Could you travel in a straight line in space and return to Earth?

Could the Universe loop around on itself, and might you come back out the other side?

“For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return.” -Leonardo da Vinci

Back when people thought the Earth was flat, it was almost heretical to suggest that traveling in a straight line for a long enough distance would eventually bring you back to your starting point. But it’s true: travel about 40,000 kilometers (or 25,000 miles) in any one direction — over the mountains, oceans and any other terrain you ran across — and return you would to where you began. It makes you wonder if space could be the same way. If you got in a rocket ship and traveled fast enough for long enough, and didn’t ruin things by colliding with a distant star or galaxy, could you eventually return to where you started?

A simulation of the structure of the Universe. If you leave one “edge” of the Universe and return through another, you just might live in a recurrent Universe. Image credit: NASA, ESA, and E. Hallman (University of Colorado, Boulder).

It’s not as crazy as it sounds. We might think that the Universe is infinite, going on forever in all directions, but the evidence we have for the size and shape of the Universe is very much finite. For one, it’s only been 13.8 billion years since the Big Bang, and so we can only see the amount of space that 13.8 billion years of light traveling through the…