Final answer:
Physics dictates that the expansion of the universe and its critical density determine whether the universe will expand indefinitely or eventually contract. Current evidence indicates that the universe's mass density is below critical levels, suggesting continuous expansion, especially with the discovery of dark energy accelerating the expansion.
Step-by-step explanation:
The concept of the expansion of the universe is a fundamental principle in cosmology, which is a branch of astronomy and thus falls under the subject of Physics. The expansion rate of the universe and its relation to critical density determine the various possible outcomes for the long-term evolution of the universe. According to models, if the actual mass density of the universe is greater than this critical density, the universe will eventually stop expanding and begin to contract, leading to a closed universe. Conversely, if the density is less than the critical density, the universe will continue to expand indefinitely. Moreover, recent observations suggest that dark energy is causing the universe to expand at an accelerated rate, which indicates that we live in a universe that will continue to expand forever.
The critical density is the mass per unit volume that, if achieved, would result in the expansion rate slowing down and eventually halting after an infinite amount of time. However, current observational evidence implies the universe's mass density is below this threshold. The universe is considered homogeneous and isotropic on a large scale, meaning that it looks the same in every direction and at every place. This assumption allows us to use simplified models to describe the universe's expansion and predict its fate. Although earlier models suggested a potential for future contraction, the discovery of dark energy has led to a consensus that the universe will likely expand forever.