Einstein's General Relativity is a theory of gravity proposed by Albert Einstein in 1915. It states that gravity is not a force between masses, but rather a curvature of space and time caused by the presence of mass and energy.
Einstein's Special Relativity, proposed in 1905, deals with the laws of physics in inertial frames of reference, while General Relativity extends these laws to non-inertial frames, such as those experiencing acceleration.
The Cosmological Constant, also known as Lambda, was originally introduced by Einstein to achieve a static universe. However, it was later discarded as unnecessary and is not included in the current formulation of General Relativity.
Einstein's General Relativity has been tested and confirmed through various experiments and observations, including the bending of light by massive objects, the precession of Mercury's orbit, and the gravitational redshift of light.
While Einstein's General Relativity has been incredibly successful in explaining and predicting many phenomena, it does have limitations. It does not account for quantum mechanics and cannot be used to describe the behavior of particles on a very small scale. Additionally, it does not account for the accelerating expansion of the universe, which has led to the development of theories such as dark energy and modified gravity.