When the ancients looked into the night sky they thought the heavens revolved around the earth and mankind. over the centuries this view has changed radically.
We discovered we lived on a planet orbiting a star within the solar system and the solar system was found to be part of the Milky Way galaxy. Later we learned that our universe was filled with billions of other such galaxies - but could it be that we're committing the same error as our ancestors by thinking the universe contains everything there is? Could it be that we live in a multiverse?
There are a number of different theories about what the multiverse could be. One proponent of the idea of the multiverse is Dr Tegmark of MIT. Dr Tegmark suggests a four fold classification of possible types of multiverse. The first type of multiverse is just an extension of what we already know our universe expanding into infinity rather than ending at the limits of our vision.
We can look back almost to the beginning of time to the edge of the observable universe, but we can see no further. So the space beyond that distance known as the Hubble radius is literally out of sight. But that doesn't mean there isn't anything there.
Because the expansion of the universe has stretched space, astronomers are able to see out to a distance of about 42 billion light years. How far things extend beyond this is unknown. If they stretch to infinity there could be numerous isolated universes cut off from one another by their own Hubble radius - depending on the observers vantage point.
To understand the second type of multiverse in Dr Tegmark system it is first necessary to understand how the universe was formed and the theory of inflation. It was first conceived of by Alan Guth in 1979 and then later refined and expanded upon by Andrei Linde who had some key insights.
This is one of the ideas of string theory which attempts to unify general relativity with quantum mechanics. The thinking is that all of the solutions produced by string theory that don't match up with what we can see in our own universe, may actually represent reality in other universes.
The anthropic principle is the idea that our universe is fine-tuned to allow humans to live. A small fiddle with the strength of gravity for example and life as we know it would not exist - a coincidence that does not sit easily with scientists. The concept of a multiverse neatly addresses this problem within the infinite number of universes that could exist we are simply living in the one we are able to.
In the third type Dr Tegmark multiverse in the first the laws of physics are the same from one to another. In this type though the component universes are separated not by distance but by time. At every moment within such a multiverse all of the possible futures allowed by the uncertainties of quantum mechanics actually happen.
In the many worlds theory of the multiverse the entirety of the universe acts like the quantum photon, but instead of having two potential future states, every possible outcome would be manifested so our entire universe and everything within it, including you, would be constantly undergoing multiple visions into daughter universes - each with its own reality and future. Any given observer though would only see one outcome.
In the final classification, the level 4 multiverse, Dr Tegmark proposes that all coherent mathematical systems describe a physical reality of some sort. Those different systems are of necessity different universes. What this last idea 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. Observational data supporting the theory of inflation have convinced some scientists that a multiverse is possible - but the idea is still controversial.
It may be impossible to ever directly observe the multiverse but some scientists hope to eventually gather enough data supporting the theories that predict it to one day confirm its existence. If that were to happen, like the ancients before us, we would be given a whole new perspective on how the cosmos works and on our place in it.