Dear Dr. Universe,
Where does the universe end?
— Oriah, 8, Pullman
When you look up at the night sky, it can feel like the universe is a big blanket of stars above you. But unlike a blanket, the universe doesn’t have corners and edges. Far beyond what humans can see, the universe keeps going. As far as humans know, it never stops.
When I saw your question, I went straight to my friend Michael Allen to learn more. He is a senior instructor of physics and astronomy at Washington State University.
The universe is bigger than the biggest thing you’ve ever seen. It’s bigger than the biggest thing this cat can imagine. It’s so big that even your question has more than one very big answer.
Allen explained that you can think of the universe kind of like a rubber band. If you look at a rubber band’s flat surface, you can see it has no beginning and no end. It keeps going around and around in a loop.
Imagine you drew dots on that rubber band. If you pull on the rubber band, what happens? The rubber band stretches, and the dots move further apart. The universe is like that. The distance between all its galaxies, planets and stars is stretching all the time, like dots on a rubber band. It never ends, but it’s also constantly expanding.
Scientists don’t think there is a true edge of the universe. But there’s an end to what humans can see of the universe. This is called the edge of the observable universe. It’s the farthest we can see, based on how we get information from light.
Everything you see depends on light bouncing off objects. Light reflects off the things around you and your eye absorbs it. When you look at your hand, you see your hand in that exact moment.
But when you look at a star, you’re actually seeing that star in the past.
That’s because the light has to travel a very long time to reach your eyes. The farther away the star, the longer it takes. It takes light from the nearest star, the Sun, eight minutes to get to our eyes. Light from the next nearest star, Proxima Centauri, takes about four years to get to us.
Light moves very fast — about 186,000 miles per second — but the universe is very big. So the farthest edge of the observable universe is the oldest light we can see: about 13.8 billion years in the past.
But that edge is just what we can see from Earth. Earth isn’t the center of the universe. It’s just one location. The edge of the observable universe depends on where you are. If we were somewhere else in the universe, we would have a different view.No matter where you are, you can think of yourself as a time traveler of sorts. When you gaze up at the stars, you’re looking up at the past.