Albert Einstein did not make any definitive statements about the beginning of the universe as described by the Big Bang theory, which gained widespread acceptance after his lifetime. When Einstein formulated his theory of general relativity, the prevailing view was that the universe was static and unchanging.
However, after the discovery of the expanding universe by Edwin Hubble in the 1920s, Einstein modified his equations by introducing the cosmological constant to accommodate a static universe. This adjustment was a way to counteract the gravitational collapse predicted by his original equations.
Later, when evidence for the expanding universe became more robust, Einstein abandoned the cosmological constant, considering it a mistake. He admitted that the introduction of the cosmological constant was his "greatest blunder."
While Einstein's theory of general relativity provides a framework to understand the dynamics of the expanding universe, it does not offer a detailed explanation of its origins or the precise events at the beginning. The Big Bang theory, which suggests that the universe originated from an extremely hot and dense state around 13.8 billion years ago, emerged later as a result of observations and theoretical developments by scientists like Georges Lemaître and others.
Einstein's contributions to cosmology primarily revolved around developing the mathematical framework of general relativity, which describes the gravitational interactions on a large scale. Explaining the precise nature of the universe's beginning fell outside the scope of his work.