the cosmological principle (which says that basic physical properties are the same throughout the universe) and Albert Einstein’s General Theory of Relativity of 1915-1916, which conceives of gravity as a curvature in space and time.
nThe beginning before 13.7 billion yeas.
nThe Planck era was extremely brief—just 10-43 seconds
nDuring this period, all four forces of the universe—gravity, electromagnetic energy, and the weak and strong nuclear forces—were theoretically equal to one another, implying that there may have been just one unified force.
The Planck era was extremely unstable, with the four forces quickly evolving into their current forms, starting with gravity and then the strong nuclear force (what binds protons and neutrons together in the nucleus of an atom), the weak nuclear force (associated with radioactive decay, it is some 100 times weaker than the strong force), and finally electromagnetic energy.