The theoretical physicist from Caltech, Sean Carroll gave a talk to the American Astronomical Society in January 2017 on the topic of what we know and don't know about the beginning of the universe. He has generously posted a copy of his presentation on his blog, preposterousuniverse.com. In this talk, Dr. Carroll speculates about how the gaps in what we don't know may be filled in by presenting a systematic classification of the main ideas developed over the last few years about what may have occurred before our universe began and brought our universe into existence. (Dr. Carroll does point out that to say our universe "came into existence" sounds like a process within time, but that time as we know it actually had a beginning with our universe.) In previous posts I have already discussed many of the things Dr. Carroll covers in his talk including (1) that our universe was in a state with very low entropy at its beginning, (2) that something like the Big Bang occurred about 13.8 billion years ago but we don't know what actually happened in the first trillionth of a trillionth of a trillionth of a second because (3) we don't have a quantum theory of gravity which may describe the initial conditions of our universe, even though (4) the equations of classical general relativity predict that our universe had an actual beginning.
Let's review what we do know: (1) About 13.8 billion years ago the universe was very hot and dense and was expanding rapidly while decelerating; (2) classical general relativity predicts that there was an actual beginning of our universe in a singularity; and (3) our early universe was in a very low entropy state which is quite hard to explain since low entropy is associated with an ordered, and improbable state. The last point presents tremendous challenges for any naturalistic proposal about how our universe came into existence.
In regards to what we don't know, Dr. Carroll presents four different classes of models about the space-time origin of our universe: (1) a bouncing model, (2) a cyclic model, (3) a hibernating model, or (4) a reproducing model.
