Thank you for the opportunity to speak here.
I would like to offer some thoughts on our understanding of our place in the universe in the light of a fundamental paradigm shift that is unfolding in theoretical cosmology. This shift concerns the epistemic roots of big bang cosmology. It has emerged from my work on quantum cosmology with Jim Hartle and Stephen Hawking, which I have elaborated on in my book On the Origin of Time (Hertog, 2023).
The idea that time began in a big bang goes back to the discoveries of Georges Lemaître, the illustrious former President of this Academy. Lemaître famously predicted the expansion of the universe on the basis of Einstein’s theory of general relativity. Tracing the expansion backward, he speculated that time must have had a genuine origin in what he called a primeval atom. Remarkably, Lemaître realized back then that it required a rather special history of expansion, in casu an extended period of very slow expansion, for the universe to be habitable.
Since those early days, the strange biophilic character of the universe has become even more intriguing. We now understand that the effective laws of nature have a long list of life-engendering properties, from the strength of the particle forces to the composition of the universe, to the size of the fluctuations in the microwave background. Twiddle ever so slightly with any of these and habitability would often hang in the balance. It is almost as if the cosmos has known all along that one day, it would be our home. What should we make of this?
Traditionally, most scientists from Einstein to the earlier Hawking, regarded the mathematical relationships that underpin the laws of physics as transcendental Platonic truths. In which case, the answer to the riddle of cosmic design – to the extent that it is an answer – is that it is a matter of mathematical necessity. The universe is the way it is because nature had no choice.
Around the turn of the 21st century, an entirely different explanation emerged. This one had its roots in a series of surprising discoveries that suggested that at least some properties of the physical laws might not be carved in stone, but instead be the accidental outcome of the particular manner in which the early universe cooled after the big bang. From the species of particles to the strength of forces to the amount of vacuum energy, it became apparent that the universe’s biofriendly laws were forged in a series of random transitions during its earliest moments of expansion. Reasoning along these lines, cosmologists began to envisage a multiverse, an enormous inflating space with a variegated patchwork of island universes, each with its own physics. This led to a sweeping change of perspective on the idea of our universe being fine-tuned for life. Even though most universes would be sterile, in some the laws of nature are bound to be just right for life. Multiverse aficionados argued that it is not a profound mathematical truth that renders the universe biofriendly, but simply excellent local cosmic weather.
Yet there is a problem buried in this reasoning: The multiverse is itself a Platonic construct. Multiverse cosmology postulates immutable metalaws governing the whole. But these metalaws do not specify in which one of the many universes we are supposed to be. Without a rule that relates the metalaws of the multiverse to the local laws within our island universe, the theory gets caught in a spiral of paradoxes that leaves one without verifiable predictions at all. Invoking the anthropic principle hardly helps, because your anthropic perspective might select one biofriendly patch, with this set of properties, whereas mine might pick out another patch, with a different set of local laws, and no objective rule at hand to decide which one is correct.
In the light of this conundrum, Hawking grew increasingly skeptical of the multiverse as the basis of a solid scientific paradigm. Can we do better? Yes, we found out, but only by relinquishing the idea, inherent in multiverse cosmology, that our theories can take a God’s-eye view, as if somehow standing outside the cosmos. It is an obvious and seemingly tautological point: our cosmological theory must account for the fact that we exist within the universe. “We are not angels who view the universe from the outside,” the later Hawking proclaimed. So we set out to turn cosmology inside-out and rethink its basic framework from an observer’s perspective.
This required adopting a quantum outlook onto the universe. The key role of the observer has been recognised since the discovery of quantum theory. Before a particle’s position is observed, there is no sense in even asking where it is. It doesn’t have a definite position, only possible positions described by a wave function that encodes the likelihood that the particle, if it were observed, would be here or there. Of course, quantum observations are by no means restricted to those made by humans. Such observations could be made by a dedicated detector, the environment, or even through interaction with a lone photon.
Thinking about the universe as a quantum system introduces a subtle backward-in-time element into cosmology. The past exists as a wave function that describes not a single history but a spectrum of possibilities that morph into a definite reality only when the future to which they give rise has been fully settled, i.e., observed.
This leads one to view what went on in the very earliest stages of the universe as a process akin to that of natural selection on Earth, with an interplay of variation and selection playing out in this primeval environment. Variation happens because random quantum jumps cause frequent small excursions from deterministic behavior and occasional larger ones. Selection enters because some of these excursions, especially the larger ones, can be amplified and frozen-in through quantum observations, giving rise to new rules that help shape the subsequent evolution. The interaction between these two competing forces in the furnace of the big bang produces a meandering branching process in which dimensions, forces, and particles first diversify and then acquire their effective form when the universe expands and cools. It is as if the collective quantum observations retroactively fix the outcome of the big bang. For this reason, Hawking referred to our hypothesis as “top-down cosmology”, to drive home the point that we read the fundamentals of the universe ex post facto, somewhat like how biologists reconstruct the tree of life.
Top-down cosmology ties in with remarkable developments in theoretical physics that have brought to the fore the holographic nature of gravity. For quite some time now, theorists have speculated that gravitational systems may be holograms made up of quantum entangled particles located on a surface in spacetime. In a cosmological setting, it turns out that it is the dimension of time that holographically pops out. From a holographic viewpoint, one ventures back in time by taking a fuzzy look at the hologram (Fig 1). It is like zooming out, an operation whereby one discards more and more of the entangled information that the hologram encodes. Eventually, however, one runs out of qubits, and that would be the big bang. That is, holography suggests that Lemaître’s primeval atom is an epistemic origin where not only time, but also the physical laws themselves disappear. This is very different to the old Platonist view that the laws of nature are immutable and transcendent. Holographic cosmology radiates the view that it is not the laws as such that are fundamental, but their capacity to change.
The upshot of all this is a profound revision of what cosmology ultimately finds out about the world. For almost a century, we have studied the history of the universe against a stable background of fixed (meta)laws of Nature. But the quantum outlook that Hawking and I developed reads the universe’s history from within and as one that includes, in its earliest stages, the genealogy of the physical laws (Fig 2). By weaving observership into its mathematical fabric, quantum cosmology appears to encapsulate the limitations of what can be known about the universe.
A finitude of this kind has been anticipated by philosophers like Hannah Arendt. To Arendt, the pursuit of science and technology from an Archimedean standpoint, stripped from all anthropomorphic elements and humanistic concerns, was an alienating and ultimately self-defeating paradigm. She argued that if we begin to look down upon the world and our activities as if we are outside it, then our actions will ultimately lose their deeper meaning. “The stature of man would not simply be lowered by all standards we know of but will have been destroyed” (Arendt, 1958). This is the paradox. In our attempt to find the ultimate objective scientific truth, we risk ending up smaller, not larger.
The later Hawking’s top-down cosmology, rooted in a profoundly quantum outlook onto the universe, amounts to a response to Arendt’s concern. It is a call emerging from the depths of fundamental physics to view our universe as our home and the laws we discover to describe it anchored in our relation with the cosmos. One can hope this could be the seed of a new worldview in which man’s knowledge and creativity will once again revolve around their common center.