What propelled the Big Bang into motion?

A brief (10^(-32) sec) exponential growth period known as inflation preceded the Big Bang. This expansion is necessary to explain why the universe is 91 billion light years in size but is only 13.8 billion years old, meaning that two galaxies 20 billion years apart (one in front of us and the other behind) could not have communicated because they could not equalize their temperatures without being in contact. In 1979, Alan Guth proposed that a faster expansion was required, and that period of exponential growth became known as inflation.

Guth proposed that the universe was in an initial state that was not the state of lowest energy but was instead reached through quantum tunneling, which is the capacity a quantum system has to pass through an energy barrier and emerge on the other side, if the new state has lower energy. Particle physicists saw the universe cooling off from a high temperature symmetric state where all interactions are unified, down to our less symmetric universe, a process familiar from condensed matter physics.

The Big Bang occurred, and the universe expanded rapidly at first, then more slowly after that.

Even though current theories suggest that the universe underwent a rapid expansion from a highly dense and hot state, the precise trigger or cause of this initial expansion remains an open question in cosmology, the Big Bang's mechanism and exact cause are still the subject of theoretical research and debate among physicists.