Imagine a universe where you could point a spaceship in one direction and eventually return to where you started. If our universe were a finite donut, then such movements would be possible and physicists could potentially measure its size. Examining light from the very early universe, astrophysicists have determined that our cosmos may be multiply connected, meaning that space is closed in on itself in all three dimensions like a three-dimensional donut. Such a universe would be finite, and our entire cosmos might only be about three to four times larger than the limits of the observable universe, about 45 billion light-years away.

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