What are comoving distances?

Comoving distances are like the distance between two friends on a moving walkway at the airport, but you imagine they’re not walking, just staying still.

Imagine you and your friend are both standing on a long, slow-moving walkway. The walkway is moving forward, but neither of you is walking. If someone from outside looks in, it seems like you're both moving. But to you, the distance between you stays the same, even though everything around you is shifting.

That’s what comoving distances are like: they measure how far apart things are as if the universe wasn’t expanding. It's like saying, "If the universe had stopped growing, how far would it be from one place to another?"

When scientists look at the universe, it’s constantly getting bigger, like a balloon that keeps inflating. Things seem farther apart because of this growth. But comoving distances take that expansion into account and let us see what the real, constant distance would be if everything stopped growing.

It's like measuring how far apart you and your friend are on the walkway before it starts moving, so you can understand the true space between you, not just how it looks when everything is shifting.