by Paul Davies
Since eternal inflation was mooted in the 1980s, it has grown steadily in popularity, not least because it avoids the vexed issue of a universe coming from nothing. The essential idea is reminiscent of the steady state theory, but instead of particles of matter being created, entire bubble universes come into being. Each bubble starts with a big bang, and embarks on a life cycle of beginning, middle and (maybe) an end. The overall system, however, is eternal. What we have all along been calling ‘the universe’ is actually just ‘our universe’ – a tiny fragment of a much vaster, more complex ‘multiverse’.
Where are all those other bubbles? Most cosmologists think they are much too far away for us to ever detect them. In the simplest multiverse variant, the bubbles fly apart from each other much faster than they themselves expand, so there isn’t any risk of our universe bumping into another one. However, some versions of the multiverse do feature cosmic encounters, an alarming prospect that I shall come back to later.
While the multiverse idea undoubtedly has its attractions – such as sidestepping the problem of a singular, magical origin of one universe – it comes with its own uncomfortable philosophical issues. If there really are an infinite number of universes, then there will be others out there identical to our own, in every respect, including duplicate versions of you and me. In fact, there will be an infinity of such replicas. The same basic problem arises even in a single universe. Roughly speaking, in an infinite universe with uniform laws and conditions, if something can exist it will exist, and exist infinitely many times. That something includes you and your entire life history, meaning that in an infinite universe or an infinite multiverse there will be a limitless number of other yous, identical in every conceivable respect. Furthermore, for every identical you there will be vastly more almost-yous.
To help get to grips with this dizzying idea, think of tossing a coin. What are the chances of obtaining heads a million times in a row? Extremely small, but not strictly zero. Infinity beats not-quite-zero every time. In an infinite number of trials, a million heads in a row is assured – not just once, but an infinite number of times. Same is true of a billion heads, or a trillion. If you think that life is a lottery and you are the jackpot, well, the odds are certain that the same jackpot will come up somewhere else, sometime else.
Of course, none of this reasoning applies if the universe isn’t actually infinite but just very, very big. It’s not all that hard to work out how big it would need to be to contain at least one other you. To be safe that we are not discussing just a momentary replication but an entire identical life history, we should consider an exact replica of the whole observable universe, because any bit of the universe can cause a small disturbance on a human being. The cosmologist Max Tegmark did the sums and determined that the volume of space you would need to explore before having a good chance of encountering a precise match – Universe 2. 0 – is a volume of space out to a distance of 10 to the power 10120 metres; that’s 1 followed by a trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion zeros. For comparison, our observable ‘Universe 1.0’ is only 10 trillion trillion metres across. With numbers like these, it’s clear you are unlikely to actually run into any of your duplicates. Nevertheless, most people find the very notion of duplicate beings disturbing. Which one is the ‘real you’?
There is a much-discussed variant of this large numbers game that can be applied to infinite time rather than infinite space. If the universe is set to endure for ever, isn’t that time enough for another you to be reconstituted from scratch? I mentioned the quantum vacuum as otherwise empty space seething with temporary particles, flitting in and out of existence randomly. But randomly means there is always a probability, albeit exceedingly small, that the quantum vacuum will generate something more organized than a sea of half-real electrons, quarks, neutrinos and photons. In principle, the quantum vacuum could create a human being – any human being – or maybe just their brain – though this luckless individual probably wouldn’t last long enough to know much joy before dissolving back into the quantum quagmire. The study of this offbeat subject has even been dignified with a name: Boltzmann brains, after Ludwig himself, who first toyed with the idea of an infinite universe randomly arranging itself into one containing a population of humans.
To avoid such disconcerting scenarios, one could throw out the very notion of infinity. Scientists have always had an uneasy relationship with the concept, given that it can have such troublesome consequences. Perhaps infinity is good for mathematics but has no place in the real universe? Some cosmologists have argued as much. It’s hard to see how the matter can ever be resolved by observation, because we can only ever measure finite quantities. An actual infinity can be grasped by the human mind but never encountered in reality. It is an idealization, useful for computational purposes, but of dubious applicability to the real world.
So much for philosophy. What about actual science? Is there any conceivable evidence for a multiverse? Possibly. In Chapter 19, I suggested that the laws of physics we know today might, in fact, be fossils from the big bang, forged in the hottest phase of existence when the most basic aspects of our universe were being established. In the case of more familiar fossils, such as dinosaurs, they come in many different species: you might find a brontosaurus here and a triceratops there. If the laws of physics are a type of fossil, then we can imagine different laws of physics in other bubble universes. If the ‘universe experiment’ is endlessly repeated, we can imagine that each bubble universe would come complete with its own set of laws, and that every variant, every possibility, would be instantiated somewhere (an infinity of times).
Given that we are stuck in this universe, with the laws it has, how can we possibly obtain evidence for other universes with different laws? Well, there’s a rough and ready line of reasoning based on statistics. Let me give an analogy. In the United States bears are brown or black. In the far north of Canada they are white. Polar bears have an advantage in being camouflaged against the snow. There are two explanations for why polar bears are white. One is that they just got lucky. They were white anyway, and happily found themselves living in a snowy environment. The other explanation is that there was a large ensemble of bears of a range of colours, and evolution selected the fittest colour for the frozen north. In science, luck is sometimes the right explanation, but if something looks suspiciously, awfully, lucky, then the chances are that we have overlooked a selection process. In that vein, evidence for the multiverse theory might be found if there seems to be something peculiar or special or really lucky about the laws in our universe – the one we call home. Is there?
Is the universe a fake?
In 1641, the philosopher René Descartes floated the notion that the universe might be a fake, conjured up by a cunning and malicious demon to deceive us. The basic idea has been revived recently in light of artificial intelligence and ever more lifelike virtual reality. If you believe consciousness is a product of physical processes then, given sufficient resources, it must be possible to simulate those processes in a computer and to create a fake virtual world for the simulated beings to inhabit, as in The Matrix series of movies. Many scientists and philosophers find this plausible. So how can we be sure that our universe is real, and not a simulation on somebody’s super-duper computer? The idea gives an intriguing twist to the multiverse theory: given that fake universes are far easier to make than real ones, the fakes might well outnumber the genuine articles, so that most conscious beings will in fact be living in a simulation.
Well, how about the fact that it is home?
22. The Goldilocks Enigma
There’s a story of a man who has fallen on hard times. His business has gone bust, his wife has left him and, to cap it all, he’s just dropped his freshly buttered toast on the carpet. Gloomily reaching down, he’s astonished to see the toast has fallen buttered side up. Could this be a sign? Has his luck begun to turn? He carefully picks up the toast, puts it in a zipl
ock bag, and hurries off to see his local priest. ‘Father,’ asks the man, showing him the toast, ‘what is the meaning of this? Are there better times ahead?’ The priest peers at the toast, then examines it with a magnifying glass. Reaching for his Bible, he suggests the man comes back in an hour. ‘Well, father,’ asks the man on his return, ‘has my luck changed?’ ‘No,’ replies the priest, shaking his head sorrowfully. ‘You buttered the wrong side, that’s all.’
We find this story amusing because we all have a natural desire to seek meaning even in the minutiae of our lives, to find a deeper significance beyond our humdrum existence. Astrology, mysticism, shamanism and religion all address this basic human need. But are we fooling ourselves? It is an ever present danger. One of the sharpest schisms in science erupted after a group of scientists claimed to have found hidden meaning in the fine details of the laws of physics. This stormy dispute goes right to the heart of the place of human beings in the great cosmic drama.
The argument began in the 1950s, when Hoyle was trying to work out how carbon – the life-giving element – was made inside stars by nuclear collisions. At first it didn’t look possible – there was no way a carbon nucleus could form from the fusion of two lighter nuclei. But as carbon is an abundant element there had to be a reaction pathway. Hoyle guessed there must exist a happy coincidence in the arrangement of nuclear states that enabled carbon to form from rare simultaneous encounters of three helium nuclei. Experiment proved him right. ‘It seemed as if a super-intellect had been monkeying with the laws of physics,’ he later wrote, noting that if the strength of the nuclear force were just a bit weaker or stronger, the coincidence would go away and the universe would be devoid of carbon, and therefore life. To Hoyle it was as if the universe were, to use his pithy description, ‘a put-up job’. Following this lead, distinguished physicists and astronomers began trawling through the details of other physical laws, looking for more examples of where the cosmic toast had fallen buttered side up. What if gravity was a bit weaker, or electrons a bit heavier? Suppose the universe were expanding faster or slower, or the big bang was hotter? Some of these changes turn out not to matter much as far as the existence of life is concerned, but others would be decidedly lethal.
As the list of propitious coincidences grew, scientists became uneasy. Like in the story of Goldilocks, our universe seems to be ‘just right’ for life. It looks too much like divine providence, another Big Fix. Then a convenient explanation came to hand: the multiverse.* Among the countless universes churned out by the multiverse’s creation mechanism, surely some fraction will have life-encouraging laws replete with happy coincidences, purely by chance? Obviously, we couldn’t find ourselves living in an uninhabitable universe, so the freakish bio-friendliness of this one is no surprise really. We have selected it by our very existence.
An explanation of this type is unusual in science, and it sparked heated debate. Some physicists, dreaming of a final theory of everything, hate the multiverse. They hope their theory will one day describe a unique universe with unique laws – thus demonstrating that it couldn’t have been otherwise – and if that universe just happens to permit life, well, that’s simply the way it is. A third group, including some of a religious persuasion, see the multiverse explanation as an untestable cop-out. They also dismiss the unique final theory as a case of promissory triumphalism, given that there is no such theory in existence.
This entire three-way spat revolves around something termed the anthropic principle, which is a misleading designation because it’s not intended to refer to human beings specifically, but to any sentient life form. At one level, the anthropic principle is innocuous enough; it merely says that the universe we observe has to be consistent with the existence of life. Thus, for example, it’s no surprise that we live on a planet near a stable star. We wouldn’t expect to be observing a region of the universe from intergalactic space. But sometimes it takes on a more mystical flavour, hinting that the laws of physics have been ‘monkeyed’ with, to use Hoyle’s evocative description. In the context of the multiverse, however, it makes no difference how rigged our universe looks because there are plenty of universes to go round, and there will always be one somewhere that will be inhabited and celebrated, no matter how quirky it is.
I haven’t conducted a straw poll, but there is a list of very distinguished physicists and cosmologists who are totally persuaded that we do indeed live in a universe delicately poised in the Goldilocks zone, and that – theological explanations being anathema – there must be a multiverse. My own feeling is that, even if a multiverse exists, it still doesn’t explain everything. In the eternal inflation version of a multiverse, for example, there needs to be a universe-creating mechanism – a bubble generator – based on some physical laws. And the inflating superstructure itself uses the laws of quantum physics and general relativity. The origin of those laws remains unexplained. You could cook up any number of different multiverse models with different overarching laws and different bubble universe generators. So, the problem is just shifted up a level: instead of ‘Why this universe?’ one can ask, ‘Why this multiverse?’ There may be no end to this ontological paper trail.
23. What’s Eating the Universe?
When COBE uncovered a sky full of splodges, theoretical cosmologists began poring over the data. Were the splodges random? Were the bigger splodges more pronounced than the smaller ones, and, if so, did the variation follow a mathematical law? How consistent were the patterns with the inflation theory? More satellites were launched, with better equipment, producing a deluge of additional data. Today, cosmologists think they have a good overall grasp of splodgology. But, frustratingly, there are persistent anomalies.
Among the many splodges in the heat map of the sky, one in particular stands out as enigmatic. Located in the constellation of Eridanus in the Southern hemisphere, it is about five degrees across – the equivalent of ten full moons – representing an enormous volume of space. What is odd about this particular splodge is that it is far colder than it should be – nearly eight times colder than the average temperature variation in the CMB. Its origin remains a mystery. At first sight, it looks as if a cosmic giant has taken a huge bite out of the universe, leaving a super-void. Astronomers have long accepted that the universe we can see – the stars, gas and dust – is being slowly but inexorably eaten by supermassive black holes lurking in the centres of galaxies. But the cold spot in the Southern sky suggests something on a much, much larger scale, a blemish in the very structure of the cosmos itself.
No easy explanation works, so there have been some wild speculations to explain it. One of these, championed by Laura Mersini-Houghton, posits an awesome collision involving another universe crashing into ours, leaving a fossilized scar in sky. Such a titanic encounter would generate copious quantities of gravitational waves and produce a distinctive pattern of polarization in the CMB. So far, searches for this pattern have come up empty. But in the multiverse of many bubbles, universes crashing into neighbours is just one of several available cosmic catastrophes, limited only by the ingenuity of theorists. Not only might our universe be bumped by another; it could be gobbled up wholesale, swallowed in toto by a bigger, more powerful universe blundering into ours. Or vice versa.
Even ignoring the menace from without, there are also threats from within. One idea that has been around since the 1970s is that our universe might eat itself from the inside out. This could happen at any time, on account of a general feature of quantum systems. The story of the self-devouring universe unfolds something like this. When an atom is excited, i.e. in an elevated energy level, then it will make a transition (it will ‘decay’, to use the correct technical term) to a lower level, emitting a photon. But what’s good for atoms is good for the quantum vacuum too. If, as most scientists believe, dark energy has a quantum origin (that is, it is the energy of the quantum vacuum – see pp. 54–6), then, like atoms, there might be many possible values or levels of vacuum energy. Our universe happen
s to be in one of these vacuum energy levels, but it may not be the lowest. The worry is, an excited, or elevated, state of the quantum vacuum wouldn’t be completely stable. There would always be a risk it would transition to a lower energy state – the vacuum could ‘decay’ – thereby releasing a stupendous amount of energy. If this happened anywhere in the universe, the consequences would be apocalyptic. A tiny bubble of the new, lower energy vacuum would spread out at nearly the speed of light, with the released energy concentrated into the bubble wall. As this boundary expanded it would destroy everything in its path. There may be no warning: we might know our universe was being wrecked only when the wall arrived and annihilated us – and everything else – faster than the speed of thought.
There is an eerie variant to this scenario. The expanding bubble might contain, not a lower energy quantum vacuum, but nothing at all. It would be a hole in space, not a black hole, but a bubble of no-space, expanding uncontrollably, sweeping all before it and eventually gobbling up the entire universe, leaving only nothing: space totally engulfed by spacelessness. The basic idea of space vanishing without warning in this alarming manner was first suggested in 1982 by theoretical physicist Edward Witten from the Institute of Advanced Study in Princeton (where Einstein used to work), based on his analysis of string theory. He described it thus: ‘A hole spontaneously forms in space and rapidly expands to infinity, pushing to infinity anything it may meet.’ Envisage space as the substance in a Swiss cheese. Now imagine the holes in the cheesy matrix growing ever bigger until there is no cheese left.