Some things occur just by chance. Mark Twain was born on the day that Halley’s comet appeared in 1835 and died on the day it reappeared in 1910. There is a temptation to linger on a story like that, to wonder if there might be a deeper order behind a life so poetically bracketed. For most of us, the temptation doesn’t last long. We are content to remind ourselves that the vast majority of lives are not so celestially attuned, and go about our business in the world. But some coincidences are more troubling, especially if they implicate larger swathes of phenomena, or the entirety of the known universe. During the past several decades, physics has uncovered basic features of the cosmos that seem, upon first glance, like lucky accidents. Theories now suggest that the most general structural elements of the universe — the stars and planets, and the galaxies that contain them — are the products of finely calibrated laws and conditions that seem too good to be true. What if our most fundamental questions, our late-at-night-wonderings about why we are here, have no more satisfying answer than an exasperated shrug and a meekly muttered ‘Things just seem to have turned out that way’?
It can be unsettling to contemplate the unlikely nature of your own existence, to work backward causally and discover the chain of blind luck that landed you in front of your computer screen, or your mobile, or wherever it is that you are reading these words. For you to exist at all, your parents had to meet, and that alone involved quite a lot of chance and coincidence. If your mother hadn’t decided to take that calculus class, or if her parents had decided to live in another town, then perhaps your parents never would have encountered one another. But that is only the tiniest tip of the iceberg. Even if your parents made a deliberate decision to have a child, the odds of your particular sperm finding your particular egg are one in several billion. The same goes for both your parents, who had to exist in order for you to exist, and so already, after just two generations, we are up to one chance in 1027. Carrying on in this way, your chance of existing, given the general state of the universe even a few centuries ago, was almost infinitesimally small. You and I and every other human being are the products of chance, and came into existence against very long odds.
And just as your own existence seems, from a physical point of view, to have been wildly unlikely, the existence of the entire human species appears to have been a matter of blind luck. Stephen Jay Gould argued in 1994 that the detailed course of evolution is as chancey as the path of a single sperm cell to an egg. Evolutionary processes do not innately tend toward Homo sapiens, or even mammals. Rerun the course of history with only a slight variation and the biological outcome might have been radically different. For instance, if the asteroid hadn’t struck the Yucatán 66 million years ago, dinosaurs might still have run of this planet, and humans might have never evolved.
It can be emotionally difficult to absorb the radical contingency of humanity. Especially if you have been culturally conditioned by the biblical creation story, which makes humans out to be the raison d’être of the entire physical universe, designated lords of a single, central, designed, habitable region. Nicolaus Copernicus upended this picture in the 16th century by relocating the Earth to a slightly off-centre position, and every subsequent advance in our knowledge of cosmic geography has bolstered this view — that the Earth holds no special position in the grand scheme of things. The idea that the billions of visible galaxies, to say nothing of the expanses we can’t see, exist for our sake alone is patently absurd. Scientific cosmology has consigned that notion to the dustbin of history.
So far, so good, right? As tough as it is to swallow, you can feel secure in the knowledge that you are an accident and that humanity is, too. But what about the universe itself? Can it be mere chance that there are galaxies at all, or that the nuclear reactions inside stars eventually produce the chemical building blocks of life from hydrogen and helium? According to some theories, the processes behind these phenomena depend on finely calibrated initial conditions or unlikely coincidences involving the constants of nature. One could always write them off to fortuitous accident, but many cosmologists have found that unsatisfying, and have tried to find physical mechanisms that could produce life under a wide range of circumstances.
Ever since the 1920s when Edwin Hubble discovered that all visible galaxies are receding from one another, cosmologists have embraced a general theory of the history of the visible universe. In this view, the visible universe originated from an unimaginably compact and hot state. Prior to 1980, the standard Big Bang models had the universe expanding in size and cooling at a steady pace from the beginning of time until now. These models were adjusted to fit observed data by selecting initial conditions, but some began to worry about how precise and special those initial conditions had to be.
For example, Big Bang models attribute an energy density — the amount of energy per cubic centimetre — to the initial state of the cosmos, as well as an initial rate of expansion of space itself. The subsequent evolution of the universe depends sensitively on the relation between this energy density and the rate of expansion. Pack the energy too densely and the universe will eventually recontract into a big crunch; spread it out too thin and the universe will expand forever, with the matter diluting so rapidly that stars and galaxies cannot form. Between these two extremes lies a highly specialised history in which the universe never recontracts and the rate of expansion eventually slows to zero. In the argot of cosmology, this special situation is called W = 1. Cosmological observation reveals that the value of W for the visible universe at present is quite near to 1. This is, by itself, a surprising finding, but what’s more, the original Big Bang models tell us that W = 1 is an unstable equilibrium point, like a marble perfectly balanced on an overturned bowl. If the marble happens to be exactly at the top it will stay there, but if it is displaced even slightly from the very top it will rapidly roll faster and faster away from that special state.
This is an example of cosmological fine-tuning. In order for the standard Big Bang model to yield a universe even vaguely like ours now, this particular initial condition had to be just right at the beginning. Some cosmologists balked at this idea. It might have been just luck that the Solar system formed and life evolved on Earth, but it seemed unacceptable for it to be just luck that the whole observable universe should have started so near the critical energy density required for there to be cosmic structure at all.
And that’s not the only fine-tuned initial condition implied by the original Big Bang model. If you train a radio-telescope at any region of the sky, you observe a cosmic background radiation, the so-called ‘afterglow of the Big Bang’. The strange thing about this radiation is that it is quite uniform in temperature, no matter where you measure it. One might suspect that this uniformity is due to a common history, and that the different regions must have arisen from the same source. But according to the standard Big Bang models they don’t. The radiation traces back to completely disconnected parts of the initial state of the universe. The uniformity of temperature would thereforealready have had to exist in the initial state of the Big Bang and, while this initial condition was certainly possible, many cosmologists feel this would be highly implausible.