What's Eating the Universe? Read online

  Admittedly, the CMB isn’t a relic of the very instant of creation; it’s what’s left of the glow present at about 380,000 years after the big bang, an epoch about 0.003 per cent of the present age of the universe. Expressed differently, we can look back 99.997 per cent of the way to the beginning. That’s undeniably impressive, but can we do better? Conventional radio and optical telescopes can’t help much, because the universe was an opaque fiery plasma before that point. We can’t see into it for the same reason that we can’t see inside the sun. But telescopes aren’t the only way to uncover a record of our deep past: fossilized chemical elements are also important remnants of bygone times. George Gamow, the nuclear physicist who put the big bang theory on the map, reasoned that if there was a primordial phase when the temperature of the universe was millions of degrees, then a range of nuclear reactions would have occurred, opening up the prospect that their products might be identified in the chemical elements around us. In the 1960s, astronomers attempted to reconstruct the nuclear story in detail. At one second after the big bang, they calculated, the universe was too hot for nuclei to exist. Space was filled with a dense soup of untethered neutrons, protons, electrons and neutrinos. Soon thereafter the universe had cooled enough for neutrons and protons to interact, react, combine and rearrange. That phase didn’t last long – just a few minutes. Once the temperature fell below about a hundred million degrees, nuclear reactions ground to a halt, leaving about a quarter of the primordial material as helium, almost all the rest as hydrogen, plus a smattering of deuterium. These numbers turn out to closely match astronomical observations. The element helium, then, can be regarded as a cosmic fossil dating from about three minutes after the origin of the universe.*

  Buoyed by this success, cosmologists sought out yet earlier fossils. Neutrinos offered an intriguing possibility. They would have been made in prodigious quantities in the hot early universe, and, because they interact so weakly with ordinary matter, they will have travelled to us untrammelled from a second or so after the big bang. Like the CMB, there should be a cosmic background of neutrinos too: four neutrinos for every eleven CMB photons, to be precise. Which implies that the material universe is bathed in a sea of neutrinos that outnumber atoms by many billions to one. Although the primordial neutrinos collectively exert a measurable gravitational brake on the expansion of the universe, directly detecting them individually looks to be a lost cause, so tiny is their effect on matter. Still, it’s creepy to think that every second, our bodies are being penetrated (albeit harmlessly) by trillions of these cosmic fossils.

  Another great leap backwards came in the 1980s with the development of the inflationary universe scenario. Here I had a modest hand to play myself. As I explained in Chapter 9, the essential idea of inflation is that the universe jumped in size by an enormous factor a split second after its birth, when quantum effects would have been important. By a happy coincidence, a student of mine, Timothy Bunch, had already worked out the properties of the quantum vacuum – the void seething with evanescent energy I mentioned in Chapter 11 – in precisely such a scenario. When the inflation theory became popular soon after, theorists were ready with the so-called Bunch-Davies vacuum to calculate what pattern of quantum density fluctuations should have been imprinted on the universe when inflation ended. A few years later they were able to match up those predicted variations with the splodges mapped by COBE, and later recorded in much higher resolution by a European satellite called Planck. The concordance was, and remains, very good. If the inflationary theory is right, it means that quantum fluctuations in the very early universe are writ large and frozen in the sky for all to see. The variegated blotches that graced the front pages of the world’s newspapers on 14 January 1990 are fossil remnants laid down just a temporal sliver away from the very origin of the cosmos.

  There is a postscript to this remarkable story. The quantum calculation that Tim and I did considered a simple type of field (for example, electromagnetic) in a background of expanding space described by general relativity. We ignored the quantum properties of gravity itself, because there was no good theory of it. But physicists have long been obsessed with detecting quantum effects of the actual gravitational field (see ‘The holy grail of quantum gravity’, below). Although the theory of quantum gravity remains a work in progress (and string theory is one contender), enough is known to work out the expected signature of quantum gravity effects emanating from the inflation phase. It would show up as distinctive twisting patterns in the CMB splodges – specifically in the pattern of polarization. Great excitement thus attended the announcement in 2014 that a special telescope in Antarctica had detected a matching pattern from a patch of sky above the South Pole. Alas, the result turned out to be caused by foreground dust in the Milky Way; it didn’t come from the dawn of creation after all. Still, the obscure field of palaeo-cosmology is in its infancy, and quantum gravitational effects may yet be found.

  That’s not necessarily the end of cosmic fossil hunting. It’s possible that the laws of physics themselves are fossils from the primordial era. As I explained in the previous chapter, many of the known features of the forces of nature and the list of subatomic particles have no rhyme or reason. Some physicists insist there really is no reason – it was just the way the cosmic cookie crumbled. And it’s true that the universe is replete with ‘frozen accidents’ – the size of the Earth, the pattern of craters on the moon, the number of planets in the solar system. The same might be true of the masses of subatomic particles and the strengths of the forces between them. Even the number of forces and the number of spatial dimensions – all might have fallen out arbitrarily as the universe cooled, with no deeper explanation for why they are as they are. They are just random accidents, forged in the heat of creation and frozen for ever after. If that is so, then the laws of physics themselves could be time capsules from the earliest conceivable moments of history.

  Figure 16. Theory suggests that the electromagnetic radiation around the CMB splodges should be polarized in a distinctive pattern that could embed clues from the earliest phase of the universe, just after the big bang. The lines in the figure depict the direction of polarization.

  The holy grail of quantum gravity

  The twentieth century’s greatest triumphs in physics were quantum mechanics and Einstein’s general theory of relativity. Unfortunately, the two theories are incompatible.

  Quantum mechanics successfully accounts for the workings of three of the four fundamental forces. It describes electromagnetism especially brilliantly, by treating the force between charged particles as due to the exchange of photons – quantized packets of light. Not so for gravity. You can attempt to apply quantum mechanics to gravity in the same way, by considering the exchange of ‘gravitons’ – hypothetical quanta of the gravitational field. But calculations based on general relativity soon run into trouble, yielding an unending series of infinite or meaningless answers. There have been many ingenious attempts to circumvent this problem by modifying the theory of gravity, string theory being one such approach.

  A further difficulty is that, even with a theory that yields finite answers, the effects of quantum gravity are likely to be incredibly small and hard to detect. On general grounds, physicists expect significant quantum effects of gravity to manifest themselves on a tiny length scale of 10−33 centimetres, often called the Planck length after Max Planck, who first defined it. The Planck length is twenty powers of ten smaller than a proton and is the scale at which strings operate in string theory. The corresponding timescale is 10−43 seconds – the Planck time. Assuming this expectation is correct, it implies that at these ultra-microscopic dimensions, space itself would be profoundly different, perhaps forming a maelstrom of shifting complex foam. When a black hole evaporates to a Planck size, its behaviour is likely to change in important, but as yet unknown, ways. Likewise, in the absence of an agreed workable theory of quantum gravity, we can say very little about the behaviour of the universe before the Planc
k time.

  The success in using physics to explain the cosmos as far back as the first split second must count as one of humanity’s greatest achievements. Nevertheless, people still ask questions about the ultimate origin of the universe. It’s one thing to give a good account of what the universe was doing at some moment just after it sprang into existence. But what about the originating event itself? What has science got to say about that?

  20. Can the Universe Come from Nothing?

  Children have a disconcerting habit of asking their parents where babies come from. Mostly they are fobbed off with vague replies. It’s something of a surprise that before the invention of the microscope, even adults didn’t really know the answer. In fact, the very topic was somewhat delicate and contentious in Christian Europe, because the Church taught that God alone could bring forth new life. Then, in 1677, Antonie van Leeuwenhoek discovered sperm cells, and the fascinating story of fertilization, zygotes and embryo development followed.

  On the whole, science gives a good account of origins – babies, hurricanes, mountains, planets, stars . . . But what about the origin of the entire universe? Where did it come from? What caused it to come into being? And can scientific reasoning even be applied to grand existential questions of that nature?

  Explanations for cosmic existence fall into two broad categories. The first is that the universe popped into being from nothing. The second is that something always existed – maybe not the universe we see now, but not nothing, anyway. Logically, it seems it has to be one or the other. During my career, sentiment has swung back and forth. When I was a teenager, the steady state theory, which posited a universe with no origin, was very popular. By the time I began doing my own research in cosmology, it had been thoroughly eclipsed by the big bang theory, which assumed a beginning at a finite moment in the past. Though the big bang idea was very successful, few cosmologists wanted to be drawn on awkward questions like how, or why, a big bang happened in the first place. Many of them simply declared it to be beyond the scope of science and not a job for cosmologists. Leave it to the philosophers, was a common reply.

  Well, what did philosophers have to say on the subject? Not very much, as it happened. One of the twentieth century’s leading philosophers of science, Bertrand Russell, engaged in a BBC radio debate with an English Jesuit priest, Fr Frederick Copleston, in 1948, and was asked about God and the origin of the universe. This is the downbeat exchange that followed:

  Copleston: . . . But your general point then, Lord Russell, is that it’s illegitimate even to ask the question of the cause of the world?

  Russell: Yes, that’s my position.

  Copleston: If it’s a question that for you has no meaning, it’s of course very difficult to discuss it, isn’t it?

  Russell: Yes, it is very difficult. What do you say – shall we pass on to some other issue?

  Russell wasn’t being evasive. There is indeed a fundamental problem with the conventional idea of a cause being applied to the whole universe: it carries the implication that there was something there before the big bang, something to make it happen. But that would not have been the case if the big bang marked the origin of time itself. The idea that time began with the universe was suggested by St Augustine of Hippo, the fifth-century Christian theologian, who proclaimed that ‘the world was made with time and not in time.’ This ruse was to avoid the tricky question of what God was doing before creating the universe, but Augustine was right to regard time as part of the physical world and therefore to have come into existence with the physical world.

  The possibility that space and time can have edges or boundaries, technically known as singularities, with literally nothing beyond, is an integral feature of modern gravitational theory. There’s a singularity right there in Friedman’s original 1921 analysis of an expanding universe based on general relativity: it describes a universe that springs into being from a state of infinite density, expanding infinitely fast, implying that there was no time (or space) before the big bang. (There is also a singularity at the centre of a black hole, according to Schwarzschild’s original solution of general relativity.) However, the singular origin wasn’t taken too seriously for many decades because Friedman’s model was very simplified, describing a perfectly smooth universe in which all matter is squeezed into a single point at the beginning. Perhaps a slightly irregular arrangement would avoid this singular crush? In the late 1960s, ingenious mathematical analyses by Roger Penrose and Stephen Hawking proved that a singularity of some sort at the start of the universe can’t be dodged by minor changes to the geometry of space or the distribution of the cosmological material. These theorems were so powerful they spawned an entire field of research into singularity formation and structure, an accomplishment deservedly recognized by the award of the 2020 Nobel Prize in Physics to Penrose. Who would have guessed that the edge of nothing could be such a fertile area of mathematical complexity?

  If time did not exist before the big bang, the concept of a prior physical cause is meaningless. Asking what was there before the big bang is, as Hawking once put it, like asking what lies north of the North Pole. The answer is ‘nothing’, not because there is a mysterious Land of Nothing there, but because there is no such place as ‘north of the North Pole’. In the same vein, there is no such place (or time) as ‘before the big bang’. It would be ‘nothing’ (no-thing) in the same sense as the nothing lying north of the North Pole.

  The ‘nothing-before’ version of the big bang theory reached its zenith in the 1980s with the work of Hawking in collaboration with James Hartle. They bravely tried to explain how nothing can become something without violating the laws of physics. To do this, they appealed to that magic box of wonders, quantum mechanics. It had long been realized that time can’t be simply subdivided into ever finer intervals: quantum processes will make something weird occur on a short-enough timescale – the so-called Planck time (see p. 109). Particle physicists often detect effects that take a mere trillion trillionth of a second and, so far, nothing untoward has been seen concerning the nature of time. But, according to theory, at a billion billionth of this already slender temporal slice, the very concept of time breaks down. If time itself can’t be traced smoothly back to the origin, then maybe it’s possible to avoid time abruptly ‘switching on’ at an initial singular moment; time may have ‘congealed’ from some fuzzy quantum ‘pre-time’. Hartle and Hawking bolstered this general idea with detailed calculations based on quantum gravity. Although many scientists dismissed their model as a bit of mathematical chicanery, Hartle and Hawking at least demonstrated the possibility of giving an account, entirely within the framework of physical law, of a universe that has a finite age, yet for which no miraculous or singular first event was involved. If that essential idea can be made to work, we could follow the historical record back to the epoch when time simply evaporated away. By any account, that would truly be the end of history.

  Explanations for the origin of the universe based on physical laws, like that of Hartle and Hawking, tacitly assume that the laws are in some sense ‘already there’ to permit the universe to come out of nothing. Some sceptics say, ‘Oh, so the universe didn’t come from nothing, after all. The laws of physics were there before the big bang!’ But that’s not really true since there was no ‘before’. It’s more accurate to say that the laws of physics transcend space and time; they exist in a mathematical realm that is not part of the physical universe. Some distinguished scientists do indeed take that position. They argue that in the matter of existence, the set of physical laws are the primary entity and have within them universe-creating capabilities. Others disagree and deny that one can think of laws as ‘things’ that exist independently of the physical universe. They reason that the laws of physics are relationships between physical things that necessarily exist only after the origin of the universe. But you can’t have it both ways. Either unexplained laws are already ‘out there’ and account for how the physical cosmos (lawfully) appeared s
pontaneously from nothing, or laws + universe together burst into being ready-made – a joint package of marvels with no explanation whatsoever. It’s Catch 22.

  If this discussion makes your mind reel, don’t worry. Scarcely was the ink dry on Hartle and Hawking’s paper when sentiment swung back, and cosmologists started suggesting that maybe the big bang wasn’t the beginning of time after all.

  21. How Many Universes Are There?

  On the list of Really Big cosmic Big Questions, they don’t come much bigger than this: how many universes actually exist? As the answer is unlikely to be, say, 153, the choice would seem to be 1, 2 or infinity. We can be sure there is at least one. In Chapter 12, I floated the notion of a matching Anti-World to balance everything, so that would make two. How about infinity? If the big bang was a natural event, surely it could happen more than once? Might there be many bangs scattered throughout space and time? Why would there be any limit on this process? The argument is persuasive, and these days most cosmologists I know think that there is indeed an infinity of universes.

  Such a scenario happens naturally in the inflationary theory. In the original version, there is a bang, then a stupendous antigravity force seizes the new-born universe and enormously inflates it, ironing out any preceding messiness. After that, inflation stops and the standard big bang story unfolds. But if inflation erases all record of the cosmic past, how do we know there was any beginning at all? What we have been calling the big bang could simply be what came after inflation. If so, what might have come before inflation? Well, maybe more inflation, back to eternity. Eternity being a long time, if there is a probability, however small, of an exit from eternal inflation into a regular universe, then our universe cannot be the only example. Taking a god’s-eye-view of this ‘eternal inflation’ scheme, the system as a whole just inflates and inflates, endlessly and frenetically. But within the overall superstructure, universes pop up randomly, separating out from the inflating region like bubbles in a fizzy drink.