The ultimate fate of the universe is a topic of intense scientific investigation and speculation. Whether the universe will stop expanding or continue to expand indefinitely depends on its overall energy density and the nature of dark energy. The possibilities can be broadly classified into three scenarios: a closed universe, an open universe, or a flat universe.
Closed Universe: If the overall energy density of the universe is high enough, gravitational forces may eventually overcome the accelerating expansion and cause the universe to stop expanding. In this case, the universe would eventually start contracting, leading to a "Big Crunch" where all matter and energy in the universe would collapse into a singularity.
Open Universe: If the energy density is low, or if dark energy's repulsive effect continues to dominate, the universe may expand forever, never coming to a halt. In this scenario, the universe would continue to expand and cool down indefinitely, eventually resulting in a state called the "Heat Death" or "Big Freeze." In this state, all stars would burn out, galaxies would become isolated, and the universe would become cold and dark.
Flat Universe: The current observations suggest that the universe is very close to flat, which is consistent with the idea of a universe that expands forever, but at a gradually decelerating rate. In this case, the expansion would continue but approach an asymptotic value, where the rate of expansion becomes extremely slow but never actually stops.
Determining which scenario will unfold depends on precise measurements of the universe's energy density, including dark energy, as well as the ongoing observations and experiments in cosmology. At present, the expansion of the universe shows no signs of stopping, and the most recent observational data supports the idea of an accelerating expansion due to dark energy.
As for the time scale, it is challenging to estimate precisely when any of these scenarios might occur. The rate of expansion and the influence of dark energy depend on its properties, which are still not fully understood. Therefore, predicting the future of the universe and the fate of its expansion remains an active area of scientific investigation.