There seems to be a confusion in your question. The theory of general relativity, developed by Albert Einstein, is not commonly referred to as "The Big Bang Theory." "The Big Bang Theory" is a term used to describe the prevailing scientific explanation for the origin and evolution of the universe, which involves an initial singularity and subsequent expansion.
Einstein's theory of general relativity, published in 1915, revolutionized our understanding of gravity and the nature of spacetime. It provides a mathematical framework to describe the gravitational interactions between masses and the curvature of spacetime. However, it does not directly address the origin of the universe.
The concept of the Big Bang as an explanation for the origin of the universe emerged much later, in the 20th century, based on observations and theoretical developments in cosmology. The term "Big Bang" was coined by astronomer Fred Hoyle in 1949 as a somewhat dismissive term for the idea. However, the name stuck and is now widely used to describe the prevailing scientific model for the early universe.
It is important to note that while Einstein's general relativity does not explicitly address the origin of the universe, it has been used to support the Big Bang theory. By applying the equations of general relativity to the early universe, scientists have developed models that suggest an initial singularity—a point of infinite density and temperature—from which the universe expanded.
It is also worth mentioning that Einstein initially introduced a cosmological constant term in his equations to account for a static, non-expanding universe. However, when it was later discovered that the universe was indeed expanding, Einstein abandoned the cosmological constant. The concept of an expanding universe was a precursor to the development of the Big Bang theory.
So, to summarize, Einstein's theory of general relativity is not directly referred to as "The Big Bang Theory." The term "Big Bang Theory" emerged as a separate scientific explanation for the origin and evolution of the universe, based on observations and theoretical advancements in cosmology.