Final answer:
Mathematical solutions of Einstein's field equations indicate that the material universe has expanded in size over time, a conclusion supported by redshift observations and the theory of general relativity.
Step-by-step explanation:
According to mathematical solutions of Einstein's field equations for the theory of relativity, the material universe has expanded in size over time. This conclusion was reached following the observation of redshifts in distant galaxies, which indicated that the universe was not static but expanding. Initially, Einstein introduced the cosmological constant as a repulsive force to balance gravitational attraction and suggest a static universe; however, it was later understood that the universe indeed expands. Notably, understanding about dark energy and the mass density of the universe being less than the critical density suggest that the expansion of the universe is likely to continue indefinitely.