I know. The universe has been around for 13.8 billion years and is likely to survive for another hundred billion years or more. Plus, where would the universe retire to? Florida isn't big enough. But it is time to retire the twenty-five hundred year old scientific idea of the universe as the single volume of space and time that contains everything. Twenty-first century cosmology strongly suggests that what we see in the cosmos—stars, galaxies, space and time since the big bang—does not encompass all of reality. Cosmos, buy the condo.
What is the universe, anyway? To test your knowledge of the universe, please complete the following sentence. The universe
(a) consists of all things visible and invisible—what is, has been, and will be.
(b) began 13.8 billion years ago in a giant explosion called the big bang, and encompasses all planets, stars, galaxies, space and time.
(c) was licked out of the salty rim of the primordial fiery pit by the tongue of a giant cow.
(d) All of the above.
(Correct answer below.)
The idea of the universe as an observed and measured thing has persisted for thousands of years. Those observations and measurements have been so successful that today we know more about the origin of the universe than we do about the origin of life on earth. But the success of observational cosmology has brought us to a point where it is no longer possible to identify the universe—in the sense of answer (a) above—with the observed cosmos—answer (b). The same observations that establish the detailed history of the universe imply that the observed cosmos is a vanishingly small fraction of an infinite universe. The finite amount of time since the big bang means that our observations only extend a little more than ten billion light years from earth. Beyond the horizon of our observation lies more of the same, space filled with galaxies stretching on forever. No matter how long the universe exists, we will have access to only a finite part, while an infinite amount of universe remains beyond our grasp. All but an infinitesimal fraction of the universe is unknowable.
That's a blow. The scientific concept, universe = observable universe, has thrown in the towel. Perhaps that's OK. What's not to like about a universe that encompasses infinite unknowable space? But the hits keep coming. As cosmologists delve deeper into the past, they find more and more clues that, for better or worse, there is more out there than just the infinite space beyond our horizon. Extrapolating backwards before the big bang, cosmologists have identified an epoch called inflation, in which the universe doubled in size many times over a tiny fraction of a second. The vast majority of spacetime consists of this rapidly expanding stuff. Our own universe, infinite as it is, is just a 'bubble' that has nucleated in this inflationary sea.
It gets worse. The inflationary sea contains an infinity of other bubbles, each an infinite universe in its own right. In different bubbles the laws of physics can take different forms. Somewhere out there in another bubble universe, the electron has a different mass. In another bubble, electrons don't exist. Because it consists not of one cosmos but of many, the multi-bubble universe is often called a multiverse. The promiscuous nature of the multiverse may be unappealing (William James, who coined the word, called the multiverse a 'harlot'), but it is hard to eliminate. As a final insult to unity, the laws of quantum mechanics indicate that the universe is continually splitting into multiple histories or 'worlds,' out of which the world that we experience is only one. The other worlds contain the events that didn’t happen in our world.
After a two millenium run, the universe as observable cosmos is kaput. Beyond what we can see, an infinite array of galaxies exists. Beyond that infinite array, an infinite number of bubble universes bounce and pop in the inflationary sea. Closer by, but utterly inaccessible, the many worlds of quantum mechanics branch and propagate. MIT cosmologist Max Tegmark calls these three kinds of proliferating realities the type I, type II, and type III multiverses. Where will it all end? Somehow, a single, accessible universe seemed more dignified.
There is hope, however. Multiplicity itself represents a kind of unity. We now know that the universe contains more things than we can ever see, hear or touch. Rather than regarding the multiplicity of physical realities as a problem, let's take it as an opportunity.
Suppose that everything that could exist, does exist. The multiverse is not a bug, but a feature. We have to be careful: the set of everything that could exist belongs to the realm of metaphysics rather than of physics. Tegmark and I have shown that with a minor restriction, however, we can pull back from the metaphysical edge. Suppose that the physical universe contains all things that are locally finite, in the sense that any finite piece of the thing can be described by a finite amount of information. The set of locally finite things is mathematically well-defined: it consists of things whose behavior can be simulated on a computer (more specifically, on a quantum computer). Because they are locally finite, the universe that we observe and the various multiverses are all contained within this computational universe. As is, somewhere, a giant cow.
Answer to quiz: (c)