Cosmos - Carl Sagan [142]
Increase all dimensions by one, and you have the situation that may apply to us: the universe as a four-dimensional hypersphere with no center and no edge, and nothing beyond. Why do all the galaxies seem to be running away from us? The hypersphere is expanding from a point, like a four-dimensional balloon being inflated, creating in every instant more space in the universe. Sometime after the expansion begins, galaxies condense and are carried outward on the surface of the hypersphere. There are astronomers in each galaxy, and the light they see is also trapped on the curved surface of the hypersphere. As the sphere expands, an astronomer in any galaxy will think all the other galaxies are running away from him. There are no privileged reference frames.* The farther away the galaxy, the faster its recession. The galaxies are embedded in, attached to space, and the fabric of space is expanding. And to the question, Where in the present universe did the Big Bang occur? the answer is clearly, everywhere.
If there is insufficient matter to prevent the universe from expanding forever, it must have an open shape, curved like a saddle with a surface extending to infinity in our three-dimensional analogy. If there is enough matter, then it has a closed shape, curved like a sphere in our three-dimensional analogy. If the universe is closed, light is trapped within it. In the 1920’s, in a direction opposite to M31, observers found a distant pair of spiral galaxies. Was it possible, they wondered, that they were seeing the Milky Way and M31 from the other direction—like seeing the back of your head with light that has circumnavigated the universe? We now know that the universe is much larger than they imagined in the 1920’s. It would take more than the age of the universe for light to circumnavigate it. And the galaxies are younger than the universe. But if the Cosmos is closed and light cannot escape from it, then it may be perfectly correct to describe the universe as a black hole. If you wish to know what it is like inside a black hole, look around you.
We have previously mentioned the possibility of wormholes to get from one place in the universe to another without covering the intervening distance—through a black hole. We can imagine these wormholes as tubes running through a fourth physical dimension. We do not know that such wormholes exist. But if they do, must they always hook up with another place in our universe? Or is it just possible that wormholes connect with other universes, places that would otherwise be forever inaccessible to us? For all we know, there may be many other universes. Perhaps they are, in some sense, nested within one another.
There is an idea—strange, haunting, evocative—one of the most exquisite conjectures in science or religion. It is entirely undemonstrated; it may never be proved. But it stirs the blood. There is, we are told, an infinite hierarchy of universes, so that an elementary particle, such as an electron, in our universe would, if penetrated, reveal itself to be an entire closed universe. Within it, organized into the local equivalent of galaxies and smaller structures, are an immense number of other, much tinier elementary particles, which are themselves universes at the next level and so on forever—an infinite downward regression, universes within universes, endlessly. And upward as well. Our familiar universe of galaxies and stars, planets and people, would be a single elementary particle in the next universe up, the first step of another infinite regress.
This is the only religious idea I know that surpasses the endless number of infinitely old cycling universes in Hindu cosmology. What would those other universes be like? Would they be built on different laws of physics? Would they have stars and galaxies and worlds, or something quite different?