Particle horizon

Imagine you have a very big field, and you are standing in the middle. As far as your eyes can see, you can see the grass, trees, and other things. But if you go to a place that is far away from you, you won't be able to see it with your eyes. This is because it is outside of your "horizon" - which means the farthest point you can see.

Now, let's talk about space. Just like how you have a horizon on Earth, space also has a horizon. It is called the particle horizon. But instead of trees and grass, it is the distance beyond which we cannot see anything in space.

This is because the light has not had enough time to reach us yet. Light is the fastest thing we know, but it still takes time to reach us from far away. The particle horizon is the limit to how far we can see with the light that has had enough time to reach us since the beginning of the universe.

So, when scientists study the universe, they can only look as far as the particle horizon. Anything beyond it is a mystery to us. As time goes on, the particle horizon also expands, which means we can see more and more of the universe.