How many multiverses are there? The universe we can observe is not unique, but there are billions of others. In other words, the universe would be part of a larger “multiverse.” As a result, some well-known scientists have spoken of a super-Copernican revolution. According to this idea, not only is the Earth just one planet among many, but the universe itself is insignificant on a cosmic scale, one among countless other universes governed by their own laws.
The term multiverse has several meanings. We can explore the space surrounding us to a distance of about 42 billion light-years, which is the distance light has traveled since the Big Bang if cosmic expansion is taken into account.
However, there is no reason to assume that the universe ends at this cosmological horizon. It could continue indefinitely beyond that, through a succession of zones similar to our observable region. The distribution of matter would differ from one region to the next, but the physical laws would be identical.
Almost all cosmologists accept this multiverse view as a collection of similar regions. Max Tegmark, a cosmologist from Sweden, has called it a “level 1 multiverse.”
An infinity of different universes
Some go even further. They imagine an infinity of different universes, subject to different physical laws, with different histories, and even spaces that do not have the same number of dimensions. Most would be barren, but some would be teeming with life. One of the main proponents of this “Stage 2” multiverse is the Russian cosmologist Alexander Vilenkin, who paints a dramatic picture of an infinite set of universes containing an infinite number of galaxies, an infinite number of planets, and an infinite number of people who are your doubles reading this article.
Such representations are not new and are found in many cultures throughout history. What is new is the idea that the multiverse is a scientific theory, with all that implies in terms of logical rigor and confrontation with experience. This idea makes me skeptical. I do not believe that the existence of these other universes has been or can ever be proven. In my opinion, the proponents of the multiverse not only expand our understanding of physical reality but implicitly redefine what is meant by “science”.
Proponents of the stage 2 multiverse hypothesis have no shortage of ideas to explain or highlight this proliferation of universes. These universes could, for example, be in very distant regions of space that have not expanded at the same rate as ours, as predicted by the perpetual inflation model of Alan Guth, Andrei Linda, and others. These universes could have existed before ours, as suggested by the cyclic model of Paul Steinhardt and Neil Turok. Or they could exist in parallel, as other realizations of the quantum states of the universe, as David Deutsch and Michael Lockwood argue. These universes could also be completely decoupled from our spacetime, as suggested by Mr. Tegmark and Dennis Sciama.