On the Origin of Time

Stephen Hawking's Final Theory




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April 11, 2023 | ISBN 9780593452011

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About the Book

NEW YORK TIMES BESTSELLER • Stephen Hawking’s closest collaborator offers the intellectual superstar’s final thoughts on the cosmos—a dramatic revision of the theory he put forward in A Brief History of Time.

“This superbly written book offers insight into an extraordinary individual, the creative process, and the scope and limits of our current understanding of the cosmos.”—Lord Martin Rees

Perhaps the biggest question Stephen Hawking tried to answer in his extraordinary life was how the universe could have created conditions so perfectly hospitable to life. In order to solve this mystery, Hawking studied the big bang origin of the universe, but his early work ran into a crisis when the math predicted many big bangs producing a multiverse—countless different universes, most of which would be far too bizarre to ​harbor life.

Holed up in the theoretical physics department at Cambridge, Stephen Hawking and his friend and collaborator Thomas Hertog worked on this problem for twenty years, developing a new theory of the cosmos that could account for the emergence of life. Peering into the extreme quantum physics of cosmic holograms and venturing far back in time to our deepest roots, they were startled to find a deeper level of evolution in which the physical laws themselves transform and simplify until particles, forces, and even time itself fades away. This discovery led them to a revolutionary idea: The laws of physics are not set in stone but are born and co-evolve as the universe they govern takes shape. As Hawking’s final days drew near, the two collaborators published their theory, which proposed a radical new Darwinian perspective on the origins of our universe.

On the Origin of Time
offers a striking new vision of the universe’s birth that will profoundly transform the way we think about our place in the order of the cosmos and may ultimately prove to be Hawking’s greatest legacy.
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Praise for On the Origin of Time

“[A] wonderful book about Stephen Hawking's Hawking’s ‘biggest legacy’.”Spectator

“Truly mind-stretching . . . Immensely immensely rewarding.”—The Times

“Why is our universe the way it is? How did everything begin? How might it end? Thomas Hertog probed these overwhelming questions in collaboration with Stephen Hawking, achieving a privileged perspective into how, struggling against daunting physical odds, Hawking’s imprisoned mind yielded astonishing insights even in his later years. This superbly written book offers insight into an extraordinary individual, the creative process generally, and the scope and limits of our current understanding of the cosmos.”—Lord Martin Rees, Emeritus Professor of Cosmology and Astrophysics, University of Cambridge, and author of Just Six Numbers

“Like his mentor and colleague Stephen Hawking, Thomas Hertog has never shied away from being ambitious in theorizing about the universe. This sweeping book provides an accessible overview of both what we know about cosmology and some audacious ideas for moving into the unknown. It is an introduction to Hawking’s final theory, but also a glimpse into even grander theories yet to come.”—Sean Carroll, author of The Biggest Ideas in the Universe: Space, Time, and Motion

“Stephen Hawking’s final theory is lucidly explained in this splendidly accessible book. Author Thomas Hertog, one of Hawking’s closest collaborators, gives us a vivid insight into Hawking as both a brilliant physicist and an astonishingly determined human being.”—Graham Farmelo, Churchill College, University of Cambridge, and author of The Strangest Man

“A beautifully written, thought-provoking account of both the physics and the personalities involved in Hawking’s visionary struggle to comprehend the cosmos. Thomas Hertog has provided a fascinating insider’s view.”—Neil Turok, co-author of Endless Universe
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On the Origin of Time

Chapter 1

A Paradox

Es könnte sich eine seltsame Analogie ergeben, daß das Okular auch des riesigsten Fernrohrs nicht größer sein darf, als unser Auge.

A curious correlation may emerge in that the eyepiece of even the biggest telescope cannot be larger than the human eye.

—Ludwig Wittgenstein, Vermischte Bemerkungen

The late 1990s were the culmination of a golden decade of discovery in cosmology. Long regarded as a realm of unrestrained speculation, cosmology—the science that dares to study the origin, evolution, and fate of the universe as a whole—was finally coming of age. Scientists all over the world were buzzing with excitement about spectacular observations from sophisticated satellites and Earth-based instruments that were transforming our picture of the universe beyond recognition. It was as if the universe was speaking to us. These developments posed quite a reality check for theoreticians, who were told to rein in their speculation and flesh out the predictions of their models.

In cosmology we discover the past. Cosmologists are time travelers, and telescopes their time machines. When we look into deep space we look back into deep time, because the light from distant stars and galaxies has traveled millions or even billions of years to reach us. Already in 1927 the Belgian priest-astronomer Georges Lemaître predicted that space, when considered over such long periods of time, expands. But it wasn’t until the 1990s that advanced telescope technology made it possible to trace the universe’s history of expansion.

This history held some surprises. For example, in 1998 astronomers discovered that the stretching of space had begun to speed up around five billion years ago, even though all known forms of matter attract and should therefore slow down the expansion. Since then, physicists have wondered whether this weird cosmic acceleration is driven by Einstein’s cosmological constant, an invisible ether-like dark energy that causes gravity to repel rather than to attract. One astronomer quipped that the universe looks like Los Angeles: one-third substance and two-thirds energy.

Obviously, if the universe is expanding now, it must have been more compressed in the past. If you run cosmic history backward—as a mathematical exercise, of course—you find that all matter would once have been very densely packed together and also very hot, since matter heats up and radiates when it is squeezed together. This primeval state is known as the hot big bang. Astronomical observations since the golden 1990s have pinned down the age of the universe—the time elapsed since the big bang—to 13.8 billion years, give or take 20 million.

Curious to learn more about the universe’s birth, the European Space Agency (ESA) launched a satellite in May 2009 in a bid to complete the most detailed and ambitious scanning of the night sky ever undertaken. The target was an intriguing pattern of flickers in the heat radiation left over from the big bang. Having traveled through the expanding cosmos for 13.8 billion years, the heat from the universe’s birth reaching us today is cold: 2.725 K, or about –270 degrees Celsius. Radiation at this temperature lies mainly in the microwave band of the electromagnetic spectrum, so the remnant heat is known as the cosmic microwave background radiation, or CMB radiation.

ESA’s efforts to capture the ancient heat culminated in 2013 when a curious speckled image resembling a pointillist painting decorated the front pages of the world’s newspapers. This image is reproduced in figure 2, which shows a projection of the entire sky, compiled in exquisite detail from millions of pixels representing the temperature of the relic CMB radiation in different directions in space. Such detailed observations of the CMB radiation provide a snapshot of what the universe was like a mere 380,000 years after the big bang, when it had cooled to a few thousand degrees, cold enough to liberate the primeval radiation, which has traveled unhindered through the cosmos ever since.

The CMB sky map confirms that the relic big bang heat is nearly uniformly distributed throughout space, although not quite perfectly. The speckles in the image represent minuscule temperature variations indeed, tiny flickers of no more than a hundred-thousandth of a degree. These slight variations, however small, are crucially important, because they trace the seeds around which galaxies would eventually form. Had the hot big bang been perfectly uniform everywhere, there would be no galaxies today.

The ancient CMB snapshot marks our cosmological horizon: We cannot look back any farther. But we can glean something about processes operating in yet earlier epochs from cosmological theory. Just as paleontologists learn from stone fossils what life on Earth used to be like, cosmologists can, by deciphering the patterns encoded in these fossil flickers, stitch together what might have happened before the relic heat map was imprinted on the sky. This turns the CMB into a cosmological Rosetta Stone that enables us to trace the universe’s history even farther back, perhaps as far back as a fraction of a second after its birth.

And what we learn is intriguing. As we will see in chapter 4, the temperature variations of the CMB radiation indicate that the universe initially expanded fast, then slowed down, and, more recently (about five billion years ago), began accelerating again. Slowing down appears to be the exception rather than the rule on the scales of deep time and deep space. This is one of those seemingly fortuitous biofriendly properties of the universe, for only in a slowing universe does matter aggregate and cluster to form galaxies. If it hadn’t been for the extended near-pause in expansion in our past, there would, again, be no galaxies and no stars, and thus no life.

In effect, the universe’s expansion history was at the center of one of the very first moments in which the conditions for our existence slipped into modern cosmological thinking. This moment occurred in the early 1930s, when Lemaître made a remarkable sketch in one of his purple notebooks of what he called a “hesitating” universe, one with an expansion history much like the bumpy ride that would emerge from observations seventy years later (see insert, plate 3). Lemaître embraced the idea of a long pause in the expansion by considering the universe’s habitability. He knew that astronomical observations of nearby galaxies pointed to a high expansion rate in recent times. But when he ran the evolution of the universe backward in time at this same rate, he found that the galaxies must all have been on top of one another no more than a billion years ago. This was impossible, of course, for Earth and the sun are much older than that. To avoid an obvious conflict between the history of the universe and that of our solar system, he imagined an intermediate era of very slow expansion, to give stars, planets, and life time to develop.

In the decades since Lemaître’s pioneering work, physicists have continued to stumble across many more such “happy coincidences.” Make but a small change in almost any of its basic physical properties, from the behavior of atoms and molecules to the structure of the cosmos on the largest scales, and the universe’s habitability would hang in the balance.

Take gravity, the force that sculpts and governs the large-scale universe. Gravity is extremely weak; it requires the mass of Earth just to keep our feet on the ground. But if gravity were stronger, stars would shine more brightly and hence die far younger, leaving no time for complex life to evolve on any of the orbiting planets warmed by their heat.

Or consider the tiny variations, one part in a hundred thousand, in the temperature of the relic big bang radiation. Were these differences slightly larger—say one part in ten thousand—the seeds of cosmic structures would have mostly grown into giant black holes instead of hospitable galaxies with abundant stars. Conversely, even smaller variations—one millionth or less—would produce no galaxies at all. The hot big bang got it just right. One way or another it set off the universe on a supremely biofriendly trajectory, the fruits of which would not become evident until several billion years later. Why?

Other examples of such happy cosmic coincidences abound. We live in a universe with three large dimensions of space. Is there anything special about three? There is. Adding just a single space dimension renders atoms and planetary orbits unstable. Earth would spiral into the sun instead of tracing out a stable orbit around it. Universes with five or more large space dimensions have even bigger problems. Worlds with only two space dimensions, on the other hand, may not provide enough room for complex systems to function, as figure 3 illustrates. Three dimensions of space seems just right for life.

Moreover, this uncanny fitness for life extends to the universe’s chemical properties, which are determined by the properties of elementary particles and the forces acting between them. For example, neutrons are a tad heavier than protons. The neutron-to-proton mass ratio is 1.0014. Had it been the other way around, all the protons in the universe would have decayed into neutrons shortly after the big bang. But without protons there would be no atomic nuclei and hence no atoms and no chemistry.

About the Author

Thomas Hertog
Thomas Hertog is an internationally renowned cosmologist who was for many years a close collaborator of the late Stephen Hawking. He received his doctorate from the University of Cambridge and is currently professor of theoretical physics at the University of Leuven, where he studies the quantum nature of the big bang. He lives with his wife and their four children in Bousval, Belgium. More by Thomas Hertog
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