The Big Bounce
In a very recent paper, Dr. Martin Bojowold of Penn State University suggested that our universe arose out of the collapse (or Big Crunch) of an earlier universe. So rather than a Big Bang arising out of a singularity, our universe may have begun with a Big Bounce.
There's nothing new about the underlying idea. It has been suggested before, both in science fiction and in cosmology. However, Bojowold has done the mathematics using Loop Quantum Gravity (which is an approach to unifying general relativity with quantum physics.) What was once just speculation now has a theoretical foundation.
This does not mean the idea has replaced the Big Bang, but it is possible that with time and further research it will.
If the Big Bounce does become the generally accepted theory, it answers the question of "What caused our universe." It negates the cosmological argument, at least with respect to our universe. Those that insist that a causeless event is not possible will have an immediate cause other than blindly pointing at a hypothetical deity.
Of course, what we still won't know is what (if anything) caused that preceding universe. Was it part of an infinite chain of collapsing and expanding universes? Had that preceding universe been in existence for an eternity before it collapsed? Was it an uncaused event? Or was there an even more unlikely "god" at some further remove?
- Martin Bojowald. What happened before the Big Bang? Nature Physics, July 1, 2007