We've known the size of Earth since the time of the ancient Greeks. The sun, solar system and Milky Way? No problem. But when it comes to the size of the universe, we haven't got a clue. "It's weird: the size of the observable universe is one of the more precisely known quantities in astronomy, but the size of the whole universe is one of the least well-known," says Scott Dodelson, a cosmologist at Fermilab in Batavia, Illinois. One way to think about the size of the observable universe is to consider how far light emitted at the big bang could have travelled by now. According to our best cosmological models, that distance is about 46 billion light-years. This is the cosmic "horizon", a sort of three-dimensional equivalent of the 2D-horizon we see on Earth.
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