determined that the Milky Way was one among "hundreds of thousands" of other galaxies, which Hubble called "island universes." Today, we know that galaxies are numbered in the hundreds of billions. Still, from one to hundreds of thousands was quite a jump.
A new cosmic vision was inevitable: just as Copernicus had removed the Earth from the center of the cosmos in 1543, Hubble had removed the Milky Way from the center of the Universe in 1924. We can call this the Second Copernican Revolution. There was no longer a center to the Universe; as Earth had lost its central role with Copernicus, the Milky Way had lost its centrality with Hubble.
As this was not enough, in 1929 Hubble and Humason demonstrated that the galaxies were receding from one another with speeds proportional to their distances. The conclusion, even more shocking, including to Einstein, was that the Universe was not static: it was expanding, the space between the galaxies stretching as if it were a rubber sheet.
For the first time in thousands of years of astronomy, the cosmos became dynamic: going backwards in time, there had to be a moment when galaxies were on top of one another occupying a small volume of space — the moment of "creation." If Hubble was right, cosmology would become mythic, facing questions very close to those of religious creation stories: if the Universe has a history, how did it begin? What or Whom started it? Why did it start?
The situation became even more interesting when, in 1927, the Belgian priest-physicist Georges Lemaître proposed that the Universe appeared from the decay of a giant primordial nucleus. Lemaître had invented a scientific model of "creation." And even if he insisted that there was no relation with Genesis, it was hard not to relate the two. For a few years, no one really took Lemaître seriously. But after Hubble discovered the expansion, his ideas begun to spark growing interest in cosmological models.
Since then, cosmology has been struggling with the question of the "beginning" of everything. In 1948, three British physicists proposed an alternative, the "steady-state model," where the cosmos would be eternal: matter would be created at the same rate that the expansion diluted it so that, on average, things remained the same in spite of the apparent expansion. However, in the 1960s, it was the model with a beginning that "won," due to the incontrovertible evidence gathered from the so-called cosmic microwave background, the left-over radiation from the epoch when hydrogen atoms were first formed, about 400,000 years after the "beginning." The steady-state model couldn't explain the existence of the radiation and was, for the most part, discarded. Since then, much stronger evidence makes the conclusion inevitable: our Universe did have a beginning dating back to 13.7 billion years ago. Is cosmology stuck with a creation event then?
Not if our Universe is not unique.
For the past two decades or so, theorists have been exploring the possibility that our Universe is part of a "multiverse," which would be eternal and infinite in spatial extension. If that's the case, our creation event becomes less interesting, one among possibly infinitely many others. Once again, a unique aspect of the cosmos is pushed aside and made part of a much larger whole, thus losing its centrality.
As I have written here recently, at present we have no evidence that the multiverse exists or, worse, that we can come up with observational tests of its existence. Still, if the theory proves out one day, it will change everything, again.
As Earth became just another planet in the First Copernican revolution and the Milky Way just another galaxy in the Second, our Universe would become just another universe among countless others, each with its properties, private histories, and creation events. This would, among all of its remarkable consequences, be essentially a Third Copernican revolution, now removing the centrality of our Universe in favor of an eternally-existing multiverse.