None of them.

However, if this is from an American book, the answer will be Hubble.

Father Georges Lemaitre was working with Hubble when they noticed all galaxies appeared to be moving away from each other. The first explanation that was tried was that of an explosion (1927).

However, Georges Lemaitre built a mathematical model to show that space itself had to change in the presence of mass. Either it had to contract or to expand.

Given the observations he had made with Hubble, the easy conclusion was "expand".

Einstein had also built a mathematical model for the behaviour of space, but he had added a "cosmological constant" to prevent the model from predicting an expansion (Einstein wanted space to be static).

In the early 1930s, Father Lemaitre had a discussion with Einstein and, at the end, they both agreed that space was expanding.

It is Lemaitre's mathematical model that later became the Big Bang theory.

However, all it says is that SPACE expands. We do not know that the universe expands.

There are some geometrical configurations under which the universe does not need to expand even if it is already "full" of expanding space.