General Relativity is a theory of gravity that explains the large-scale structure of the universe, while Quantum Cosmology is a theory that attempts to unify quantum mechanics and general relativity in order to understand the origin and evolution of the universe at a very small scale.
According to General Relativity, gravity is not a force between masses, but rather the curvature of spacetime caused by the presence of massive objects. This curvature causes the path of light to bend as it passes through the distorted spacetime, resulting in the observed phenomenon of gravitational lensing.
Currently, there is no complete theory that reconciles General Relativity and Quantum Cosmology. Many physicists believe that a theory of quantum gravity, which would merge these two theories, is necessary for a complete understanding of the universe.
General Relativity predicts that the universe is expanding due to the presence of dark energy, a mysterious force that is causing the expansion of the universe to accelerate. This theory has been supported by observations of distant galaxies and the cosmic microwave background radiation.
The Cosmological Constant was first introduced by Albert Einstein in his theory of General Relativity to explain a static universe. However, the discovery of the expanding universe led Einstein to abandon this constant. In recent years, the Cosmological Constant has been revived to explain the observed acceleration of the universe's expansion.