OUR COSMIC HABITAT III: WHAT LIES BEYOND OUR HORIZON?
Then assuredly the world was made, not in time, but simultaneous with time. For that which is made in time is made both after and before some time – after that which is past, before that which is future. But none could then be past, for there was no creature by whose movements its duration could be measured. But simultaneously with time the world was made.
St Augustine
HOW BELIEVABLE IS THE BIG BANG STORY?
The Big Bang theory has lived dangerously for more than thirty years. Various measurements could have refuted it if they had turned out differently. Here are five of them:
• Astronomers might have discovered an object whose helium abundance was zero, or at any rate well below 23 per cent of that of hydrogen. This would have been fatal, because fusion of hydrogen in stars can readily boost helium above its pre-galactic abundance but there is no way of converting all the helium back to hydrogen.
• The background radiation measured so accurately by COBE might have turned out to have a spectrum that differed from the expected ‘black body’ or thermal form.1
• Physicists might have discovered something about neutrinos that was incompatible with the Big Bang. In the ‘fireball’, neutrinos would outnumber the atoms by a huge factor – around a billion – just as the photons do. If each neutrino weighed even a millionth as much as an atom, they would, in total, contribute too much mass to the present universe – more, even, than could be hidden in dark matter. As discussed in Chapter 6, the actual masses (if not zero) seem to be too low to embarrass the theory. But they could have turned out higher.
• The deuterium abundance could have been out of line with the amount expected to survive from the Big Bang.
• The temperature fluctuations over the sky could have implied a value of Q that was incompatible with what is inferred from the present-day structure in the universe, rather than, as discussed in Chapter 8, being consistent with a value of 1/100,000.
The Big Bang theory has survived these tests. The grounds for extrapolating back to the stage when our universe had been expanding for a second (when the helium began to form) deserve to be taken as seriously as, for instance, inferences from rocks and fossils about the early history of our Earth, which are equally indirect (and less quantitative).
Perhaps we can deepen our understanding, and even ‘explain’ the key cosmic numbers, by extrapolating still further back – not just into the first second but into the first tiny fraction of a second.
We can confidently go back a bit closer to the Big Bang, but not much. For the first millisecond we are less sure of the physics because everything would have been denser than a neutron star. Very hot and dense conditions can be simulated, on a microscopic scale, by experiments that crash together very energetic particles. But there are limits to how far back this technique can take us. Not even the giant Large Hadron Collider, being built at CERN in Geneva, will achieve the energies that all the particles in the Big Bang had during the first 10−14 seconds. Many crucial features of our universe could have been imprinted when the cosmic clock was reading 10−35 seconds, or even less. In these contexts, each factor of ten on the cosmic clock in the age of the universe – each extra zero after the decimal point – is likely to be equally eventful and should count equally. The leap back from 10−14 seconds to 10−35 seconds is then bigger (in that it spans more factors of ten) than the timespan between the three minute threshold when helium was formed (about 200 seconds after the Big Bang) and the present time (3 × 1017 seconds, or ten billion years). In this perspective, there is plenty of action at even earlier stages.
A time chart of some key stages in the expansion of our universe.
Right back at the beginning, the mysteries of the cosmos and the microworld overlapped. To probe these mysteries, we need to relate gravity, the dominant force on large scales, to the other forces that govern individual particles. This is still unfinished business. But the various forces and particles of the subatomic world are now seen to fall into a pattern.
Early in the nineteenth century, Michael Faraday realized that electricity and magnetism were intimately linked: a moving magnet generated electric currents; a moving electric charge, conversely, created a magnetic field. This principle underlies electric motors and dynamos. In 1864 James Clark Maxwell codified Faraday’s discoveries into a famous set of equations, which expressed how a changing electric field generates a magnetic field, and vice versa. In empty space, these equations have solutions where the electric and magnetic fields oscillate. This is what light is: it’s a wave of electric and magnetic energy (as are radio waves, X-rays, and the rest of what we now call the electromagnetic spectrum).
This left just two distinct forces: electromagnetism (perceived as a single force) and gravity. Even Faraday yearned for a unification between gravity and electromagnetism, although he realized that it was premature. A hundred years on, Einstein spent his later years seeking a deep connection between these two forces. This was still a vain quest. Indeed, we now realize that it was doomed because he didn’t then know about the short-range forces that govern atomic nuclei: the ‘strong’ or nuclear force that binds the protons and neutrons together in atomic nuclei (and determines our number ε); and the ‘weak’ force, important for radiative decay and neutrinos. In the somewhat harsh view of his most distinguished biographer, the physicist Abraham Pais, Einstein ‘might as well have gone fishing’ for the last thirty years of his life.
The challenge is now to unify four forces: the three that govern the microworld – electromagnetism, the nuclear force, and the ‘weak’ force – and the force of gravity. The first modern step towards this unification was associated with the names of Sheldon Glashow and Steven Weinberg in the US, Gerard t’Hooft in Holland, and the Pakistani physicist Abdus Salam. The outcome of their work was to show that the electric and magnetic forces (unified by Maxwell) are themselves linked to an apparently quite different force – the so-called ‘weak’ force important for neutrinos and radioactivity. These forces would have been the same in the very early universe; they acquired distinctive identities only after the universe had cooled below a critical temperature of about 1015 degrees (which happened when it was 10−12 seconds old). The biggest accelerators can simulate these temperatures, and Salam and Weinberg were vindicated when experiments at CERN discovered new particles that they had predicted.
In the 1950s and 1960s, so many new kinds of particles were discovered (supplementing the familiar electrons, neutrons and protons) that there seemed a risk that particle physics would become like stamp collecting. But patterns were discerned; the subatomic particles could be grouped into ‘families’, rather as the atoms in the periodic table fall into ‘periods’ and ‘groups’. In 1964, Murray Gell-Mann and George Zweig, two American theorists, introduced the ‘quark model’. Quarks have charges that are 1/3 or 2/3 that of the electron. Experimental support came from Jerome Friedman, Henry Kendall and Richard Taylor, who used the newly commissioned Stanford Linear Accelerator to crash electrons into protons. They found that the electrons scattered as though each proton was made up of three ‘point charges’, carrying respectively 2/3, 2/3 and – 1/3 of the total charge. One counterintuitive aspect of the ‘quark model’, however, is that an isolated quark can never be dislodged even though, inside a proton, the quarks behave as though they are free. (All attempts to detect fractionally charged particles have failed.) By the late 1970s, most of the ‘particle zoo’ had been explained in terms of nine types of quark.
The so-called ‘standard model’ that emerged in the 1970s has brought impressive order into the microworld. The electromagnetic and ‘weak’ forces have been unified; and the strong or nuclear forces have been interpreted in terms of quarks, held together by another kind of particle called a ‘gluon’. But nobody has taken this as the final word: the number of elementary particles remains bewilderingly large, and the equations still involve numbers that have to be determined by experiment and can’t be derived from theory alone. In particular, the ‘gluon’ interpretation does not pin down the strength of nuclear forces, crucially manifested in our basic number ε = 0.007.
The next goal after unifying the electromagnetic and weak forces is to bring in the nuclear force, and thereby achieve a so-called ‘grand unified theory’ (GUT) of all the forces governing the microphysical world (although these theories are still not grand enough to include gravity, which poses a still greater challenge). A stumbling block is that the grand unification is thought to occur at a temperature of 1028 degrees. This is a million million times higher than experiments can presently reach – and to achieve the requisite energies would need an accelerator far bigger than our Solar System. It is hard, therefore, to test these theories on Earth.
Their distinctive consequences in our low-energy world are vestigial: for instance, protons, the main ingredient of all stars and planets, would very slowly decay – an effect that could be important in the remote future but is insignificant now. Everything, however, would have been hotter than 1028 degrees for the first 10−35 seconds. Perhaps the early universe was the only place where the requisite temperature for unifying the forces could even be reached. This ‘experiment’ shut down more than ten billion years ago, but did it leave fossils behind, just as most of the helium in the universe survives from the first few minutes? It seems that it did: indeed, the favouritism of matter over antimatter (discussed in Chapter 6) may have been imprinted at this ultra-early stage. Even more important, the vast scale of the universe, and the fact that it is expanding at all, may be determined by what happened in those brief initial instants.
THE ‘INFLATION’ CONCEPT
Two fundamental questions about our universe are: ‘Why is it expanding?’ and ‘Why is it so big?’ We can trace out what happens during the expansion, and we can extrapolate right back to the first few seconds (and corroborate this with the helium and deuterium abundance). But the so-called Big Bang theory is really a description (and a quite successful one) of what happened after the Big Bang. It says nothing about what set up the expansion in the first place. Another puzzle is: ‘Why does our universe have the overall uniformity that makes cosmology tractable, while nonetheless allowing the formation of galaxies, clusters and superclusters?’ And, still further: ‘What imprinted the physical laws themselves?’
One basic mystery (discussed in Chapter 6) is why our universe is expanding, after ten billion years, with Ω still not too different from a value of one. Our universe has neither collapsed long ago, nor is it expanding so fast that its kinetic energy has overwhelmed the effect of gravity by many powers of ten. This requires Ω to have been tuned amazingly close to a value of unity in the early universe. What made everything start expanding in this special way? Why, when we observe remote regions in opposite directions, do they look so similar? Or why is the temperature of the microwave afterglow almost the same all over the sky?
These mysteries would be solved if all parts of our present universe had synchronized and co-ordinated themselves very early on, and then accelerated apart – and this is the key postulate of the ‘inflationary universe’ theory. The (then) young American physicist Alan Guth put forward this idea in 1981. As so often happens in science, there were several precursors, especially the theories of Alex Starobinski and Andrei Linde in the Soviet Union and Katsumoto Sato in Japan, but Guth made the arguments clear enough to convince most of us that this was indeed a crucial insight. His book The Inflationary Universe2 recounts the ‘eureka moment’ when the idea dawned on him, and how a lively community of theorists debated and developed it further. (Guth also offers frank sociological insight into the American academic scene, from the perspective of a young researcher seeking a niche in an overcrowded profession.)
According to the ‘inflationary universe’ theory, the reason why our universe is so big, and why gravity and expansion are so closely balanced, lies in something remarkable that happened very early on, when our entire observable universe was literally of microscopic size. At the colossal densities that then prevailed, a ‘cosmic repulsion’, rather like an enormously strong λ, came into play and overwhelmed ordinary gravity. The expansion was ‘kicked into overdrive’, leading to runaway acceleration, so that an embryo universe could have inflated, homogenized, and established the ‘fine-tuned’ balance between gravitational and kinetic energy.
All this is supposed to have happened within about 10−35 seconds of the Big Bang! The conditions that prevailed back then are far beyond what we can test experimentally, and the details are therefore speculative. We can nonetheless make guesses consistent with other physical theories and with what we know about the later universe.
The idea behind the ‘inflation’ theory is compellingly attractive because it seems to show how an entire universe could evolve from a tiny ‘seed’. This is deemed to have happened because the expansion is exponential; it doubles, then doubles, and then doubles again . . . Mathematical formulae (unless they are very long and complicated indeed) generally don’t yield huge numbers. The only natural way for a ‘modest’ number to generate a gigantic one – such as 1078, the total number of atoms in our observable universe – is if it is ‘in the exponent’ (to use mathematical jargon), so that it tells how many times the size doubles. Each time a sphere doubles its radius, its volume goes up by a factor of eight (in ordinary Euclidean space); only a hundred of these doublings would be needed in order to reach a number like 1078.
This is just what is proposed as happening during the ‘inflationary’ phase of our universe. The fierce repulsion that drove inflation must have switched off, allowing the universe, having by then enlarged enough to encompass everything that we now see, to embark on its more leisurely expansion. This transition converted the huge energy latent in the original ‘vacuum’ into ordinary energy, generating the heat of the fireball and initiating the more familiar expansion process that has led to our present universe.
The concept of inflation has been boisterously debated ever since it was first proposed twenty years ago. It has been through many variants, based on different assumptions about how the pressure, density and so forth behaved under conditions far beyond anything that we can study directly. But the general idea will surely retain its appeal unless a better one comes along. At the moment, if offers the only credible explanation for why our universe is so large and so uniform. It suggests why the universe is expanding at such a seemingly fine-tuned rate, so that it could heave itself up to dimensions of ten billion light-years.
CAN WE TEST THE INFLATION THEORY?
If a wrinkled surface is stretched by a huge factor, then the curvature reduces until any deviations from flatness are imperceptible. The analogue of ‘flatness’ in cosmology is an exact balance between (negative) gravitational energy and (positive) expansion energy. This is the firmest generic prediction of inflation. Is it fulfilled? The simplest kind of flat universe is one in which Ω is exactly unity. The evidence in Chapter 5 that atoms and dark matter contribute only 0.3 of the critical density seemed at first sight to be a setback. Theorists therefore seized enthusiastically on the claim that the expansion is accelerating, because the energy associated with the number λ must then be added in. Our universe seems indeed to be ‘flat’ (though the more cautious among us may say the jury is still out, and await a definitive verdict within a few years). The ‘mix’ of stuff that makes up the critical density is four per cent atoms and about 25 per cent dark matter, the rest is the ‘vacuum’ itself.
This evidence of ‘flatness’ is moderately encouraging. It at least motivates us to seek further tests, especially ‘diagnostics’ that might reveal details of what happened during inflation. Most detailed ideas about the ultra-early universe have a short shelf-life. The first 10−35 seconds is as uncertain today as was the physics of one second after the Big Bang when Gamow and other pioneers first explored the cosmological origin of the elements. Their first ideas were wrong in important respects, but were corrected and put on a firm footing within a decade or two. Maybe we can share similar hopes about a symbiosis between ultra-high-energy physics and cosmology in the next decade.
Helium formation in the first few minutes involved nuclear reactions and atomic collisions of a kind that can be reproduced experimentally. In contrast, the processes during the inflationary era that determine fundamental cosmic numbers such as Q are too extreme to be simulated terrestrially, even in accelerators. That makes the new challenge more daunting. On the other hand, that very fact provides an extra motive for studying the very early universe. It may offer the firmest tests of new unified theories because it is the only place where energies are high enough for the distinctive consequences of these theories to be manifested. When astronomers are trying to understand cosmic phenomena, they normally utilize discoveries made by physicists in the lab. Perhaps they can now return the compliment by discovering some fundamentally new physics. There are already other instances of this – for instance, neutron stars extend our knowledge of dense matter and strong gravity. But most extreme of all is the Big Bang itself. In the 1950s, cosmology was outside the mainstream of physics – only a few ‘eccentrics’ like Gamow paid any attention to it. In contrast, cosmological issues now engage the interest of many leading mainstream theoretical physicists. And that surely gives us grounds for optimism.
Microscopic ‘vibrations’, imprinted when our universe was smaller than a golfball, inflate so much that they now stretch across the universe, constituting the ripples that develop into galaxies and clusters of galaxies. Theorists still haven’t shown whether inflationary models can ‘naturally’ account for Q = 10−5 characterizing the amplitude of these ripples; it depends on some physics that is still anything but ‘battle-tested’. But we can learn something about the details (and rule out some options) because specific variants of inflation make distinctive predictions. Measurements with the MAP and Planck-Surveyor spacecraft, and surveys of how galaxies are clustered, will offer clues about the inflationary phase, and teach us things about ‘grand unified’ physics that can’t be directly inferred from experiments at ‘ordinary’ energy levels.
Along with the fluctuations that develop into galaxies and clusters, the inflation is thought to generate ‘gravitational waves’ – oscillations in the fabric of space itself, criss-crossing the universe at the speed of light. Objects encountered by such waves feel a gravitational force that pulls them first one way and then the other; they ‘shake’ slightly as a result. The effect is minuscule and its detection in reality poses a formidable technical challenge. The European Space Agency’s LISA project (standing for Laser Interferometric Space Array) is planned to deploy a set of spacecraft in orbits around the Sun, separated by several million kilometres. The distances between them would be monitored by laser beams to a precision of a millionth of a metre.
Even LISA may not prove sensitive enough to ‘feel’ these primordial vibrations. It is therefore a comfort to its designers that other signals should be easier to detect. An intense burst of gravitational waves would, for instance, be generated whenever two black holes collided and coalesced. We expect such events to occur from time to time. Most galaxies harbour a central hole as massive as millions of stars. Pairs of galaxies often collide and merge (we see many such events in progress); whenever this happens, the holes in the centres of the two participating galaxies spiral together.
We can therefore look forward soon to empirical probes of the inflation era. Even if we don’t know the appropriate physics, we can calculate the quantitative consequences of specific assumptions of the theory (the value of Q, the gravitational waves, etc). We can then compare these with the observations, and thereby at least constrain the possibilities.
Any ‘fossils’ of that ultra-early era would be important as missing links between the cosmos and the microworld. One interesting possibility (which loomed large in Guth’s mind when he was developing his theory) is that magnetic monopoles might have survived from the early universe. Faraday and Maxwell showed the intimate relation between electricity and magnetism, but there was (as they well realized) one key difference between these two forces: positive and negative electric charges exist, but ‘north’ and ‘south’ magnetic poles don’t seem to come separately. Magnets are dipoles (with two poles) rather than monopoles (with one); and if we chop up a dipole we never get two monopoles, merely smaller dipoles. Despite many ingenious searches, nobody has ever ‘caught’ a monopole.
Modern theories suggest that monopoles could exist, but they may be immensely heavy (a million billion times heavier than a proton). Because of the high mass, it would need an immense concentration of energy to make them – the kind of energies that prevailed in the very early universe but not thereafter. There are very few monopoles in our present universe – magnetic fields pervade interstellar space, and these would be ‘shorted out’ if there were a population of monopoles. Guth was puzzled by the absence of monopoles because it seemed that they would unavoidably have been produced in the early universe – indeed, his best guess was that their total collective mass would amount to millions of times more dark matter than there actually is. An important bonus of inflation (if it occurred after the monopoles formed) is that it would dilute the putative monopoles, and thereby account for their apparent absence today.
Monopoles are a kind of ‘knot’ in space – in the jargon of the subject, they are ‘topological defects’. Even more interesting are defects in the form of lines rather than points – regions of space that get knotted into tubes far thinner than an atom. They would either make closed loops, like elastic bands, flailing around at nearly the speed of light, or else stretch right across the universe. Some cosmologists have speculated that these defects in space could be the seeds for cosmic structure – in effect, that they contribute to Q. This idea attracted interest in the early 1990s, but turned out to be incompatible with the details of the galaxy clustering that was subsequently mapped out. But these loops could still exist, and they are so extraordinary in their properties (thinner than an atom, but so heavy that each kilometre could weigh as much as the Earth) that astronomers should make every effort to find one.
Miniature black holes are another interesting possibility. A hole the size of a single atom would be as massive as a mountain. As we’ve seen in Chapter 3, this is a direct result of N being so large: gravity is so feeble that it can’t overwhelm other forces on the atomic scale unless the mass of N atoms is packed into the volume of one. Conceivably the ultra-early universe generated the requisite pressures to make them. Even though no present-day process could provide this degree of implosion, maybe some future high-tech civilization could do so – an especially fascinating prospect if combined with the other speculation that, within a black hole, a new universe may sprout and inflate into a new (possibly infinite) space–time disconnected from ours.
FROM ‘NOTHING’?
It may seem counterintuitive that an entire universe ten billion light-years across (and which probably spreads even further beyond our horizon) can have emerged from an infinitesimal speck. What makes this possible is that, however much inflation has occurred, the universe’s net energy can still be zero. Everything has energy mc2, according to Einstein’s famous equation. But everything also has negative energy because of gravity. We need energy to escape from Earth’s gravity – the burning of enough rocket fuel to reach a speed of 11.2 kilometres per second. Down on the Earth’s surface we therefore have an energy deficit compared with an astronaut in space. But the deficit (technically called ‘gravitational potential energy’) due to everything in the universe added together could amount to minus mc2. In other words, the universe makes for itself a ‘gravitational pit’ so deep that everything in it has a negative gravitational energy that exactly compensates for its rest-mass energy. So the energy cost of inflating our universe could actually be zero.
Cosmologists sometimes claim that the universe can arise ‘from nothing’. But they should watch their language, especially when addressing philosophers. We’ve realized ever since Einstein that empty space can have a structure such that it can be warped and distorted. Even if shrunk to a ‘point’, it is latent with particles and forces – still a far richer construct than the philosopher’s ‘nothing’. Theorists may, some day, be able to write down fundamental equations governing physical reality. But physics can never explain what ‘breathes fire’ into the equations, and actualizes them in a real cosmos. The fundamental question of ‘Why is there something rather than nothing?’ remains the province of philosophers. And even they may be wiser to respond, with Ludwig Wittgenstein, that ‘whereof one cannot speak, one must be silent’.
BEYOND OUR HORIZON TO THE MULTIVERSE
The long-range forecasts sketched in Chapter 7 were actually based on an assumption that we can’t test, namely that the parts of the universe beyond our present horizon resemble those we see. If you were in the middle of an ocean, you wouldn’t expect land to lie immediately over the horizon; but you’d know that the ocean wasn’t unending and would eventually be bounded by a continent. Likewise, we may be mistaken in thinking that our universe extends uniformly without limit. We could perhaps be living in a low-density bubble, big enough that its edge lies far beyond our present horizon, yet surrounded by a still larger region that will eventually collapse on top of us. If so, our remote descendants would revise the ‘forecast’ of perpetual expansion when the higher-density material loomed within their horizon. A drastic change only just beyond our horizon would be unlikely; on the other hand, we have no warrant to extrapolate all the way to infinity.
The most important implication of inflation is that it grandly and dramatically enlarges our perspective on the universe. To explain the universe that we see, there must have been enough inflation to account for the 1078 atoms within range of our telescopes. But that’s just a minimum. It may take a long time to stop the inflation once it has started (theorists refer to this as the problem of the ‘graceful exit’ from inflation). Indeed, most versions of the theory suggest that the number of ‘doublings’ should be far more than is needed to account for our observable universe. In Chapter 1, we imagined a succession of views of our universe, each taken ten times further away than the last. Twenty-five frames took us to the limit of our present vision, starting from the everyday human scale. This limit is set, essentially, by how far light has been able to travel in the ten billion years or so since the first galaxies formed. But inflation theorists envisage a universe so much larger that it would take millions of frames, each a leap by a factor of ten, to reach any ‘edge’. This stupendous expanse of space is (to me at least) impossible to grasp. The leap in scale from the microworld to our horizon is as nothing compared with the leap beyond that to the real limit of our universe. Though not infinite, our domain of space and time extends far beyond what we can see. The time before light reaches us from the ‘edge’ is then a number of years written not just within ten zeros, nor even with a hundred, but with millions.
But this isn’t all. Even this colossal universe, whose extent requires a million-digit number to express it, may not be ‘everything there is’. It is the outcome of one episode of inflation; but that episode – that Big Bang – may itself just be one event in an infinite ensemble. Indeed, this is a natural consequence of the ‘eternal inflation’ espoused especially by the Russian cosmologist Andrei Linde. According to this scenario, which requires specific (though still speculative) assumptions about the physics at extreme densities, the cosmos may have had an infinite past. Patches where inflation doesn’t end always grow fast enough to provide the seeds for other Big Bangs. There are variants on these speculations, in which an episode of inflation could be triggered inside a black hole, creating new domains of space and time disjoint from our own.
At this point, let me add a semantic note about the definition of ‘universe’. The proper definition of ‘universe’ is, of course, ‘everything there is’. I am arguing in this chapter that the entity traditionally called ‘the universe’ – what astronomers study, or the aftermath of our Big Bang – may be just one of a whole ensemble, each one maybe starting with its own Big Bang. Pedants might prefer to redefine the whole ensemble as ‘the universe’. But I think it is less confusing to leave the term ‘universe’ for what it has traditionally connoted, even though this then demands a new word, the ‘multiverse’, for the entire ensemble of ‘universes’ – a concept to which I’ll return in Chapter 11.