Answer :
Final answer:
Theories related to the universe's expansion describe boundaries that are ever expanding, dependent on whether the universe is open or closed, alongside the critical density and cosmological constant.
Explanation:
Like space, theories that defy classification have boundaries that are ever expanding. This expansion can be understood in the context of whether the universe is open or closed, concepts central to cosmological theories. The critical density of the universe and the cosmological constant are crucial in determining the universe's fate, whether it will expand forever or eventually stop. Theories related to the curvature of space, influenced by general relativity, discuss an open universe, which is negatively curved, suggesting that travel in any direction could, in principle, continue indefinitely. Conversely, a closed universe is positively curved, meaning traveling far enough in any direction might lead to returning to the original point. Therefore, the boundary conditions of theoretical models reflect the ongoing nature of cosmic expansion.
