In my field of fundamental physics and cosmology the idea most ready for retirement is that the big bang was the first moment of time.
In popular parlance the big bang has two meanings. First, big bang cosmology is the hypothesis that our universe has been expanding for 13.7 billion years from an extremely hot and dense primordial state-more extreme than the centre of a star or indeed anywhere now existing. This I have no quarrel with-it is established scientific fact which has been elaborated into a detailed story which narrates the expansion of the universe from an extremely uniform and dense hot plasma to the beautifully varied and complex world that is our home. We have detailed theories which pass numerous observational tests which explain the origins of all the structures we see from the elements to galaxies, stars, planets and the molecular building blocks of life itself. As in any good scientific theory there are questions still to be answered, such as the precise nature of the dark matter and dark energy which are prominent actors in the story, or the very interesting question of whether there was a very early phase of inflationary exponential expansion, but these do not suggest the basic picture could be wrong.
What concerns me is the other meaning of the big bang, which is the further hypothesis that the ultimate origin of our universe was a first moment of time at which our universe was launched from a state of infinite density and temperature. According to this idea, all that exists or has ever existed is 13.7 billion years old. It makes no sense to ask what was before that because, before that, there was not even time.
The main problem with this second meaning of big bang is that it is not very successful as a scientific hypothesis because it leaves big questions about the universe unanswered. It turns out that our universe has to have started off in an extraordinarily special state for the universe to evolve to anything like our universe. The hypothesis that there was a first moment of time turns out to be remarkably generic and unconstraining as it is consistent with an infinite number of possible states in which the universe might have started out. This is due to a theorem proved by Hawking and Penrose, that almost any expanding universe described by general relativity has such a first moment of time. Compared to almost all of these, our own early universe was extraordinarily homogeneous and symmetric. Why? If the big bang was the first moment of time there can be no scientific answer because there was no before on which to base an explanation. At this point theologians see their opening and indeed have been lining up at the gates of science to impose their kind of explanation-that god made the universe and made it so.
Similarly, if the big bang was the first moment of time there can be no scientific answer to the question of what chose the laws of nature. This leaves the field open to explanations such as the anthropic multiverse which are unscientific because they call on unobservable collections of other universes and make no predictions by which their hypotheses might be tested and falsified.
There is however a chance for science to answer these questions, which is if the big bang was not the first moment of time, but rather a transition from an earlier era of the universe-an era that can be investigated scientifically because processes acting then, through the time before the big bang, gave rise to our world.
For there to have been a time before the big bang, the Hawking-Penrose theorem must fail. But there is a simple reason to think it must, which is that general relativity is incomplete as a description of nature because it leaves out quantum phenomena. Unifying quantum physics with general relativity has been a major challenge for fundamental physics, one on which there has been much progress in the last thirty years. In spite of the absence of a still definitive solution to the problem, there is robust evidence from quantum cosmology models that the infinite singularities that force time to stop in general relativity are eliminated, turning the big bang-in the sense of a first moment of time-into a big bounce, which allows time to continue to exist before the big bang, deep into the past. Detailed models of quantum universes show a prior era ending with a collapse, where the density increases to very high values but, before the universe becomes infinitely dense, quantum processes take over which bounce the collapse into an expansion-launching a new era that could be our expanding universe.
There are presently several scenarios under study for what happened in the era before the big bang and how it transitioned to our expanding universes. Two hypothesize a quantum bounce-and go under the name of loop quantum cosmology and geometrogenesis. Two others-due respectively to Roger Penrose and Paul Steinhardt with Neil Turok, describe cyclic scenarios in which universes die giving rise to new universes. A fifth posits that new universes are launched when quantum effects bounce black hole singularities These scenarios offer insights as to how the laws of nature that govern our universe might have been chosen, and may explain also how the initial conditions of our universe evolved from the previous universe. The important thing is that each of these hypotheses make predictions for real, doable observations by which they might be falsified, and distinguished from each other.
During the Twentieth Century we learned a great deal about the first three minutes of our expanding universe (in Steven Weinberg's phrase). During this century we can look forward to gaining scientific evidence of the last three minutes of the era before ours, and learning how physics before the big bang gave rise to the birth of our universe.