The Most Beautiful Idea in Science You’ve Never Heard of: Quantum Inflation
During my final year studying physics, I did my best to ensure that I was going to leave university having covered, as broadly and deeply as possible, the foundations of humanity’s understanding of physical reality — searching for the most fundamental patterns in the natural world, patterns which we can describe with astonishing accuracy using mathematics. I will use this personal experience as a narrative to build you up to a revelation that awaited me, a breathtaking synthesis of deep physics ideas called quantum inflation, which is surely one of the greatest discoveries in all of science.
On the one hand, this quest for understanding meant going as small as possible, addressing questions like: what is everything made of? What are the fundamental building blocks of physical reality? How do they interact and give rise to the macroscopic world that we inhabit? This meant studying quantum mechanics, particle physics, statistical thermodynamics, and quantum field theory (the current foundation level of our understanding of Nature). At the underlying quantum level, matter behaves very strangely indeed — so strangely that no human can be said to understand it, despite the fact that the theory works (makes predictions), really, really well… and in fact quantum mechanics (specifically quantum electrodynamics) is the best tested theory — i.e. in best agreement with experimental data — in the history of science, and its principles underpin numerous modern technologies.
On the other hand, it meant going as big as possible, asking: what does the universe look like on the very largest scales? How does it behave on these scales, and how do we gather and process information — with telescopes etc. to test and inform these theories? How did it evolve to this state since its creation? This big picture (or rather, biggest picture) investigation involved the study of cosmology, Einstein’s relativity, universal evolution… from the cosmos’ rapid transformation and expansion after the “Big Bang” to its ultimate fate after unimaginable aeons… a probable heat death scenario. You can review the timeline of the universe’s evolution here.
One might take the view that there is really only one thing in existence: the entire inextricably-interconnected-since-the-moment-of-creation Universe. Scientists call this Nature. Cosmology is attractive because it looks at this Nature thing in its entirety (or as close as we can get, given that some regions will forever be inaccessible to our observations), free from humanity’s bias to the very small scales of metres and minutes.
As an experimental science, cosmology recently has advanced to such an incredible degree — indeed it is in something of a golden age — that is has begun to provide answers to deep questions that humans have asked since the dawn of philosophy: how old is the universe? (as we know it, 13.7 billion years, though something might have existed before the Big Bang…) Why does the universe look the way it does? (Keep reading) Was it always this way? (No!)
Naturally, this going big and going small felt like two separate investigations, of the miniscule and the colossal, going in opposite directions. But then the craziest thing happened: they overlapped… I entered an area of cosmology, in the inconceivably violent and rapid furnace of the early universe, where we have to use both the theories of the biggest and smallest phenomena to understand what was going on. To explain how, I’ll do my best to lay out the minimum physics knowledge necessary, covering some of modern physics’ central ideas — which I hope will be interesting in itself!
Turning Back the Clock
A key feature of our understanding of the universe is that it is expanding, in every direction, at an accelerating rate (we know that mainly by mapping occurences of a certain type of supernova (stellar-death explosion) that has a regular brightness, allowing us to know their distance away from us, and then seeing that they are all receding from us, and at an increasingly faster rate than they used to be!). So when we rewind the clock to study the early universe, we end up studying a much smaller cosmos. Imagine playing a video of somebody blowing up a balloon in reverse. Same principle: earlier time, smaller balloon. Go back far enough, and you can study the universe when it was the size of a planet, then a grape, then an atom… and smaller still, until you reach the point where the universe was so small that you need to use quantum mechanics and quantum field theory — our best theories of the incredibly tiny — to understand it!
Origins of the Big Picture
Looking at the night sky today, what does the overall state of the universe look like? Physicists seek to know the origin and structure of all phenomena, and this extends to the very largest things in the universe: galactic superclusters which constitute the cosmic web. Building up to this structure, the basic size-hierarchy goes like this: “Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the “cosmic web”.” Observable universe 101.
Now how would one answer the question: where do the largest structures (galactic superclusters organised into a filamentous cosmic web) in the universe come from? Why is the matter in the universe — stars clustered into galaxies — distributed in this way, and not evenly spaced out?
Why is the cosmos more like a piece of holey cheese instead of a solid block? Where did the holes come from?! Why the irregularities? Some processes in the earlier, smaller universe must have caused these structures to form…
As mentioned previously, rewinding the clock to the early universe is our primary tactic in our search for the origins of these superstructures. Their origins are far weirder than we ever imagined. In fact, the answer lies in a very alien feature of the microscopic quantum world called quantum vacuum fluctuations.
Quantum Vacuum Fluctuations
Heisenberg’s famous uncertainty principle states roughly that “the better we know a particle’s momentum, the less certainly we know it’s location, and vice versa.”. It’s a basic limitation about how much we can know about very small — quantum — systems. But it’s not just theory: squeezing particles into small volumes (so we know their location better) makes them move (we are less certain about their momentum). It’s weird as hell, but that’s what happens. (If you’re new to quantum mechanics, get used to that).
There is a lesser known “sister” uncertainty principle involving energy and time: in a given volume of space, the more precisely we measure a time interval, the less confident we can be about the amount of energy in that space. Again, this isn’t just theory.. over very short time periods, we get energy fluctuations in empty space! The iron principle of conservation of energy is actually violated, if only for a short enough time. The simplest example of such a fluctuation is when a particle-antiparticle pair pop into existence extremely briefly before annihilating one another* (this allows energy conservation to be violated without electric charge conservation being broken too).
Pop into existence — from nowhere. From pure void, pure vacuum, stuff appears. Then it disappears. Something from nothing — and back to nothing. Totally weird, but that’s what happens… (incidentally, this sounds like something advanced meditators say about the objects of consciousness: there is nothing, then something appears, then it fades into nothing. This is why some Buddhists talk about the “void” underlying reality. Anyway…)
In fact, these fluctuations are happening everywhere, all the time. The bottom layer of physical reality has been described as a “seething quantum foam”, which looks something like this:
This isn’t just theory: quantum fluctuations have measurable consequences in our world, as in when two metal plates are pulled towards one another during the Casimir effect. (They are also responsible for black hole evaporation, when a particle-antiparticle pair appears on the event horizon and one falls in, but the other doesn’t, allowing information to “escape” the black hole’s event horizon, so conserving entropy, as famously shown by Stephen Hawking.)
The Jewel of Cosmology: Quantum Inflation
So, back to our mysterious cosmic web, the superstructures of today’s universe for whose origins we are diligently searching… and now we’re getting very close indeed.
The universe at 10^–36 seconds of age was utterly utterly minuscule — it had expanded just a teeeeeny bit from the initial singularity. At this stage, then, some quantum fluctuations occurred, as they usually do at very small scales: some blips of energy randomly bubbled up (yes, from nowhere) and disturbed the lovely perfect evenness of the baby universe.
A little more energy in a given region means that it is a little more dense that the surrounding region — so these fluctuations basically caused some areas of the early universe to be denser than others. Hold that thought.
Just before these quantum energy fluctuations could disappear again, as they always do, a period of expansion called inflation began, taking these short-lived, tiny fluctuations and stretching them outwards in every direction until they are colossal…
In an obscenely rapid expansion, occurring in the timeframe of 10^-36 and 10^-32 seconds after the Big Bang singularity, the universe was blown up to at least 10^26 times it’s prior size. To get an idea about space itself expanding: if you draw a picture on a deflated balloon, then blow it up, that picture will be massively stretched and enlarged — an imprint of a once much smaller picture. This means that those little pockets of energy density and scarcity, caused by quantum fluctuations just before inflation occurred, were magnified — blown up — by this expansion into very large features of the universe. And as the universe continued to expand, the dense areas got denser, collapsing inwards due to gravity, and the empty areas got emptier as space stretched outwards in all directions. If these fluctuations didn’t happen, then, all energy would be totally evenly distributed, and there wouldn’t be any structure in the universe — and no us to observe it!
(This wild piece of theory is the leading theory to fit current experimental datasets. Analysis of the Cosmic Microwave Background (CMB), the light energy leftover from the Big Bang, still bouncing around at 2.3K (-270.85 ° celsius), is in great agreement. We await some other important evidence, like the signatures of primordial gravitational waves imprinted on the polarisation patterning of the CMB, during inflation. Inflation also explains why the universe is “spatially flat”. Einstein’s relativity allows for curved spacetimes… but we observe a flat universe… why?! In short, the extreme expansion of inflation will have “flattened” out any curvature of space that was present beforehand — a little like how blowing up a wrinkly balloon smooths out its surface.)
Following inflation, much of the universe’s energy, which was previously driving the expansion, was transformed into matter in the form of the familiar particles of the Standard Model, (giving us all the ingredients for familiar forms of energy like light, atoms, etc.) — a mysterious process known as reheating. Much of this matter became hydrogen, the simplest atom (one proton, one electron). When giant hydrogen gas clouds collapse under gravity, they heat up, triggering nuclear fusion and thus forming stars.
One might think of the pre-inflation density variations — again, the result of quantum fluctuations — a little like the dust particles that allow water molecules to clump into droplets in Earth’s atmosphere: an initial seed is needed for the clumping together. In the case of clumped-together hydrogen clouds that ignite into stars, all the way up to galaxy clusters and the cosmic web, the original seeds for these structures were pre-inflation quantum vacuum fluctuations! This fact is the essence of quantum inflation. It tells us, astoundingly, that the very largest structures of today’s physical world were seeded by the very smallest phenomena we know of, when the universe was inconceivably young and miniscule. In this way, science has actually arrived at an explanation as to why the universe has any large scale structure at all, and isn’t just a dark, diffuse sea of microscopic particles. What’s more, these seed phenomena were random quantum phenomena, and one might conceive of an infinitude of alternative scenarios in which the quantum fluctuations occurred at a different magnitude to produce a cosmic web of different appearance.
The cosmic web pictured above is an immortalised fossil of one the earliest phenomena ever to occur in the universe, some unimaginably small random quantum fluctuations that occurred in the first 10−36 seconds (!) after the Big Bang. An ancient imprint of the primordial universe. That modern physics has been able to make this vastest of connections is, surely, one of its greatest achievements.
(If you made it to the end of this article, I’ve no doubt that you’ll enjoy reading about the utterly strange feature of inflation: that the theory tells us that this process will occur eternally in different “pockets” of the universe, infinitely seeding new universes as part of a wider multiverse. In a nutshell: if the universe inflates in one area, there will be little patches on the side of this that don’t inflate… when they do, there will still be patches on the side of this patch uninflated… and so on ad infinitum, infinitesimal regions of undifferentiated singularity inflating… seeding entire universes like bubbles. Each of these “bubble universes” will have a different structure, due to the different random quantum vacuum fluctuations that occur before inflation!)