the expansion of the universe, starting with the Big Bang, is a well-attested physical phenomenon. But over the past 400 years the universe has also undergone a different sort of expansion—a mental one. This began with a big bang too, the shattering in the early 17th century by astronomers such as Galileo Galilei and Johannes Kepler of the crystal spheres hitherto supposed to have held heavenly bodies on their proper courses. That led people to realise the so-called fixed stars, the celestial backdrop on which the movement of the planets is played out, are vastly farther away than had been believed, which led in turn to an understanding that the Milky Way, as seen in the night sky, is actually the view from Earth of a gigantic system of stars, of which the sun is a single, lowly member.
For a time the Galaxy, as this star system became known from the Greek name for the Milky Way, was thought to be the whole universe. Then, about 100 years ago, as telescopes grew in size and power, astronomers came to realise it is only one of many such groups of stars, and the mental picture expanded again, to where it is today—namely a galaxy-filled space which dates back 13.8 billion years, and whose evolution through that period is now understood in some detail.
One of the leading proponents of multiverses is Max Tegmark of the Massachusetts Institute of Technology. Dr Tegmark suggests a fourfold classification of possible types of multiverse. It has to be said that only three of these four seem comprehensible to mere mortals. But they are a good place to start.But the question of what constitutes universality has not been laid to rest. Some physicists suspect that just as the Galaxy—once thought unique—is merely an example of a general phenomenon, so too the universe may not be reality’s final frontier. Their idea is that there is not so much a universe as a multiverse. Indeed, there may be more than one sort of multiverse. These are grand claims, and hard to test. But, if true, they might solve some of the most puzzling questions of existence.
Worlds within worlds
The simplest Tegmark multiverse is an infinite extension of the familiar. Modern telescopes can see a long way, but the finite speed of light, and the finite age of the universe, mean they can peer only at things within a limited radius. Were space static, this horizon, known as the Hubble radius, would be 13.8 billion light-years away. In fact, because of the expansion of space after the Big Bang, the Hubble radius is 42 billion light-years.
How far things stretch beyond the Hubble radius no one knows. But some theories suggest they stretch to infinity. If that were true, then all permitted arrangements of matter might exist somewhere. They might even exist in infinite numbers. There might be an infinite number of Earths with readers reading this article on them. In effect, these places, cut off from one another by their own Hubble radii, would be isolated universes as the term is currently understood by science.
That may sound mind-boggling, but it is pedestrian compared with the second type of Tegmark multiverse. The first type assumes the laws of physics are the same everywhere. The second suggests they can vary from one universe to another. Tinkering with physics’s laws would change the nature of reality, so these universes would be different—perhaps very different—from each other.
In the third type of Tegmark multiverse, as in the first, the laws of physics are the same from one to another. In this type, though, the component universes are continually separating from each other as time passes. At every moment within such a multiverse, all possible futures allowed by the uncertainties of quantum mechanics actually happen somewhere, and that somewhere constitutes a new universe.
The final type of multiverse Dr Tegmark proposes is one in which any and all coherent systems of mathematics describe a physical reality of some sort. What this translates to in practice is hard to conceive of. It is more the province of metaphysics than physics. But the other three types of multiverse, though they push the bounds of physical theory, do not overstep them. Moreover, if the second and third type did turn out to be true, each would solve a profound problem of reality that is hard to deal with if the observable universe really is the be-all and end-all of everything.