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 PhD 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.
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