In the beginning it was simple. We lived on a perfect plane laid out under the heavens or, later in our history, a round orb at the centre of the universe.

We discovered other worlds orbiting our sun, which usurped our place at the centre of all things, then other suns with other worlds orbiting them and eventually other galaxies of suns outside our own.

What then, if the boundaries of our observable universe are not the limit of all things but merely the latest horizon beyond which we cannot yet see. What if there are other universes, an infinity of universes, beyond our own?

Some will be very similar (except that you might have wings or eight legs like a spider or your tear ducts produce Irn Bru) others will governed by entirely different laws of physics and even mathematics (gravity might be repulsive and atoms the size of an ostrich egg to pick but a couple of relatively conservative examples)

Where might these exotic universes be found? Perhaps within our own space-time: if space-time is flat and infinite, all possible things must exist somewhere within it. There must be, beyond the observable part of our own universe, repetitions of and variations on our own world.

Or consider this: we believe that soon after the Big Bang, the universe went through a period of rapid inflation. What if some parts of space continued inflating while others, such as our own universe, stopped, creating isolated bubble universes within larger space-time?

What if every time a choice is made, multiple universes form, accommodating each possible outcome? This would lead to an infinity of universes.

Perhaps there are other dimensions: imagine an ant on a flat surface: it can move left and right, forwards and backwards but cannot leave the surface: similarly, we might be bound within the dimensions of our universe, unaware of other worlds located within further dimensions. Again, this hyperspace would contain an infinity of universes.

This then, worlds beyond worlds, space beyond space, is the multiverse.