Albert Einstein Professor in Science, Departments of Physics and Astrophysical Sciences, Princeton University; coauthor: Endless Universe
The Big Bounce

2016 was the Year of the “Big Bounce.  

Everyone has heard of the Big Bang, the idea that, about fourteen billion years ago, the universe emerged from nothingness through some sudden quantum event into an expanding space-time filled with hot matter and radiation. Many know that the Big Bang alone cannot explain the remarkably uniform distribution of matter and energy observed today or the absence of curves and warps that one might expect after a sudden quantum event. In order to account for these observationsan enhancement has been added, a brief epoch of superluminal expansion, known as inflation, that immediately follows the bang. Inflation added to the Big Bang was supposed to explain how the turbulent and twisted conditions following a bang could have been stretched out, leaving behind a smooth universe except for a pattern of tiny variations in temperature and energy.  

But neither the Big Bang nor inflation are proven ideas, and there are good reasons to consider an alternative hypothesis in which the Big Bang is replaced by a Big Bounce. In a universe created by a Big Bang followed by inflation, there immediately arises a number of obvious questions: 

  • What caused the Big Bang?  

  • If the Big Bang is a quantum-dominated event in which space and time have no certain definition, how does the universe ever settle down to a classical space-time described by Einstein’s theory of general relativity in time for inflation to begin? 

  • Even if the universe manages to settle into a classical space-time, why should it do so in the exponentially fine-tuned way required for inflation to begin? 

  • Why don't we observe large-amplitude gravitational waves? If some way were found for inflation to begin, one would expect that the same high-energy inflationary processes that produce fluctuations in temperature and density also generate gravitational waves with large enough amplitude to have been detected by now. 

  • How does inflation end? The current idea is that inflation is eternal and that it transforms the universe into a “multi-mess” consisting of infinitely many patches or universes that can have any conceivable properties with no principle to determine which is more probable.  

All of these questions that have been known for decades and that theorists have failed to answer despite best efforts immediately become moot if the Big Bang is replaced by a Big Bounce. The universe need not ever be dominated by quantum physics and the large-scale structure of the universe can be explained by non-inflationary process that occurred during the period of contraction leading up to the bounce. This includes avoiding the multi-mess and producing fluctuations in temperature and density without producing large-amplitude gravitational waves or isocurvature fluctuations that would conflict with observations. 

If that is the case, then why haven’t astrophysicists and cosmologists jettisoned the Big Bang and embraced the Big Bounce? The answer in part is that many astrophysicists and cosmologists understand inflation as it was first introduced in the 1980s, when it was sold as a cure-all, and do not appreciate how thorny the questions listed above really are. But perhaps the bigger reason is that, before 2016, there was no theory of the bounce itself and so no Big Bounce theory to compare to. Attempts to construct examples of bounces consistent with quantum physics and general relativity generally led to instabilities and mathematical pathologies that made them implausible. Some even believed they could prove that bounces are impossible. 

2016 was the Year of the Big Bounce because, depending on how one counts them, at least four different theories for producing a stable, non-pathological bounces have been introduced by different groups around the world. Each uses different sets of reasonable assumptions and principles and each suggests a smooth transition from contraction to expansion is possible. This is not the place to go into details, but let me briefly describe one specific case discovered by Anna Ijjas and me at the Princeton Center for Theoretical Science earlier this year. In this theory of the Big Bounce, quantum physics is always a minor player, even near the bounce. At each moment, the evolution of the universe is well-described by classical equations that are well-defined and can be solved on a computer using the same sort of tools of numerical general relativity that were introduced to study mergers of black holes and employed in the recent discovery of gravitational waves by the LIGO collaboration. With this approach, the Big Bounce becomes calculable and prosaic.  

And once one knows that a Big Bounce is possible, it is hard to go back to considering the Big Bang again. The notion that time has a beginning was always strange, and, as the list above illustrates, it has created more problems in explaining the universe than it has solved.  

The time is ripe for the Big Bounce to become the new meme of cosmology.