What Triggered the Big Bang?

Answer

That is a fundamental question to which we do not have a clear answer.

This can seem very confusing, since physics is about discussing effects from causes, and if we don’t know the cause, how dare we make a statement about the effect? If we come to the conclusion that there was a big bang, it is because of a coherent set of phenomena that indicate it, such as the expansion of the universe, the fact that we see an upper limit on the age of objects in the universe, the fulfilled prediction that there is a cosmological background of radiation, and the explanation it offers for the composition of matter. The big bang scenario has proven to be a very close paradigm for understanding the universe as a whole, but it does indeed raise questions on an even more fundamental level.

Of course, people are looking for answers to your question, in the sense that there is a physics that can frame the occurrence of big bangs in a deeper fundamental scheme. But even then, fundamental questions will arise about the meaning of that schema at an even more fundamental level. We are like little children who ask a new, further, why question for every answer to a why question. And since we ourselves, with our material reality and even our concepts, are products of the universe, it is probably an illusion to ever want to have the last word.

You ask your question in terms of concepts introduced by the universe, such as mass and energy. Also ‘sudden’ is not a neutral word, as it refers to an event in time, and that time too is a concept we have learned from our universe.