Universe is vast and mostly empty. Even if many of the billions of
planets we suspect are out there have life, most of the cosmos is
uninhabitable, and those worlds are unreachable by any means we know.
That’s just within our galaxy, which is one of about 100 billion in
the part of the universe we can see (the “observable universe”),
which itself is only part of a bigger universe, which may extend
infinitely in space. Then there’s the possibility, arising as a
consequence of several theories, that that universe is but one part
of a multiverse. All these things understandably can lead to feelings
That’s the entry point Adam Frank used for his recent Nautilus post: Is this universe at all special, especially in light of the possibility that it’s just one small part of a multiverse? If our observable universe is one of many, but the only one that can support life, does that somehow restore us to special status? Frank connects this question with the Copernican principle, a concept central to modern cosmology.
In the four centuries since his major published astronomical work, Copernicus has become a symbol for scientific revolution: changing the way we think about the world from received wisdom to empirical knowledge. The Copernican principle is actually an extension of Copernicus’ ideas rather than something he wrote. The central pillar of this idea is that not only is Earth not the center of the solar system, it doesn’t occupy a privileged position in the Universe as a whole. A bug-eyed alien astronomer from planet Gleethnorp in a distant galaxy would count more or less the same number of galaxies in the observable universe that we do, scattered in more or less the same way. What if we wanted to look at galaxies not from planet Gleethnorp but from an entirely different “pocket” universe—would things look the same there as they do in our local cosmos? It’s unlikely. Many multiverse models include the idea that the other pocket universes separate from ours might have different physical parameters, meaning they might be completely unable to support life. The bit of the cosmos we inhabit has just the right values for certain physical properties—the relative strengths of forces, for example—for life to exist. The question of why our universe seems so perfectly set up for us to be here is called the “fine-tuning” problem. A slight alteration to some parameters would result in a universe incapable of bearing life, or devoid even of stars and galaxies. That is still in keeping with the Copernican principle, since other pocket universes aren’t connected to ours, and no position within any of them is special. Nevertheless, as Frank wants to know, is our pocket universe special, somehow privileged?
We have no more evidence for multiverses than we have evidence for life beyond Earth—though it’s reasonable to think both exist.
The multiverse is profoundly uncomfortable to many people, but it’s not just an idea (or a “meme,” as Frank puts it). Inflation—the extremely rapid expansion of our cosmos during its very earliest moments—almost certainly would produce regions that have never been in contact with the one containing our observable Universe. In that sense, we get a multiverse as a consequence of the theory with no extra assumptions, as each of those pockets would be a parallel universe, inaccessible from ours, and possibly having different physical parameters than ours. That could solve the fine-tuning problem, if we can establish what the possible range in physical properties could be between the different pocket universes. The multiverse could include an enormous number of pocket universes with different parameters, and we just happen to be in one where stars, and star-nosed moles, can exist.
The best evidence for inflation comes from the cosmic microwave background (CMB, seen at the top of the post): the radiation left over from when the observable Universe became transparent, about 380,000 years after the Big Bang. The temperature of this radiation barely fluctuates over the entire sky, even though regions on opposite sides of the observable Universe were never in contact if the cosmos expanded at the rate we see today. Inflation explains that those regions were in contact when the Universe was very young, and the rapid expansion carried them far apart before the CMB formed. Inflation isn’t a dead certainty—most cosmologists accept that it probably happened, as observations have mostly ruled out its alternatives, though we need a new type of measurement to see its effects more directly.
A multiverse also arises in string theory if you consider each of the 10500 or so possible “vacuum states”—known as the “string theory landscape”—to be a separate pocket universe. In either the inflationary or stringy case, the existence of pocket universes is a question of physics, not metaphysics (philosophy about existence)…though as with the question of what came before the Big Bang, it may be hard to establish their existence. Personally, I’m not a fan of the string theory landscape (and neither are all string theorists), but I’ve reluctantly come to accept that the inflationary take on the multiverse is probably real—with the usual scientist’s disclaimer that I’m willing to be convinced if the evidence contradicts me. In any case, one need not invoke string theory to see the possibility of a multiverse in the observable universe we have.
We know that the universe is capable of supporting life, and that any physical parameters must be consistent with that obvious fact. Beyond that, we can’t go yet: We have no more evidence for multiverses than we have evidence for life beyond Earth—though it’s reasonable to think both exist. The uncomfortable possibility is that there are other pocket universes, but we’ll only ever know about them indirectly. That doesn’t make them any less real, just discomforting.
Fine-tuning is a bona fide puzzle, and the multiverse may not solve it, depending on what parameters are possible for other universes. Even a large number of parallel pocket universes doesn’t lead to infinite possibilities. (If you could check all of the 10500 possible universes in the string theory landscape, you’ll still never find a a low-mass object holding a high-mass object in orbit, or a universe where am I a fan of the Star Wars prequels.) So without knowing more about the finite possibilities for other universes—or indeed if they exist—it’s impossible to say whether our particular universe is special.
In any case, metaphysics didn’t invent the multiverse, and it won’t solve the fine-tuning problem.
Matthew Francis is a physicist, science writer, public speaker, educator, and frequent wearer of jaunty hats. I’m currently writing a book on cosmology, with working title Back Roads, Dark Skies: A Cosmological Journey.