I heard that the universe is expanding, but sometimes I also hear that the universe will shrink back. Is that also the case or does the universe continue to grow?

The universe is expanding. It will continue to expand if what is holding back expansion overtakes what is supporting expansion, of course.

What’s stopping the expansion? Just the gravitational attraction of all that mass that is in the universe.

What sustains the expansion?

– The expansion itself, or at least its inertia. If we add up how much mass we see and can guess, and measure the extent of expansion, then that mass appears to have too little attraction to stop the expansion. Expansion forever.

– It has gotten worse since 1999. Then convincing evidence has been found for what is now often called ‘dark energy’, an additional repulsive term that accelerates the expansion of the universe.

So the situation today strongly suggests that the expansion will not stop, and on the contrary, it will accelerate. So no indication of an eventual collapse.

And yet: making a prediction is dangerous, especially when it comes to the future. Because as long as we don’t fully understand what that dark energy is, uncertainty will remain. You can always claim that one day it will change sign and turn everything around, although that seems very ad hoc today.

The expansion or contraction of the universe has no influence whatsoever on what happens to the earth and the other planets around the sun. That is very local, and before the universe is really small again, we will have long disappeared.