Both quantum theory, which governs the subatomic realm, and Einstein’s general relativity, which describes reality at cosmic scales, are often viewed as the most important developments in 20th-century physics. But there is another finding on par with these breakthroughs: the discovery the universe is expanding and must have originated at a finite time in the past, a moment now called the big bang.
General relativity and quantum theory both became vital tools for exploring how the universe evolves. They sparked new ways to understand how galaxies, stars, planets and ultimately living creatures came into being. Yet even in the bright light of these two revolutionary theories, the big bang’s origins and earliest moments have remained shrouded in mystery. Any satisfactory explanation, it seems, would have to somehow reconcile the sometimes contradictory tenets of quantum theory with those of general relativity—while also explaining why so many observed properties of elementary particles, forces and fields appear to be fine-tuned to produce the rich diversity of phenomena in the universe we know.
For celebrated late physicist Stephen Hawking, solving these mysteries was an obsession—one he shared with his closest friends and collaborators including Thomas Hertog, a theoretical physicist who obtained his Ph.D. at Cambridge University under Hawking’s supervision with a thesis on the origin of the expansion of the universe. Today Hertog is a professor at the University of Leuven in Belgium (which is also the alma mater of Georges Lemaître, the astronomer and Roman Catholic priest who first introduced the idea of an expanding universe in 1927). Most recently Hertog was also the co-author of what has been widely reported as Hawking’s final paper: a study titled “A Smooth Exit from Eternal Inflation,” which was completed shortly before Hawking’s death and addresses how the universe might have begun.
A few days after the publication of their joint paper on April 27 in the Journal of High Energy Physics, I met with Hertog in his office at the University of Leuven. We discussed the origins and conclusions of their paper—as well as the nature of its novel methods—that include findings from string theory, one of the most dominant emerging paradigms in 21st-century physics.
[An edited transcript of the interview follows:]
We are all familiar with the big bang theory, which involves a kind of explosion that blossomed into the universe. What inspired you and Hawking to use a new and different approach to looking at the big bang?
The old big bang theory as developed by Georges Lemaître, [George] Gamow and others is based on Einstein's theory of general relativity. In this formulation the big bang was literally the beginning of time. But you could not say anything about what happened at the very beginning. The big bang was what people called a singularity—a point of infinite mass and density for which existing theories fall apart. Relativity was very good at describing the evolution of the universe once it existed but it couldn’t say anything about its origin, although it implied there was an origin. In a sense [Einstein’s theory of general relativity] predicted its own downfall. In our last paper we used quantum theory to get a handle on the beginning.
Why is it so important that we understand how the big bang happened?
The way cosmological history unfolds and how the laws of nature come into existence is heavily dependent on how exactly the universe got going. The laws of nature are not some sort of Platonic construct, separate and outside of reality. They have emerged as the universe expanded and cooled—and the way that happened depends very much on the precise physical conditions of the big bang. Now it happens that the universe we find ourselves in seems to be very delicately fine-tuned in order for complexity and life to emerge. This fine-tuning can be traced all the way back to the big bang, nearly 14 billion years ago. So something very special must have happened at that initial moment. We want to know what it was.
Could the idea of inflation—the enormous and sudden accelerated expansion of the universe after the big bang—help reveal those details?
Yes, and our theory is a possible completion of inflation. It explains how inflation could have started in the first place. Inflation in the early universe solves some major puzzles such as why the universe is large and uniform—and yet not completely uniform. The answer from inflation is that the near-uniformity is due to inflation’s amplification of quantum fluctuations in the early universe. We can still see signs of these inflated fluctuations today, in patterns of tiny variations in the big bang’s afterglow—the cosmic microwave background, emitted when the universe was only 380,000 years old. These variations went on to seed the formation of the galaxies.
Another consequence of inflation, it seems, is the creation of a multiverse; that is, of a universe incomprehensibly larger in extent than what we see within our own cosmic horizon—a universe that is infinite. Other, far-distant regions of this multiverse beyond our cosmic horizon could have different physical laws, and would be too far separated from us for any communication or interaction to take place. What are your thoughts on that?
The problem is that inflation tends to work a little too well. Once it starts it is hard to stop everywhere, at least so it was thought. If inflation is eternal, if it keeps going forever, somewhere this leads to an ever-increasing amount of space growing at an exponential rate, dotted with an infinite number of “pocket universes” growing more slowly. This is the picture that our observable universe is not all that exists, but rather [is] one pocket of infinitely many universes, forming a multiverse. As you suggested, things like the values of certain key physical constants could vary randomly among the pocket universes, which would render moot any effort to get a deeper understanding of why our own observable universe is the way it is.
In our recent work, however, we argue there is no eternal inflation and that instead our universe is approximately uniform on the largest scales.
How did you arrive at this conclusion?
First we used string theory, a theoretical framework in which the elementary pointlike particles are replaced by strings. One of the vibrational states of the string corresponds to the graviton, the quantum-mechanical particle carrying the gravitational force. This suggests string theory constitutes at least a step towards a theory of quantum gravity. We evolved our universe backwards in time in our theory, and arrived at the singularity—the moment at which Einstein’s equations break down. Rather than relying on Einstein’s theory, at this point we used a relatively new concept from string theory, called the holographic principle, to project out the time dimension and view the entire situation in a timeless fashion. [Editor’s note: The holographic principle, in a nutshell, is the notion our reality may in fact be a hologram—that is, time, space and all its contents are reducible to information encoded on a two-dimensional surface at the boundary of our observable universe.] We find this novel holographic viewpoint of the earliest phase of the universe does not lead to eternal inflation and a multiverse. Instead, a more or less unique and uniform universe emerges. Our new theory of “cosmogony,” in contrast with the multiverse idea, makes definite predictions, which should in time enable it to be tested—at least to some extent. It therefore offers the hope we can achieve a deeper understanding of what makes our universe special and habitable.
How do you counter critics of string theory, who argue it cannot be tested?
I don’t agree with this statement; it is not my intuition that string theory can’t be tested. We may already have observations based on studies of the universe’s large-scale structure and evolution that are telling us something about the nature of quantum gravity. Of course, further theoretical work will be needed to arrive at a mathematically rigorous, fully predictive framework for cosmology.
So, your paper’s key predictions depend on the reality and nature of inflation. Will that be testable?
There are the obvious observables, yes. Just as it amplified tiny quantum fluctuations in the early universe, inflation should have amplified gravitational waves in the early universe, too. Gravitational waves are ripples in spacetime, first predicted by Einstein, that were finally observed just a few years ago—but the ones we have observed come from black holes and other stellar remnants in neighboring galaxies, not from the primordial universe. These amplified gravitational waves would leave their imprint on the polarization of the cosmic microwave background. Astronomers are actively trying to detect this polarization pattern.
So you are optimistic they will succeed?
Well, our theory certainly predicts that primordial gravitational waves should be there at some level.
The model that Stephen and I proposed in our final paper only deals with a very small sector of physics. We don’t talk about particle physics. In the end, in a complete cosmology this will have to be incorporated. I am confident, however, that our work will lead to further predictions that can be tested.
Recently, in some of the press your joint research is mainly referred to as Hawking’s work.
Yes, I have seen press reports referring to our joint work as “Hawking’s final theory of the big bang.” This is understandable, and I appreciate this final tribute to my late friend and mentor. As a matter of fact, Hawking and I, often together with our fellow physicist James Hartle, have always worked as a team. We never had any notion of a “leading” author. And so in keeping with the tradition in our domain of research, we listed the authors in alphabetical order on our papers.
Notwithstanding his physical limitations, Hawking was able to do great physics. What was his secret?
I certainly think intuition played a more prominent role in his work and his thinking than with many of our colleagues—by necessity. By intuition, I also mean his ability to ask the right questions. It is as if he could sort of distance himself a little bit from the messy calculations.