Bubbles of other universes
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Could there be an infinite number of universes?

Cosmologist Anthony Aguirre from the University of California thinks our universe may not be alone.

According to Aguirre, the Cosmic Microwave Background - the left-over glow from the Big Bang - could give us clues to the Universe next door.

He believes that beyond our visible horizon, there are bubbles of other universes. If one of these bubbles were to collide with our own, then we could see a "bruise" on the fabric of space.

Horizon: How Big is the Universe is broadcast on Monday 27 August at 2100 BST on BBC Two in the UK and afterwards on the BBC iPlayer.