WHAT happened before the big bang? Cosmologists have long speculated that a universe much like ours could have collapsed in a big crunch and then bounced back into the universe we know. Now a theory that tries to reconcile the incompatible theories of general relativity and quantum physics has provided the first physically plausible model of how this could have happened. General relativity explains gravity as being caused by distortions in the fabric of space-time. But physicists have struggled to also explain gravity in terms of quantum mechanics, leaving it the only force that still lacks a clear quantum description.
A view of the universe before the big bang