In Quantum Mechanics, Quantum Mechanics has become more difficult due to the fact that our universe is expanding, rather than collapsing, being pushed apart by dark energy. This makes it harder for physicists to understand a universe that is expanding or static. However, this is the one our real world most resembles. In an expanding universe, physicists cannot square the world we experience with the way things work at the smallest levels. They are now using black holes to help them understand how quantum physics works within this expanding universe. The theory of general relativity, Einstein’s theory of gravity, suggests that space and time are linked to the evolution of the universe. Dutch physicist Willem de Sitter, who discovered that an empty universe with a cosmological constant, could take on three shapes depending on the sign of this constant, including a positive or negative sign, found that an expansion could be positively curved or negatively curved. In anti-de Sitter space, this expansion creates a horizon beyond which it is impossible to communicate with an observer living within this space. In this expanding space, the expansion creates light but only light can reach it.