What caused the Big Bang?

Jason Rhodes, Jet Propulsion Laboratory Colliding universes may have caused it. The cosmos is currently expanding, and in the distant past it was much smaller and denser. By observing radiation left over from the early days and other astronomical phenomena, we can see that the universe must have been infinitesimally small about 13.8 billion years ago. However, since our notions of time and space break down in such unimaginably dense environments, we're still trying to understand what could have caused this point to begin the expansion we see today.