Within the standard model of general relativity, an interesting fundamental question can be asked. The answer to this question does not alter the model; it just sees it in another light. Yet it is very important to see the model in this other light, as we might be in danger to forget to interprete our mathematical models according to reality.
‘Expanding universe’ is a mathematical model. So long as we are using general relativity (which is very well tested and useful), the universe looks like its space coordinates are expanding. In the standard model the space coordinates are expanding with elapsed cosmic time. Cosmic time is the time measured by the clock of an observer at rest in coordinate system of the expanding universe. An observer like that sees the cosmic microwave background as being uniform in all directions. All cosmic time clocks remain synchronized with each other as the universe expands, so they are the standard we use to measure the age of the universe.
Yet Einstein proved the phenomenon of time dilation. Time dilation has been demonstrated by flying clocks around the world, and by measuring the different times at different distances from the Earth. Therefore, if we do not correct for time dilation by using some logaritmic time scale, but use synchronized time, of course we’ll see the effect in space coordinates, and then in the mathematical model, it looks like the universe is expanding…but is it really?
It might seem to make no difference; in the usefullness of the mathematical model it is of course irrelevant. But to actually view the universe as expanding, while it could just as well be finite with borders formed by infinitely slow time, that does make a difference in the way we view reality.