While the popular theory of gravity, or general relativity, of Einstein boasts of success that has lasted for over a century, it does have its own theoretical shortcoming when it comes to explaining the Universe.
The theory of gravity has only been tested in gravity that is weak. This is unlike other physical theories that describe the three other fundamental physics forces, namely, the strong, weak, and electromagnetic nuclear interactions.
General relativity's gravity deviations are not tested nor excluded anywhere in the Universe. Theoretical physicists think that deviation is necessary.
Einstein held that the Universe started with the Big Bang. There are other singularities that can be found within black holes. Within these massive cosmic mysteries, time and space become meaningless, while pressure and energy density end up becoming infinite. These show that the theory of Einstein fails there and that a more fundamental theory should replace it.
Quantum mechanics should be able to resolve singularities in spacetime. Quantum physics typically depends on two ideas, namely, the Heinsenberg uncertainty principle that holds that no one can know a certain quantity pair's value with absolute accuracy and that point particles do not have sense.
This is sufficient enough to understand that such pathologies should not be present in a theory that embraces quantum physics and general relativity. However, Einstein's theory ends up with deviations when attempts are made to mix quantum physics and general relativity.
This means that the general theory of relativity proposed by Einstein cannot be the utmost theory of gravity. Interestingly, Arthur Eddington began looking for alternatives shortly after Einstein's theory was introduced. Eddington is known for verifying the theory during the solar eclipse in 1919.
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