The Tarantula Nebula: the panorama that astronomers can observe could be a tiny part of the aftermath of our Big Bang
An astonishing concept has entered mainstream cosmological thought: physical reality could be hugely more extensive than the patch of space and time traditionally called “the universe.” We’ve learnt that we live in a solar system that is just one planetary system among billions, in one galaxy among billions. But there are signs that a further Copernican demotion confronts us. The entire panorama that astronomers can observe could be a tiny part of the aftermath of our Big Bang, which is itself just one bang among a potentially infinite ensemble. In this grander perspective, what we’ve traditionally called the laws of nature may be no more than parochial bylaws—local manifestations of “bedrock” laws that must be sought at a still deeper level.
Astronomers might seem the most helpless of all scientists. They can’t do experiments on stars and galaxies, and human lives are far too short for us to watch most cosmic objects evolve. But there are some compensating advantages. There are huge numbers of objects in the sky, and one can infer the life-cycle of stars, just as one can infer how trees have grown and will die from one day’s wandering in a forest. Because the light from remote galaxies set out towards us billions of years ago, we can actually (unlike geologists) observe the past. Moreover, we understand enough physics to be able to simulate stars and galaxies—how they form, collide and explode—in the “virtual universe” of a computer.
Powerful telescopes can capture newly formed galaxies whose light set out when the universe was less than 10 per cent of its present age. And high-precision measurements of the “afterglow of creation”—the weak microwaves that warm intergalactic space to a temperature of three degrees above absolute zero—allow us to trace cosmic history back to a time when everything was squeezed hotter and denser than the centre of a star. Such inferences are as evidence-based as anything a geologist might tell you about the history of our Earth. We can confidently trace things back to an era just a nanosecond after the Big Bang, when every particle carried as much energy as can be generated by the huge Large Hadron Collider in Geneva, and the entire observable universe was squeezed into dimensions no larger than the solar system.
But, as always in science, each advance poses new questions. For instance, “Why is the universe expanding the way it is?” and “How did it acquire its observed mix of particles and radiation?” The answers to both lie in the brief instants when our universe was far less than a nanosecond old, and everything in it was far hotter and denser than we can simulate in the lab. We consequently lose any firm foothold in experiment and enter more speculative realms.
An issue of Discover magazine had a cover showing a sphere, with the caption: “The universe when it was a trillionth of a trillionth of a trillionth of a second old—actual size.” According to a popular conjecture, the entire volume we can see with our telescopes “inflated” from a hyper-dense blob no bigger than a tennis ball. How can we firm up such an idea?
The twin pillars of 20th century physics are Einstein’s theory of gravity (general relativity) and quantum theory. But these haven’t yet been unified. In most contexts, their domains of relevance do not overlap. Astronomers can ignore quantum fuzziness when calculating the motions of planets and stars. Conversely, chemists can safely ignore gravitational attraction between atoms because this force is about 40 powers of ten feebler than the electrical forces between them. But during the ultra-compressed earliest instants after the Big Bang, quantum fluctuations could, as it were, shake the entire universe. To tackle the fundamental mystery of what banged and how it banged therefore requires a synthesis of these two great 20th century theories.
Einstein’s theory is incomplete because it treats space and time as smooth and continuous. We know that no material can be chopped into arbitrarily small pieces: eventually, you get down to discrete atoms. Likewise, it’s thought that space has a grainy and “atomic” structure—but on a scale a trillion trillion times smaller than atoms. Theories of how space is “quantised”—superstring theory, for instance—require an understanding of this tiny scale.
Such a theory is also needed in order to elucidate one of the deepest mysteries posed by the universe. A “repulsive” force is seemingly latent even in empty space. This force is too weak to be detected on Earth, or even within our galaxy, but it overwhelms gravity and pushes galaxies away from each other at an accelerating rate. We have no idea why this force exists, and why it is so small.
How big is our universe? Observations are restricted to a volume that is huge but nonetheless finite. That’s because there’s a horizon—a shell around us, delineating the distance light can have travelled since the Big Bang. But that shell has no more physical significance than your horizon if you’re in the middle of the ocean: far more ocean lies beyond.
There are firm reasons for suspecting that the array of galaxies stretches thousands of times further than our horizon. But it could stretch far further still. The domain that astronomers call “the universe”—the space, extending more than ten billion light years around us and containing billions of galaxies, each with billions of stars and billions of planets (and maybe billions of biospheres)—could be an infinitesimal part of the totality. Indeed, space could extend so far that, somewhere, there are assemblages of atoms in all possible configurations and combinations.
We’re familiar with the idea that if enough monkeys were given enough time, they would write the works of Shakespeare. This statement is mathematically correct. However, to generate a specific set of letters as long as a book is so immensely improbable that it wouldn’t have happened even once within the observable universe. The number of failures that would precede eventual success is a number with about a million digits. Even the number of atoms in the visible universe has only 80 digits. If all the planets in the observable universe were crawling with monkeys, who had been typing ever since the first planets formed, then the best they would have done is typed a single sonnet. (Their output would include short coherent stretches from all the world’s literatures, but no single complete work.)
But if the universe stretches far enough, everything, however improbable, would happen. All conceivable chains of events could be played out somewhere, though almost all of these would occur far out of range of any observations we could make. Somewhere far beyond our horizon there would even be a replica of our Earth. (But the distance would be stupendous—measured by a number with ten to the power 100 digits—a googolplex.) The combinatorial options will encompass replicas of ourselves, taking all possible choices. Whenever a choice has to be made, one of the replicas will take each option. You may feel that a choice you make is determined. But it may be a consolation that, somewhere far out there, your avatar has made the opposite choice.
We are three-dimensional beings: we can go left or right, forward or backward, up or down, and that is all. So how is it that the extra dimensions, if they exist, are concealed from us? This may be because they are all wrapped up tightly. A long hosepipe may look like a line (with just one dimension) when viewed from a distance, but from closer up we realise that it is a long cylinder (a two-dimensional surface) rolled up tightly. From still closer, we realise that this cylinder is made from material that isn’t infinitely thin, but extends in a third dimension. By analogy, every apparent point in our three-dimensional space, if hugely magnified, may actually have some complex structure: a tightly wound origami in six extra dimensions. According to the most favoured theory, the particles that make up atoms are all made up from structures on this utterly minuscule scale—indeed they are woven from space itself.
Even more interestingly, one extra dimension may not be wrapped up at all: there may be other three-dimensional universes alongside ours, embedded in a grander four-dimensional space. Bugs crawling around on a large sheet of paper (their own two-dimensional universe) may be unaware of a similar sheet that’s parallel to it and not in contact. Likewise, there could be another entire universe less than a millimetre away from us, but we are oblivious to it because that millimetre is measured in a fourth spatial dimension and we are imprisoned in just three. (Whether a universe could have more than one time dimension is less straightforward. Certainly a language with more tenses would be needed to describe what happens in it!)
There are many variants of the “multiverse” scenario—but they remain speculative because they depend on still-unknown physics. It’s an arena where, in the fashion of ancient cartographers, we must inscribe “here be dragons.” An idea called “eternal inflation,” developed especially by the Russian cosmologist, Andrei Linde, envisages Big Bangs popping off, ad infinitum, in an ever-expanding underlying space. This process depends on the physics that prevailed when our entire observable universe was tennis-ball size. There are genuine prospects that, in the coming decades, physicists might be able to infer whether the prerequisites for eternal inflation are indeed fulfilled, and whether the aftermath of our Big Bang could be just one island of space-time in a vast cosmic archipelago.
A further question then arises: “Are the laws of physics unique?” This is a less poetic version of the famous question that Einstein once posed to his assistant, Ernst Straus: “Did God have a choice when he created the universe?” We have strong evidence that the same physical laws apply everywhere we look. When astronomers analyse the light from remote galaxies, they infer that the atoms that emitted the light behave just like atoms in a lab. And the properties of distant stars suggests that all are held together by a gravitational force of the same strength. But does this uniformity extend far beyond our horizon? And to the aftermath of other Big Bangs?
String theory suggests that the answer is no. Theorists suspect that the substratum of space—what is technically called “the vacuum”—might display immense variety. There could be more “vacuum states” than there are atoms in all the galaxies we can see. If so, different universes could be governed by different bylaws. The “cosmic numbers” that characterise particles and forces could be different—consistent with an overarching theory governing the ensemble, but not uniquely fixed by that theory. More specifically, some features may be truly universal but others not. As an analogy (which I owe to the physicist Paul Davies) consider the form of snowflakes. Their ubiquitous six-fold symmetry is a consequence of the properties and shape of water molecules. But snowflakes display a variety of patterns because each is moulded by its distinctive history and micro-environment: how each flake grows is sensitive to the temperature and humidity changes during its growth.
Many things in our cosmic environment—for instance, the exact layout of the planets and asteroids in our solar system—are accidents of history. Likewise, the cosmic numbers astronomers measure could be environmental accidents, rather than uniquely fixed throughout the multiverse. Our universe could be the result of a lucky draw of cosmic numbers conducive to the emergence of complexity and life.
Support or refutation for these conjectures must await firmer links between the theories of the very large (the cosmos) and the very small (the quantum). Physicists may eventually discover which aspects of nature are direct consequences of the bedrock theory (just as the symmetrical template of snowflakes is due to the molecular structure of water) and which are environmental contingencies (analogous to the distinctive pattern of an individual snowflake). The outcome will, I think, determine how we should interpret our own bio-friendly universe.
Four hundred years ago Johannes Kepler thought that the Earth was unique, and that its orbit was related to the other planets by beautiful mathematical ratios. We now realise that there are zillions of stars, each with planetary systems. Earth’s orbit is special only because it allows for conditions compatible with life.
I foresee an analogous conceptual shift, on a far grander scale. Our Big Bang may not be unique, any more than planetary systems are. The hope for neat explanations in cosmology may be as futile as Kepler’s numerological quest. It may disappoint some physicists if some of the key numbers they are trying to explain turn out to be no more fundamental than the parameters of the Earth’s orbit around the Sun. But that disappointment would surely be outweighed by the revelation that physical reality is grander and richer than previously thought.
During a conference at Stanford University, participants in a panel discussion were asked how strongly they’d bet on the multiverse concept. I said that, on the scale “Would you bet your goldfish, your dog, or yourself?” I was at the dog level. But Andrei Linde was far more confident—after all, he’d devoted 25 years of his life to the eternal inflation idea. The great theorist Steven Weinberg (see Prospect, December 2011) later said that he’d happily bet Martin Rees’s dog and Andrei Linde’s life.
Our cosmic environment could be richly textured, but on scales so vast that our purview is restricted to a tiny fragment. We’re not directly aware of the big picture, any more than a plankton whose universe is a litre of water would be aware of the world’s topography and biosphere. It is sensible for cosmologists to start off by exploring the simplest models. But there is no more reason to expect simplicity on the grandest scale than in the terrestrial environment, where intricate complexity prevails. It is exhilarating that this wonderful concept is now within the scope of scientific enquiry.