Here's what we know:

* Einstein initially rejected the Big Bang. He believed the universe was static and eternal, a view he even tried to enforce into his equations with the introduction of the cosmological constant. This was a theoretical "fudge factor" to counterbalance gravity and keep the universe from collapsing.

* He later came around to the expanding universe idea. This was largely influenced by Edwin Hubble's observations in the 1920s showing galaxies moving away from each other. This discovery, along with theoretical work by others, provided strong evidence for an expanding universe.

* Einstein's acceptance of the expanding universe didn't necessarily mean he fully embraced the Big Bang theory. The Big Bang, as we know it, was proposed by Georges Lemaître in 1927, and fully fleshed out later by George Gamow and his colleagues. While Einstein acknowledged the expansion, he never publicly endorsed the Big Bang theory in its entirety. He was also wary of the idea of a beginning of the universe, as it seemed to conflict with his understanding of physics.

In summary:

* Einstein did eventually accept the expanding universe, but it wasn't a complete acceptance of the Big Bang theory.

* He was hesitant to fully embrace the idea of a universe with a beginning.

* His initial rejection of the expanding universe was driven by his belief in a static universe, but later observations and theoretical work led him to change his mind.

It's important to remember that Einstein's views evolved over time, and he was always open to new ideas and evidence. While he wasn't the originator or strongest advocate of the Big Bang theory, he certainly played a role in the scientific understanding of the universe's expansion, which is a fundamental pillar of the Big Bang model.