Distances in an expanding universe become easier to understand, if you first consider a simple example. (There! I said it. First.

)

Consider a flat rubber sheet being stretched. You have marked two points on the sheet, A and B. You have a small toy car with a motor, so it moves at a fixed velocity with respect to the surface it is on. You set it down so it travels from point A to point B.

If the distance between A and B is L

_{0} when the car starts at A, and L

_{1} when the car arrives at B, the distance L the car actually traveled is between L

_{0} and L

_{1}.

The exact distance it did travel, depends on the rate of the expansion of the rubber sheet, the speed of the car, and the duration of the travel. In the case of our expanding universe, while the speed of light is constant, distances grow exponentially, because every point in space is expanding at the same rate. So, it's not like stretching the rubber sheet by moving the endpoints away from each other at a constant rate; you'd need to move the endpoints ever faster and faster. (Or, if you consider a balloon, inflate it faster and faster.)

The end result is that for short trips, L is approximately L

_{1}. The longer the trip, the closer L is to L

_{0}.