Stephen Hawking was no stranger to asking and answering big questions throughout his life, but 20 years before his passing, he had turned his mind to arguably the most fundamental question: how had the universe developed in such a way that it could be “fine-tuned” for life?
When cosmologist Thomas Hertog met Hawking at Cambridge University in June 1998, forging a relationship that would last until the great physicist’s passing in March 2018, Hawking had realized that the theory of the Big Bang he had put forward in his book A Brief History of Time was flawed, according to Hertog. Hawking was looking for a better theory, a theory of cosmic evolution.
This quest to crack cosmic design led Hawking and his collaborator, Hertog, on a theoretical journey back to the very beginning of the universe, the Big Bang, through a multiverse burgeoning with universes replete with their own physical laws, to an idea that would take physics outside its comfort zone — the idea of a very quantum beginning.
As the title of his new book, On the Origin of Time: Stephen Hawking’s Final Theory, suggests, Hertog likens this voyage to the journey Charles Darwin took on the Beagle and the development of his theory of biological evolution that emerged as a consequence, which is famously documented in the not coincidentally similarly titled book, On the Origin of Species.
“What sets Hawking’s final theory apart is that it takes a fundamentally evolutionary perspective on physics and the laws of physics,” Hertog explained to Popular Mechanics. “Darwin didn’t need a zillion other planets to study biological evolution on this planet; he started from the observations, and he tried to reconstruct the Tree of Life. We did exactly the same, Stephen and I. We looked around and tried to reconstruct the history of the universe.”
What Hertog said he and Hawking had found themselves constructing was, in effect, a tree of physical laws that was forged in the very early stages of the universe by a process that is reminiscent of the variation and selection of Darwinian evolution.
“It is a little bit as if we are proposing a kind of Darwinian revolution in cosmology,” he said.
But what shape does this cosmological tree of life take?
The Holographic Tree of Life
Hawking and Hertog formulated an adaptation of holography to solve the problem of how time arose at what cosmologists consider the beginning of the universe, the Big Bang — a period of rapid inflation 13.8 billion years ago that set the cosmic clock running and before which there was no time.
Their idea differed from the concept of standard holograms which, for example, sees a three-dimensional hologram emerge from a two-dimensional surface or screen. This isn’t the first time that a holographic theory of the universe has been put forward. Over 25 years ago, Juan Maldacena of the Institute for Advanced Study in Princeton suggested that the four-dimensional universe could emerge as a holographic projection from a lower dimensional surface. The idea was featured in a 1997 paper by Maldacena and has beguiled physicists ever since.
Hertog’s and Hawking’s idea is different as it is very clear which of these dimensions is the emergent property of the hologram. They envision a 4D universe emerging from an abstract and timeless surface, with time being the emergent dimension, akin to the third dimension of a standard hologram that arises from an encoded 2D surface.
“Holography is, in general, the idea that one or the dimension of the world we perceive is emergent from some sort of more abstract picture of reality,” Hertog explained. “What Stephen and I did was apply that idea to the Big Bang, and what we discovered was when you do so, it is not the third dimension of space that is popping out. It is the dimension of time, which is the holographic dimension, the one which is abstractly encoded.”
Thus, when one looks back in time, say by looking at light from a distant galaxy that has traveled billions of years to reach us, this is akin to “zooming out” on the hologram and making its details fuzzier in the process. This zooming out can continue until all the details of the hologram disappear altogether, which in the model of the universe suggested by Hawking and Hertog, would be the origin of time at the Big Bang.
“The crux of our hypothesis is that when you go back in time, to this earliest, violent, unimaginably complicated phase of the universe, in that phase you find a deeper level of evolution, a level in which even the laws of physics co-evolve with the universe that is taking shape,” Hertog said. “And the consequence is that if you push everything even further backward, into the Big Bang, so to speak, even the laws of physics disappear.”
A Quantum Beginning, But No Multiverse
Because quantum physics describes quantum systems as waves, and waves can overlap, creating a so-called superposition, it is possible for a quantum system to theoretically exist in overlapping contradictory states: a quantum superposition.
In the quantum Big Bang, as seen by Hawking and Hertog, at the origin of time, the infant universe is in a superposition of possible states, each representing a different evolutionary path and the emergence of different physical laws.
One can only retrospectively look back at the evolutionary path the universe has actually taken. That means the universe is “fine-tuned” for life because only in such a universe in which stars and planets could be born could intelligent life emerge that is capable of pondering its own existence.
This idea of different universal paths of evolution may sound like a multiverse, and indeed it was the same question of how the universe became attuned for life that set scientists like Andrei Linde thinking about the multiverse in the first place. Hertog pointed out, however, that Hawking’s final theory is different from multiverse theories that see “eternal inflation” proceeding in different pockets of eternal inflation, creating distinct universes with different physical laws. This is because it requires looking back at time from within the universe.
“That is very different from the idea of a multiverse in which we behave as if we’re trying to look at the multiverse from outside it and then ask, ‘which of these many universes are we in?’” Hertog said.
This creates a limitation in Hawking’s final theory that doesn’t exist in multiverse formulations. The disappearance of the laws of physics means there is a fundamental limit, a horizon, to what scientists can know. If the multiverse of Linde and other scientists exists, these other universes, Hertog said, lie beyond this knowledge horizon.
Verifying Stephen Hawking’s Final Theory
Actually looking back in time to the Big Bang with standard astronomy is also impossible, though all astronomy is essentially akin to staring back in time because of the finite speed at which light travels.
This is because the infant universe, in its hot and dense state, was opaque to light. It was only when the universe had expanded and cooled around 300,000 years after the Big Bang that electrons could bond with protons to form atoms. The disappearance of free electrons during this era of the universe, known as the epoch of recombination, allowed particles of light — photons — to freely travel the cosmos, thus making the universe transparent.
The “first light” from this event, called the last scattering, is called the cosmic microwave background (CMB). The CMB near-uniformly fills the universe and represents a fossil relic of the limit on how far back astronomers can “see.”
“The Big Bang is hidden in the mist, so we probably have to find a different way to probe this early phase of the universe in which light could not even travel,” Hertog explained.
There is another form of radiation, however, that humanity is only just beginning to explore, known as gravitational waves, which are tiny ripples in the very fabric of spacetime caused by accelerating objects of great mass.
“Gravitational waves could reach us all the way from the beginning of the universe,” Hertog added. “And perhaps there are gravitational wave signals that can reveal something about that primeval phase in the universe during which the laws of physics were branching out.”
Hertog compares this to a cosmic fossil like the CMB but in gravitational waves rather than electromagnetic radiation, and one which predates the era of recombination.
“À la Darwin, we are fossil hunting, looking for fossils representing those early days of our universe,” Hertog said. “We are in the same boat as Darwin was in the 19th century; he did not have many fossils to recreate the tree of life, and we do not have many fossils to recreate the tree of physical laws. Hopefully, by the end of the century, we’ll be in a different place.”
In addition to documenting the final theory of Stephen Hawking, which he co-engineered, Hertog also describes many of his personal interactions with the legendary physicists in the book On the Origin of Time.
Popular Mechanics asked the cosmologist what was the most invaluable lesson he learned from Hawking. Hertog pondered this for a moment and then replied: “Never give up.”
Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Rob’s articles have been published in Newsweek, Space, Live Science, Astronomy magazine and New Scientist. He lives in the North West of England with too many cats and comic books.
