The observable universe is everything light has had time to reach us from: a sphere 93 billion light-years across. Beyond it there is almost certainly more space. But some theories go much further and suggest that our whole universe may be just one of many, perhaps infinitely many, each with its own history and perhaps its own laws of physics. Together they are called the multiverse.
Four kinds of “other universes”
In 2003 the physicist Max Tegmark sorted the ideas into four levels:
- Level I: space simply goes on beyond what we can see. In an infinite universe, far enough away, every arrangement of matter would repeat.
- Level II: other regions where the constants of physics are different, born from inflation.
- Level III: the many worlds of quantum mechanics.
- Level IV: every mathematical structure exists as a universe of its own.
Bubbles of inflation
The most discussed version comes from cosmic inflation, the idea that the universe expanded with enormous speed in its first instant. In the early 1980s Paul Steinhardt and Alexander Vilenkin showed that inflation, once started, may never stop everywhere. Here and there it ends and a bubble universe forms, while the space between keeps inflating. This eternal inflation, developed further by Andrei Linde, would produce endless bubbles. As Alan Guth has put it, it is hard to build models of inflation that do not lead to a multiverse.
String theory adds to the picture. Its equations appear to allow an enormous number of possible vacuum states, often quoted as at least 10⁵⁰⁰, each giving a universe with different physics. This is called the string theory landscape.
Many worlds
A different idea comes from quantum mechanics. In 1957 Hugh Everett proposed that every time a quantum event has several possible outcomes, all of them happen, each in its own branch of reality. The name “many-worlds” came later, from Bryce DeWitt.
Is it science?
There is no evidence for any other universe. One possible test exists: if another bubble collided with ours early on, it might have left a circular mark in the cosmic microwave background. In 2011 Stephen Feeney and colleagues searched WMAP’s data and found no sign of one.
Many physicists, including George Ellis and Paul Steinhardt, argue that a theory that cannot be tested is not really science, and that the multiverse is closer to philosophy. Its supporters reply that it follows from theories that have been tested elsewhere. The debate continues.