How will astronomers know when the universe stops expanding?

The expansion of the universe is regulated by gravity and something that we do not know and we call Dark Energy. The DA is accelerating the expansion of the universe and, until we do not understand what DE is, we will not be able to make predictions about the fate of the expansion. It can expand forever or stop, at a certain moment. Right now nobody knows.

The theory that matter and energy were eternal and would recycle between Big Bangs and Crushes required that the rate of expansion should slow down and eventually reverse; the supernova studies of 1997 and 1998 were conducted to confirm this theory.

The implications are that the universe will never stop expanding, that it is an open system, and that matter and energy are not eternal or self-existent. The studies showed that the rate of expansion of the universe is actually increasing rather than decreasing as predicted.

Astronomers would be able to determine whether the universe is stopping its expansion by examining the rates at which it is stopping. Based on these studies, it appears that the universe will continue to expand.

The rate at which the universe expands will be observed over time, and if it slows down and eventually stops, astronomers will be able to determine when the universe has reached its maximum size. This can be done by measuring the distances and velocities between galaxies and by examining the effects of dark energy, dark matter, and other cosmological parameters.