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DEEP TIME
The Journey of a Particle from the Moment of Creation to the Death of the Universe and Beyond
David Darling 
Kinds of Flowers
The most beautiful experience we have is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science.
– Albert Einstein
Already fading are those strange, shifting impressions of Time Zero, those half-glimpsed images culled from the scientific lore of genesis. With a single instantaneous leap we have returned to a more familiar place: the one-second-old cosmos, with its thick, pervasive stew of protons, neutrons, electrons, photons, and neutrinos, simmering at ten billion degrees.
From this point a new phase of our adventure begins. Inevitably, mysteriously, the path to the future winds away into the gray, misty distance. And even as we strain to see where it may lead, our particle hero, restored to its protonic form, approaches once more out of the bright chaos around.
Urgently, unpredictably, our proton moves, as of old, from one brief encounter to the next. Rebounding from an electron, now from a fellow proton, it seems engaged in an endless, apparently aimless game of subatomic billiards.
Not for the last time do we ponder the prospects for this mad jumble. And yet we recall, too, that the universe has already passed through a bewildering series of transformations – in a single second! So how much more might it not evolve in the billions upon billions of years to come?
Still, factored in must be the rate of cosmic metamorphosis. And that now is noticeably on the wane. The next ten seconds bring no fresh revolution, no great new surprise. At the fifteen-second mark, though the temperature has dropped to a balmy three billion degrees, the particle mix remains the same.
A full minute goes by. And more. So that, half seriously, we begin to doubt whether nature has any creative power left following the first frenetic second.
Two minutes AG. Three minutes. (The doubt grows.) Three minutes 45 seconds AG:
And now, as it has many times before, our proton draws near to another tiny islet of matter – a neutron – collides with it, and scatters. Yet, for a lingering instant, the proton and neutron remain attached. Like two drops of liquid that touch, the nucleons momentarily fuse. Only barely is the vigor of their impact enough to prevent the strong nuclear force – operating between the proton and neutron at close range – from binding the two particles permanently together.
But even as our proton flies off, its independence narrowly preserved, the neutron from which it has just pulled free strikes another proton nearby. And this time there is no subsequent scattering. Instead, the neutron and its new partner remain tightly, securely bound together – as a nucleus of deuterium, or heavy hydrogen (H2).
At last, it seems, the temperature has fallen sufficiently for this new stage of cosmic synthesis to begin. Alongside those most primitive of nuclei, the protons (the nuclei of ordinary hydrogen), small quantities of deuterium start to appear. And not only that. Some deuterium nuclei quickly go on to collide with and absorb an additional neutron and thereby change into tritium (H3), the heaviest form of hydrogen. Others, by chance, acquire an extra proton and so transmute into a lightweight variety of helium, helium 3. In either case the normal mode of helium is but a short step away. Tritium swallows a proton and becomes helium 4 (two protons plus two neutrons); helium 3 swallows a neutron and does the same.
At three and three-quarter minutes AG vast quantities of helium 4 are being produced rapidly all over the universe. And yet therein lies a puzzle. For the fact is that helium 4 is sturdy enough to survive at temperatures of around three billion degrees. In other words, it was sufficiently cool much earlier, at only fifteen seconds AG, for ordinary helium to exist. Why then did it take so long to appear?
The answer lies with the temperature stability of the middlemen: deuterium, tritium, and helium 3 – especially deuterium. Three billion degrees is still far too hot for these weaker-bound nuclei to hold together, so that they are simply blown apart the instant they form. Although the end product, helium 4 is stable at much higher temperatures, its formation is delayed by the more fragile nature of its intermediaries.
Only as the temperature slides down to around 900 million degrees, at about 225 seconds, do deuterium and tritium and helium 3 each manage to cling together long enough for the final jump to helium 4 to occur. And then, quite suddenly, it happens: The universe is 10 percent helium, and the dramatic moment passes at which deuterium finally achieves stability. With the chain reaction process from individual neutrons and protons to helium 4 no longer chocked off at the second level (deuterium), virtually all the remaining free neutrons are gobbled up into helium nuclei.
And our proton? Despite some close shaves, it has retained its liberty throughout this early phase of cosmic nucleosynthesis. Though one in ten of the nuclei around it are now of helium 4, almost all the remainder are free protons like itself. A tiny fraction endure as deuterium and helium 3 (though not as tritium, since this is radioactive and quickly breaks up). And there is a small but dwindling tribe of nomadic neutrons.
Unlike the proton, the neutron cannot live indefinitely on its own. Bound up within a nucleus, it is secure. But alone, unattached, it must, as if it were a live grenade, quickly split apart – another strange idiosyncrasy of nature. Isolate any neutron at random and the chances are fifty-fifty that it will decay – into a proton, an electron, and an antineutrino - within just twelve minutes. Every 100 seconds from now on the remaining population of free neutrons will decline by 10 percent, until the only neutrons remaining will be those enclosed within nuclei.
Four minutes after the Big Bang: Blindingly intense radiation bathes every corner of space. The photons swarm, 100 million of them for every proton and neutron. Electrons and their antiparticles, the positrons, continue their inevitable annihilation, until all the positrons have gone and the residual electrons are roughly equal in number to the protons. There are the ghostly neutrinos and antineutrinos. And, at the other extreme of materiality, there is this new, complex thing called helium.
But why should the universe stop at the helium stage? Why not go on immediately to build still more complicated nuclei, perhaps those of carbon, oxygen, silicon, or even iron? The reason is the same as that for which the formation of helium 4 was delayed. Even when the final product was stable, certain intermediate nuclei – vital stepping-stones in the process of nucleosynthesis – were still highly unstable. Deuterium’s temperature sensitivity caused the hold-up in helium 4 manufacture. Now, for anything heavier than helium, such as lithium 6, beryllium 9, boron 10, or carbon 12, it is the unstable nuclei with five and eight nuclear particles that are the stumbling block. Only in a very different environment, in the dense, central furnaces of stars-to-come, will nuclei more elaborate than helium be able to take shape.
A half hour slips by. Our proton moves within a cosmos cooled to 300 million degrees – just fifteen times hotter than the core of the future sun. Less often now does it collide with other particles. The average density of matter has dropped to just one tenth that of water. Nucleosynthesis has come to a virtual standstill.
And again there is apparent quiescence. Again, after a sudden, frenzied burst of change and synthesis, the anxious waiting for some new, unknown step toward greater cosmic complexity. Only this time the waiting seems longer – interminably long, even by human standards.
Gone forever is the Golden Age of ultrafast transition, when the character of the whole universe could alter beyond recognition within the smallest fraction of a second, or within a few seconds, or a few minutes.
An hour elapses. A day. A year.
A thousand years!
And, all the while, space relentlessly expands, stretching further the kinetic pattern on its multidimensional surface. The density of cosmic matter, along with its temperature, continues steadily to fall. And yet, apparently, there is no change in matter’s quality.
Ten thousand years go by. And even though our proton, like the countless other protons and heavier nuclei around it, often passes close to an electron, it forms no partnership with it. Even though the proton and electron have equal and opposite charge, and are therefore powerfully drawn to one another, they fail to come together in stable alliance. X-ray and ultraviolet photons – bullets of high-grade energy – strafe the fledgling cosmos, instantly stripping away any electrons that dare to enter bound states around a nucleus. Laser-intense, ubiquitous, the young electromagnetic field tears apart anything resembling an atom. And so, for millennia upon millennia, while the universe burns this bright, there is only a writhing, thinning, electrically charged mist – a plasma – of loose nuclei and electrons.
Or so it had seemed. And yet there may be more to this young universe than simply a hot, spreading fog of particles and blazing light. For now the saga of our proton is beginning to take a strange new turn. And the prospect is slowly emerging that there may have been other things born of the Big Bang, bizarre, almost indescribable things, that have surreptitiously found their way into nature.
At first it had seemed incredible. But now there can no longer be any doubt. Our proton is being pulled, gradually, irresistibly, over thousands of years, toward ... what? Some unknown attractor. An obscure but tremendously powerful source of gravity. And it is not just our own tiny particle but all of the subatomic matter for light-years around that is streaming in toward this unnamed, previously unsuspected phenomenon. Nor is the mystery attractor unique. There are billions of others of its kind strewn about space, each busily spinning its own cocoon of hot plasma. Evidently, even as the universe continues its headlong outward rush, portions of it are being drawn together locally. Material is steadily accumulating around these newfound objects instead of being diluted more and more by the overall cosmic expansion.
Which seems promising. For while matter holds stubbornly together, there is the chance at least of it forming more complex arrangements. While protons, electrons, and the other shards of genesis remain in reasonably close contact, they can hope to interact more often and combine and so further evolve.
Yet still there are no visible clues as to the nature of our proton’s lure. Only vaguely, through enhanced perception, do we begin to sense some of its outlandish properties. At the heart of the influence, it seems certain now, is no ordinary object, no familiar aggregation of matter. On the contrary, whatever is doing the pulling is exotic in the extreme – a loop of ... something ... a light-century long, yet no thicker anywhere than a million trillion trillionth of an inch. A shoelace length of it would outweigh the Himalayas. An yet, because if the terrific tension within it, the creature fiercely writhes, like a monstrous captured eel, at speeds approaching that of light. This is a cosmic string – a long, wriggling, ultra-thin tube of energy trapped since the dawn of time.
It was at that primal moment when matter first appeared, when the strong force first split away from the electroweak, that cosmic string first flickered into reality. The process by which it did so resembled the freezing of a pond or lake. Before 10-35 s AG the vacuum of space was smooth, homogeneous, the same at all places and in all directions. Before 10-35 s AG, the scientist would say, the universe was highly symmetric. But then, as the strong force peeled away, some of that ancient symmetry was lost, was “broken,” forever.
In the same way, liquid water has a high degree of symmetry. It appears the same from whatever point or direction it is seen. Yet, as it turns to ice, water forfeits much of its regularity and simple beauty, retaining only certain preferred axes of symmetry where its molecules have lined up in orderly fashion along crystal planes.
And there is a second source of novel complexity here. For of course an ice film never forms all at once across a stretch of water. Rather, the freezing spreads out from various random spots until the whole surface is covered in patchwork style. Yet since the crystals growing at one center cannot possibly “know” what is happening elsewhere, almost certainly they will have symmetry axes pointing in a different direction from those of crystals of neighboring centers. Where the ice fanning out from two points meet, there will appear an irregularity, a discontinuity. In other words, an ice sheet will contain defects where the direction of symmetry abruptly changes.
And this is exactly what happened when the strong force “froze” out, 100 billion trillion trillionth of a second after the Big Bang. Maybe the physics involved was more esoteric, the properties less commonplace than those of a village pond in winter. But the analogy holds true. A cosmic freezing took place at 10-35 s AG. And as a result of this the universe acquired defects – wrinkles or internal boundaries – one manifestation of which is the fantastic object toward which our proton is now speeding.
Little wonder then that cosmic string should seem so extraordinary. The stuff it contains has survived unchanged from the era of the grand unification, when the energy density was inconceivably high, when all of space-time was curled up within a region far smaller than a nucleus. Cosmic string has no right to exist – does not belong – in a universe tens of thousands of years old. Yet here it is, a living fossil, playing a pivotal role in the development of more mature forms of matter.
Without cosmic string, we begin to realize, nothing much could have come of the hot breath of genesis. Simply, it would have been swept away, endlessly dissipated. The self-gravity of the cloud of particles that billowed out of the Big Bang would have been far too feeble by itself ever to cause the cloud to become clumpy. And without the prospect of some local infalls of material — of the condensation of knots and swirls of richer material – what hope could there be for more interesting structures to emerge? Structures like diamonds and oceans. Like elephants and senators. And like those of us who strive to understand how all this came about. The strange truth is surfacing that every marvel of creation owes its existence, ultimately to cosmic string.
But then, how did – and how will – cosmic string evolve? That now becomes a paramount issue. Because cosmic string, it seems, both predated and predetermined the large-scale structure of the universe. Just how did string itself evolve? Moreover, what effect is it having – what sort of material structures is it seeding – in the current universe of our particle hero, 30,000 years after the Big Bang? And what further influence will it bring to bear as time goes on?
Return almost again to the beginning of the universe. Revisit those moments just before 10-35 s AG when there is as yet no matter, no string. When there is only a vacuum. Not nothingness, but vacuum – a state of minimum energy in the absence of particles. Later, the energy of that vacuum will fall to zero. But in the earliest instants of time, the vacuum is enormously energetic, pregnant with creative potential.
Now the great moment approaches. The cosmic pond is about to freeze over. Within many scattered, patchwork domains the phase transition begins, spreading quickly outward. Inside these domains much of the latent energy of the vacuum is discharged spontaneously as a mass of subatomic particles. Matter, in huge amounts, is born. But along the narrow margins, where neighboring domains meet, fragments of the ancient, high-energy vacuum are preserved. Preserved perhaps in various forms – as null-dimensional points (particles called magnetic monopoles) and as two-dimensional sheets (domain walls) – not only as linear threads. But it is the threads that most interest us. For these wormlike “cosmic strings,” we suspect, are destined to lay the foundations of the future universe.
On our first journey here we failed even to notice the string, so overshadowed was it by the sudden, spectacular arrival of matter. But now, with our awareness trained upon it, we see the string clearly. It pervades the early universe like a tangled web. A web that has no endpoints, for its individual strands are either infinitely long or fashioned into closed loops.
Wriggling violently because of the tension within them, the strings often cross themselves or one another. And when this happens they break at the point of intersection, only to join up again instantly in some fresh configuration. Existing loops, twisting into figure eights, split in two. Infinitely long strings, repeatedly coiling back on themselves, shed loops continuously.
To begin with, these loops are small. Even squirming at near-light speed, a string takes time to curl back on itself and cross over. The closer it is to the grand freezing, the lesser the opportunity for folding back, and the shorter the resulting loops. Small loops, then, are characteristic of the early universe, larger loops of times more remote from the Big Bang.
And yet this does not mean that space gradually becomes more and more congested with string loops of every size. As soon as a new loop is formed it begins to vibrate wildly in the manner of all of its kind. Just as a plucked guitar string generates sound waves, so a vibrating cosmic string emits rhythmic waves of gravitation. Waves which, in turn, steadily waft away the string’s energy, causing the loop to shrink – and eventually to disappear. Whatever its initial size, a string loop can survive for only about ten thousand oscillations. And since smaller loops oscillate faster than larger ones, their lifetimes must be correspondingly less.
Time passes. Space grows. And the complex quivering network of cosmic string evolves. Such is the style of its evolution that the string network retains the same overall appearance. Only its scale changes. Smaller loops die out. Larger ones, as they are born, become the dominant type. And so it goes on. Steadily the average size of loops increases. As does their mass. As does the strength of their gravitational attraction.
And so to a crucial question. When did cosmic string start to affect significantly how ordinary matter evolves? When did it start to make matter clumpy? Not right away. Not immediately after the Big Bang – because then the loops of string were still too small, too lightweight, too short-lived to act as important centers of condensation. Besides, in the very beginning the material in the erupting fireball of genesis was much too hot to be shepherded and corralled into many distinct lumps. Only as the string loops grew and matter cooled could the universe become noticeably clumpy.
And now we watch this process with childlike curiosity. In imagination we hold up the young universe in our hands, as if it were a crystal ball. We turn it around and peer inquisitively into its depths as it evolves through its first few thousand years. And we gaze at the quick-spreading mist of glowing matter, at the dark, shivering veins and loops of cosmic string embedded within the mist. Wondering what will happen next. Knowing that somewhere in this precious everything-globe is our own tiny particle.
Gradually, the glowing mist within the ball changes. Countless microscopic points of more intense light begin to appear all throughout it. And we know that at the core of these bright points there must be a loop of cosmic string. A loop roughly 100 light-years long, it transpires, for this was the critical size of string needed to trigger the local infall of matter in the early universe. As soon as loops of that length were formed they began pulling matter from the regions around them. And slowly, over thousands and thousands of subsequent years, each of them drew together a great shining cloud of particles that one day would become ...
But now we seem to be falling into the cosmic crystal ball. Falling in, head over heels, and shrinking. Shrinking. So that, in a few moments, that toy we had previously held in our hands has become, once again, the real universe, 30,000 years after its birth.
Once again we see our particle hero. And we realize now, as if in waking from a dream, that we are inside one of those tiny bright lights that condensed from the glowing mist of the crystal ball. Only now it seems no longer tiny. We have the vague impression of a huge structure taking shape around us. Yet we cannot actually see it. From the vantage point of our proton we cannot see anything but a uniform haze of light. It is as if a great city were being constructed, but all of it in the midst of an impenetrable fog.
Nor is the source of that light-fog hard to find. Looking out from our particle, we notice that the billions of photons swarming around are still highly energetic. Each of them, on average, is still well able to disrupt any would-be atom. And because of this the nuclei and electrons in the huge cloud to which our proton belongs remain in their dissociated state.
But free electrons, it turns out, offer a huge target for any approaching particles of light. An electron simply “looks” much bigger to a photon when it is outside an atom than when it is inside. And so the photons habitually bump into the very specks of matter whose they have helped maintain. Each time this happens – each time a photon scatters off an electron – it adds a little to the overall pressure of light that acts on the whole plasma cloud.
Since no photon can travel far without blundering into an electron, this explains why there are no glimpses of the far-off at this stage. Zigzagging its way across space, light arrives at our proton (and at every other point in the cosmos) from all directions with equal, inscrutable brilliance. Frustratingly, for us eager tourists, the universe is totally opaque.
But not only that. Not only is the light-fog spoiling our view. Much more important, it is influencing the very evolution of the universe. The pressure of light, caused by photons continually bouncing off free electrons, is buoying up the clouds of nebulous matter that have gathered, like cotton candy, around cosmic string. Even as the gravitational force of that string, aided by the self-gravity of normal matter, strives to squeeze the clouds tighter and tighter together, so the outward pressure of radiation attempts to blow the clouds apart. The result: A balance point is reached. The nascent clouds remain intact. But they are prevented from collapsing further by the force of light within.
Years pass. The universe grows. And the cosmic temperature falls. 100,000 years AG, 200,000. Still no change. 300,000 years AG.
A third of a million years into its expansion. And the universe has seen its temperature decline to a mere 3,000 degrees. The dance of the photons is now less exuberant. And slowly, surely, it is becoming cool enough for the first atoms to form.
Not far away a proton and an electron combine. From their marriage – a hydrogen atom. But will it survive?
A photon approaches on collision course, strikes the newly-wed electron, which then absorbs the photon’s energy. Higher it jumps, away from its proton partner. Yet the electron is not thrown free. The energy it gained was not sufficient to snap the electromagnetic bond it had established with its tiny nucleus. Simply, the electron hops to a higher energy level within the atom before quickly falling back to its normal, “ground” state.
Nearby another hydrogen atom forms. Again, within moments, its electron is torpedoed by a photon. Again the same result. The photon narrowly lacked the energy it needed to sunder the electron-proton pair.
More and more the atomic population swells. Atoms of hydrogen and helium mingle thickly now with the remaining solo electrons and protons. And with the decline in the number of free electrons, there is less opportunity for photons to scatter. So the radiation pressure throughout the cloud is falling rapidly.
Now it is 335,000 years AG and our proton is no longer alone. It has found itself an electron mate and now moves in close partnership with it through the cooling, condensing mist around them. The pressure of light has eased further, leaving gravity, always waiting in the wings, virtually unopposed.
The long era of plasma is nearly over, and with it the time during which radiation and matter were inextricably coupled. A new age is bout to dawn – the age of the atom.
As it cools the universes changes dramatically in color. In the beginning, during the first few hours, it was the lurid, alien hue of gamma rays, the most energetic, highest-frequency waves of the electromagnetic spectrum. Then, steadily, the peak of its glow drifted down into the X-ray region, and more recently into the ultraviolet. Some tens of thousands of years ago the main light of the universe began to fall upon that narrow window accessible to human eyes, the most numerous photons around clustering at the violet and blue end of the optical spectrum.
Now, with its average temperature sinking just below 3,000 degrees, the cosmos shines everywhere yellow. Glorious, brilliant, golden-yellow. And with matter at last shedding its opacity to radiation, some of this aureate light is crossing vast regions of space to yield the first true cosmic vistas, the first startling glimpses of the faraway.
Between nearby scudding cloudlets of partial plasma, still resistant to the passage of light, we catch breathtaking glimpses of what lies beyond. For a few moments, or perhaps years, we gaze through these clear, temporary portals in our own supercloud to witness events in the greater world outside. Light-years away, in every direction, other bright mists of light are condensing and evolving. And, in general design, we know that they must be much like the structure in which we find ourselves.
Once more we turn inward to contemplate our own minuscule hero. From its birth out of spacetime to primal energy speck, to quark, to proton, to atomic nucleus it has progressed – even as the macrocosm around it has evolved. Nature, it appears, is growing in sophistication at every level, from the subatomic to the cosmic. Growing in sophistication, complexity, and variety. And revealing, moreover, as times goes by, a most potent, inner creative urge. Where will that irrepressible drive to unfold lead next?
Dimly we sense the movements of matter around us. The streaming of gas. The passage of giant, nacreous shapes. We feel the indomitable force of gravity drawing together the hydrogen and helium of this native cloud, for since the counteracting pressure of light has all but vanished, there is everywhere the impression of compaction and collapse.
More rapidly now does time seem to pass. Tens of thousands of years are as moments to us. Our proton, in its atomic spaceship, is caught up in some great, fast-flowing current. Yet it remains uncertain where that current will lead. Only can we be sure that the local density of gas is rising. That the cosmos, once more, is in a fervent creative mood.
And again our thoughts race out to the universe at large. So that it seems, even as we ride with our particle toward its new fate, we stay closely in touch with the changing appearance of the entire cloud about us. A hazy ball it is from the outside. A golden ball, one of billions, its boundary blurred where it meets and merges with the ambient ocean of sparser cosmic matter.
For a while it seems as if we may have circumscribed the material essence of the universe. There is ordinary matter, mostly atomic hydrogen and helium. And there is cosmic string, about which great clouds of normal matter have congregated.
But even as we wait to see what new inventions nature may bring forth, we sense that something is awry. The movement of our proton is wrong. The dynamics of the whole universe is wrong – wrong, at least, if we allow only for the gravitational effects of conventional matter and of string. There must be some other player, some other wielder of gravity whose presence we have formerly overlooked. And though apparently hidden from view, nonetheless we can be sure now that this third component, this “dark matter,” exists.
Like normal matter, and like cosmic string, dark matter must have been forged within the earliest moments of the Big Bang, around the time of the grand deunification at 10-35 s AG. But forged of what? Not of baryons – not of protons or neutrons or any near relations of these particles. That much is certain.
But then what does dark matter contain? Neutrinos? Perhaps – but only if neutrinos have mass, and that issue is far from being resolved. On the other hand, maybe dark matter is fashioned of some much more unfamiliar stuff. Depending on the exact blueprint the universe followed, species of particle unmentioned in our tale to date may have flashed into existence at time’s threshold. Among these exotica, these prime candidates for dark matter: gravitinos, photinos, and axions. Each a new, conjectured type of particle. Each with its own unique attributes, the details of which are not important to us – except in one regard. That is, the nature of these particles is such that they shun interaction with ordinary matter. They cannot be seen, cannot be detected in any way, save by their gravitational influence.
And that influence, we realize now, is very, very powerful. So powerful that to explain the way the visible contents of the universe are behaving it seems there must be at least ten times more dark matter than bright. Which is to say, no less than nine tenths of the cosmos is in a form that renders it entirely unobservable.
One hundred million years have elapsed since first we saw our proton. And how much its environment has changed in that time! No longer is the universe simple to describe, its energy and material contents spread more or less evenly about space. Now there are great evolving associations of matter, held in check by their own self-gravity, in which the density is much higher than average. And between these gatherings there are even larger gulfs where the density is low – and becoming lower still.
With vision divided between two levels, we continue to watch nature’s progression – from the viewpoint of our hardy proton and from a sweeping global perspective. Before us the microcosm and the macrocosm are simultaneously arrayed. And between each breath a million years go by.
Steadily we begin to discern the import of dark matter. We begin to appreciate that whereas cosmic string may have triggered the development of large-scale structure in the universe, it was due mainly to dark matter that this development could continue.
In hosts far greater even than those of the protons, neutrons, and electrons, the mysterious particles of dark matter assembled around cosmic string. At first these two very different subatomic creations intermingled. But then, gradually, the constituents of bright matter and of dark began to segregate. The particles of ordinary matter started to drift inward, to slide down the steep slopes of a valley – a gravitational valley – sculpted by the encompassing cloud of dark matter. For now gravity is revealed to us in this new and more lucid way. Whereas before it had been an unexplained “force,” broadcast somehow by any object with mass, now we can comprehend gravity in terms of the very geometry of space and time.
Spacetime, so our earlier voyage to genesis revealed, is comparable to an elastic skin. Empty, devoid of matter, its strange supple surface is smooth and flat. But place objects upon it and the elastic skin of spacetime stretches. Each material thing, from the humblest subatomic particle to the mightiest loop of cosmic string, creates its own depression in the spacetime skin around it – the greater the mass, the greater the resulting warp. Any other mass traversing the curved sides of such a warp will have its trajectory altered, just as a rolling marble is deflected from its original course if it comes across a dip in the floor. Seen at a superficial (human!) level, the ensuing shift in an object’s path would naturally be taken as evidence for an invisible “force” – a force of gravity. But with our awareness now focused on the mutable topography of spacetime, we grasp a deeper truth – that mass determines the local geometry of spacetime. And that the details of this geometry, in turn, influence the motion of other objects nearby.
Massive indeed is the great cloud of dark matter that now surrounds our particle and all of its sibling nuclei of hydrogen and helium. To our sensitized vision, the spacetime valley created by this unseen cloud appears like the inside of a giant bowl, the particles of ordinary matter around us like tiny beads spiraling down the sides of that bowl. Racing to the bottom of the bowl – the floor of the spacetime valley – where they will meet and unite.
Faster and faster comes the succession of events. Our own proton, with its electron companion, is moving ever more swiftly, part of a vast, warming, thickening vortex of matter. And now, within this giant, rotating cloud, this cosmic tornado, many smaller features are appearing. Streaks and swirls, self-gravitating knots and eddies are forming, breaking up, reforming, out of the haze. Held captive by their beauty and prismatic variety, we fail for a moment – for a few million years – to grasp what is happening. Then realization dawns: Deep within these condensing nebulae, the first stars are about to shine. Realization widens: The emergence of stars heralds a new stage in the development of the supercloud around us.
A momentary clearing opens between the gas and dust and glowing stellar delivery room. And through it we look beyond. Hundreds of thousands of light-years beyond. To see, by the uncounted millions, other misty, still-forming cities of stars. All across space the galaxies are being born.
All across space?
No, not quite. Nature would never simply daub the galaxies at random throughout the universe. Complexity, subtlety . . . and beauty – these lie at the core of all evolution. “Expect the unexpected!” could be the cosmic motto. The embryonic galaxies, we see, are arranged not haphazardly but in great winding ribbons. Ribbons millions of light-years long. Slender filaments, galaxy-sequined.
And of course it had to be that way. Now it all begins to make sense:
Three things were created just after the Big Bang: ordinary matter, dark matter, and cosmic string. Each, for many years, evolved independently of the others – concerned, as it were, only with its own development. There was little or no coupling. Ordinary matter spread and cooled and transformed by stages into atoms. But left alone it would have thinned and thinned endlessly. Dark matter, whatever it may be, spread and cooled likewise. And by itself, too, would have been hopelessly scattered by the outblast of genesis.
But then there was string. By its very nature, string was immune to dissipation. Ten thousand years after the Big Bang the space-wide network of cosmic string looked in essence the same as it did half a second after it was formed. It had simply grown bigger — in exact proportion to the increased size of the universe. And that property of self-similarity was vital. Because it meant that string could provide a permanent anchor, or framework, for matter to hold on to.
At first the loops of string being generated were too small, too gravitationally feeble, to counteract the explosive power of the Big Bang. But, in time, longer and more massive loops appeared. By the time the universe was several centuries old, loops big enough to start the slow nucleation of the galaxies were in existence. Only about 100 light-years long were these curious galactic seeds. Yet their gravity was compelling enough, intense enough, to draw matter in from their surroundings. As this matter was captured, its own gravitational force fell in league with that of the string. Which was essential. For as the years went by, and the pregalactic cloud of matter gathered, the string seed within it steadily withered. A piece of cosmic string 100 light-years long can last for only about a million years — a trifling instant in the lifetime of a galaxy. But after that first million years enough matter condensed to be able to continue the task of galaxy-building unaided by string.
Strange had been that symbiosis. Matter needed string to rally it together and would have progressed no further without it. String, on the other hand, would have served no purpose without matter. On its own, string can produce nothing but more string. Strange, too, that all hope for a more interesting universe to come should rest with what were, essentially, incidental defects. But then this whole reality is beginning to look more and more as if it is exquisitely intertwined, its various parts entirely dependent upon one another.
Consider, too, the relationship between normal matter and dark. Both these types were lured, indiscriminately, into the pregalactic clumps by cosmic string. Dark matter dominated – perhaps by a factor of ten or more — over its visible counterpart. Dark matter appears incapable of evolving any further by itself. But once the binding power of string had gone, the gravitational influence of dark matter was to prove decisive in ensuring the continued collapse of hydrogen and helium. Without ordinary matter, dark matter would have had no meaning. But in the absence of those vast, enveloping clouds of unseen and unseeable material, all the infinite variety of forms that atoms and molecules and planets and life can take would have been unrealized. Interconnections. Always, interconnections. Nothing in nature, it seems, exists without a purpose.
And those ribbonlike clusters of galaxies before us now? They are readily explained. For of course the string loops never stopped growing in size. We overlooked that while our attention turned inward to the development of our own protogalactic cloud. But now, as we stare through that ragged gap in our new-formed galaxy, we see clearly the filamentary arrangements of the galaxies at large. Why are they so? Because they, like the individual galaxies, have built up around cosmic string. Only in this case the loops involved were bigger. Very much bigger. Perhaps millions of light-years long.
Even as the galaxies were taking shape, neighboring protogalactic clouds fell collectively under the sway of lengthier loops of string. Several clouds clustered together along a filament. And then, as even longer loops evolved, these fledgling groups of galaxies were themselves drawn together to create bigger clusters. Clusters that to us now seem like fabulous shining rivers and tributaries coursing this way and that, spanning the light-years.
The birth of stars. The birth of galaxies. The emergence of great galaxy clusters. All this is taking place simultaneously as we look out on the universe of about 150 million AG. Matter everywhere, on every scale, is developing with vigor.
But too long have we been seduced by such grandeur. Preoccupied with the overall evolution of the cosmos, we have neglected our own trusty particle. How far has it traveled in our absence? How many new suns has it seen beam out their first, triumphant rays? This young galaxy in which it moves has become a true metropolis of stars since last we surveyed it. Billions of stars have already condensed and blazed forth within it. Many more are incubating deep inside the huge dusty cloud banks that throng the galactic interior.
And now into one of these great star-making machines our intrepid particle is tumbling. Not since the earliest days of the universe has it been engulfed by matter this dense and warm. But whereas, in the wake of genesis, particles everywhere were flying frantically apart, now, inside this nebular womb, matter is pulling itself confidently together. Over millions of years denser-than-average clumps assert themselves throughout the cloud. Then, over millions more years, these richer associations self-gravitate further into smaller, denser, warmer globules – star embryos.
Too close to such an object has our particle ventured now. So that it has become prisoner of the globule’s gravitational field. Deeper into this congealing dark the particle plunges. More and more thickly other atoms of hydrogen and helium, along with fine grains of dust, swarm and stream around it. And with the rise in density comes a rise in temperature. Until after a few more million years ...
Star birth!
From somewhere below – from the core of the globule – hot blue light is suddenly erupting. Apparently the central yolk of this collapsing stellar egg has become so dense, so hot, that now it can generate its own intense outpouring of energy by the “fusion” of hydrogen into helium. As to the details of that process, they can await description. And in any case our particle is not directly involved with them. Not in the Dantean core, but in the relatively cool, upper atmosphere of this newest of stars our hero finds itself. Quickly stripped of its solitary orbital electron in the first searing blast of stellar radiation, it roams within the topmost few thousand miles of the star as a naked proton once more.
Like a newborn baby, the infant star coughs and sputters, as if struggling to win mastery over its fiery core. Brilliant but erratic torrents of light, visible and ultraviolet, pour from its surface, rush into the surrounding space, and break upon any unconsolidated gas nearby. So is this residue of the parent nebula set aglow by the light of its progeny. Soft red and blue shines the circumstellar cloud – some of its radiation owing to starlight glanced off dust grains, the rest due to the glow of recombining atoms, like those in a neon tube, temporarily parted from their electrons by the star’s blazing ultraviolet.
But not only does light from the new sun decorate the surrounding nebula in glorious hues. It also sends powerful shock waves through the nebular gas, waves which, as they travel out, compress the gas and eventually help trigger the formation of other stars.
Perhaps, too, in due course, this star that harbors our proton would have nourished its own family of planets. Already some icy globules are condensing out of the swathes of gas and dust that remain in attendance. But how those worlds might have fared in the fullness of time, how they might have evolved under other circumstances, we shall never know. Because, even as they take shape, they move quickly toward their doom. Soon, terribly soon, a cataclysm will befall this pristine sun and its environs that will tear it apart completely.
How could we have known that as our particle journeyed around and around this nascent galaxy, its path had it ever closer to the galactic heart and to the beast that dwells there. How could we have known that only a thin veil of obscuring dust lay between its newfound stellar home and the lair of that impossible monster?
But now the veil is lifting – no, is being savagely wrenched away. And at last the creature at the core of this youthful star city is revealed.
Our shock on seeing it is like that of walkers who, as the mist around them suddenly clears, realize they have been strolling at the very edge of a precipice. Except that from this precipice there is no shrinking back.
Directly ahead, spanning half the sky, is a flat, whirling disk. Slightly edge-on we see it. A hot, fiery disk, composed of what at first looks like molten lava, until the scale of the thing bursts in on our stunned senses. Those lumps in the outer reaches of the disk are not melted rocks. They are whole stars, their contents being pillaged, their planetary systems ripped from their grasp. Farther in, nearer the hub of the disk, the material is even hotter, smoother, spinning more rapidly – here the stellar remains are beyond recognition. And at the disk’s very center? There lives the dark ruler of this hellish realm, a source of gravity so tenacious that it will let nothing, not even light, escape from its clutches. There, the star wrecker – a supermassive black hole.
How had it come to be here? This bottomless pit in space-time. This aberration of nature, less than a light-day across yet with the mass, and gravitational pull, of a hundred million suns.
Perhaps it was formed, along with many others of its kind, in the first instant of the Big Bang. Black holes great and small might have been bred in huge numbers at that extraordinary moment, when matter was so dense that even the slightest irregularity ran the risk of vanishing down a gravity chasm of its own making. But then, if this monstrous black hole really is so ancient, what relationship could it have had to the cosmic string that seeded the galaxy? How did the string and hole come to be associated? And why was the string not simply swallowed up by its even more gravitationally severe partner?
More likely, it seems, the core black hole came later, long after the string had wasted away. Perhaps it was born only when the protogalaxy had acquired most of its mass and the bulk of its ordinary matter had begun to condense at the center of the encompassing cloud of dark matter. As millions of stars’ worth of hydrogen and helium started to pile up at the galactic core, it could well have reached some critical density at which it condemned itself to ultimate collapse. And so may the dark heart of this fetal galaxy have come into being.
For stars well distanced from the galactic center, the demon that lurks there poses no threat. From afar the black hole’s gravitational field – the curvature of its spacetime crater – seems no different from that of any equivalent mass. Just as the Niagara looks like any other big river. Until it hurls itself spectacularly over the Falls.
Too late is the terrible secret of the black hole revealed. Namely that, beyond a certain point, the spacetime walls surrounding the hole plunge sheer to infinity. Once over the brink of those fearful cliffs, there is no hope of escape. No hope for the tortured, blazing matter in this strange whirlpool before us. Already it has been drawn in beyond the point of recall.
And now the young star that shelters our proton is coming to the fringes of the vortex. Is being tugged and stretched, stretched in to an egg shape, by forces it cannot resist. Other stars from the same nebular brood have already succumbed to this gravitational assault. Their gassy contents are smeared in great burning arcs around the rim of the swirling disk. And suddenly our particle, too, is cast into that infernal whirlpool. Its star has finally given up the unequal struggle and been dismembered. Looking out to the edges of the spinning disk, we see other stars and gas clouds preparing to meet the same fate. Preparing to become fodder for the insatiable beast at the core.
How big is it, this whirlpool of plundered star stuff? Ten light-years across? One hundred? Somewhere between the two perhaps. And why is it so hot? The black hole gives off no heat. Gives off nothing at all. So why should its meal burn so brightly?
Those parts of the disk closest to the black hole, we notice, are spinning the fastest. At the periphery the rotation speed is very much less. (Just as the outer planets orbit more slowly around the sun than the inner worlds.) Because of this differential rotation, a tremendous amount of friction occurs between neighboring parts of the whirlpool. The outer layers try to slow down the inner layers, the inner layers to speed up those farther out. And as a result there is a great release of energy as the competing factions slip and slide against one another – energy ultimately supplied by the inexhaustible gravitational reserves of the black hole.
Now that process is graphically revealed to us. In the midst of a small clot of ex-star matter, our particle careers on its terminal flight toward the central hole. More and more furious becomes its chase around the wheeling vortex. Faster it goes, and deeper. Becoming hotter. Faster yet. And as the doomed material around us gathers speed, we see very clearly that some of its frenzied energy of motion is being passed on, as frictional heat, to gas farther out in the disk. So is there a continuous passage of high-grade energy from the fast-moving inner parts of the disk to the more sluggish-moving outer parts. And thus is some of the black hole’s gravitational energy converted to heat and light. By proxy the darkling beast shines.
At the outer brim of the accreted disk the temperature had been only a few thousand degrees. Hot enough to make the captured gas there simmer in the infrared and visible regions of the spectrum. But as our particle is swept inward the temperature climbs. To tens of thousands of degrees it climbs. So that matter here glows white and violet and, eventually, ultraviolet.
And now the beleaguered proton has come almost to the threshold of the black hole itself. To the innermost zone of its vassal domain. Where the temperature is ten million degrees, and matter, in its death agony, lets out a final scream of X-rays.
And then?
In a few days it will be the end for our particle. Once it has crossed the invisible barrier known as the “event horizon,” which immediately surrounds the black hole, it can never again return to the normal universe. The event horizon is like a one-way valve to . . . other spacetimes? Total obliteration? Whichever it is, no news of what takes place within a black hole can ever be sent back out. The black hole is truly, definitively black.
Our proton begins its last trip around this wild carousel. In just a few hours from now, along with other ill-fated matter around it, it will penetrate the event horizon and . . .
Collision.
A chance bump with a neighboring particle has abruptly altered the proton’s trajectory. Now it is hurtling out of the whirlpool – seemingly on a direct collision course for the black hole.
Before us the monster gapes, like the mouth of a colossal, round cave, impenetrably dark. But. Our hero is heading, we realize now, not for the center of that blackness. Instead, it is veering up. Veering sharply up and away from danger. With luck it may skim the event horizon without actually crossing it. Up, up.
To safety.
And at last we take time to marvel at the scene of destruction around. Below: the fervent whirlpool, ablaze with radiation from X-rays to infrared. At the outskirts being joined by sundered stars and tattered clouds of gas and dust; at its inner margin feeding the core monster with looping, licking tendrils of multimillion-degree plasma.
And ahead? Ahead, the escape route.
Two points: This black hole spins (as perhaps all such objects do). And it is not only matter that it lures and bends to its will. Magnetic fields, as well as more tangible stars and interstellar gas, are being sucked in by its powerful gravity. As the black hole scrambles the incoming magnetic field lines and drags them around at high speed, so it operates in the manner of a superscale dynamo. Via the turbulent magnetic field, the black hole transforms some of its spin energy into a vast electrical current. The outlet for that current – two narrow channels running north and south along the hole’s spin axis. The escape route.
From each of the poles the black hole sprays a hugely energetic fountain of charged particles. And it is along one of these that our fortunate proton is now moving. Moving at very close to the speed of light. So that within just a few thousand years it has risen far above the plane of the galaxy.
Looking down, we have a fabulous bird’s-eye view of that great assemblage of stars we are leaving now forever.
Yet the view in retrospect is different – remarkably different – from what we had expected. For we had not reckoned on the enormous contribution of the central supermassive black hole to the galaxy’s total energy output. That contribution, coming mainly from the sizzling vortex of matter around the black hole, exceeds by a factor of a thousand the combined radiation of all the stars in the rest of the galaxy! Five hundred billion suns burn within this stellar commune, its luminous bulk sprawled across 150,000 light-years of space. But the misty, mellow glow from that stellar host is almost completely drowned by the searing glare of the core.
Hundreds of millions of years go by. The cosmic clock advances to one billion AG. And as we move ever farther from it, the hazy peripheral structure of the galaxy fades from sight, leaving behind just the intense central point source. More like a single, outrageously bright star it looks now. And so we call it by a new name – “quasi-stellar object.” Or simply, “quasar.”
How many other young galaxies are also passing through this extraordinary phase? Quite a number it seems. From the scattering of similar brilliant points across the sky, we might guess that at least 1 or 2 percent of all galaxies in the universe harbor active quasars at this stage. Within these brightest of galactic beacons there must be giant black holes much like that from which our proton narrowly escaped. Yet the quasar phenomenon depends not only the presence of such a beast but also on the availability of star food nearby. Where there is plenty of matter in the galactic core to stock its fiery whirlpool, the central black hole can raise the light of hundreds of trillions of stars. But in time that immediate, rich supply of nourishment must diminish. In time the innermost parts of the galaxy must become clearer of gas and dust and stars, so that gradually the black hole will be starved, its quasar light dimmed.
What we have styled “quasar,” in fact, surely marks only the upper bound of a complete, unbroken range of galactic activity. Some galaxies, perhaps the majority, may never have shone as true quasars. Perhaps at their cores lie more modest black holes (or none at all). Or perhaps they could never stoke their central engine rooms fast enough. Even so, looking around this still-immature universe, we observe that the inner region of almost every galaxy shines with unexpected brilliance. With too much brilliance, it seems, for normal starlight to be the source.
Again hundreds of millions of years go by, and that suspicion is gradually confirmed. The true quasars, by and large, are growing fainter (though occasionally one will flare up, as if it has found a fresh, temporary wealth of star fuel). And as they fade the quasars begin to appear more and more like conventional galaxies. Their superluminous cores dim, their vastly more extensive stellar suburbs come into view. And, on a lesser scale, the same happens to other galaxies. After an adolescence spent not always in the pursuit of staid convention, they settle down to a more sober adulthood.
We gaze all around at the variety of galaxies. Over millions and millions of years, as our proton coasts on alone through the void, we come to know every species of the galactic zoo. And we learn that not only in the scale of their core activity do star cities differ. There are many diverse shapes and sizes of galaxy. Some are round or ovoid with few internal markings. These “ellipticals” have used up virtually all their supply of loose gas and dust in fabricating stars, so that now they are little more than tidy arrangements of suns and space.
By contrast, a second great class of galaxy, the “spirals,” shows dazzling variety of form. The inner bulbous hub of a spiral resembles an elliptical in miniature. Here the stars are evidently very old, so that they must have formed early on in the galaxy’s history. But spreading out tens of thousands of light-years from this ancient hump is a much broader and flatter disk, bedecked with interstellar clouds and bright youthful suns. Within the disks of the spirals, winding density waves like ripples in a pond propagate outward. And, as these waves pass through regions rich in gas and dust, they stimulate the interstellar material to collapse into new stars, which then illuminate great curving arms in the disk and give the galaxy its name.
By why, we muse, have some systems become spiral, others elliptical?
Perhaps, as they formed, spirals spun around so fast that they were unable to gather up all of their star-making material into a single ball as the ellipticals did. Instead, part of their contents was whirled out as a broad, gassy pancake in which star building could proceed only at a much slower rate. How fascinating it would be to be able to explore one of these beautiful, complex systems in more intimate detail – from within.
How fortunate that now, it seems, we may have just that opportunity.
For five billion years our proton has been adrift in intergalactic space. And in all that time it has managed to weave its way between the slender islands and atolls of galaxies that populate this universe, avoiding an encounter. But now that lonely, lonely voyage may be coming to an end. Only a million light-years away is a glorious Catherine wheel of light, a spiral galaxy of over 200 billion stars.
Closer and closer it looms, spreading impressively across the sky, its brightest suns and largest nebulae making themselves individually known. Closer and closer. Entering the galaxy’s material realm now, our proton sweeps past the central bulge on an arcing path that plunges it into the outskirts of the disk. Ahead, for the first time in five billion years, lies interstellar space.
We had forgotten how well separated are the stars in a galaxy in comparison with their size.
Take fifty oranges. Scatter them throughout a sphere 10,000 miles across. That is a measure of the seclusion of neighboring stars in a galaxy. Relative to the size of object, interstellar distances are very much greater than intergalactic, though in absolute terms they are thousands or even millions of times less.
Still remote these tiny, cold points of light around us may be. Yet at least the time scale over which the background appreciably changes is now much reduced. As our proton-explorer travels on, the stars seem to drift steadily past, whereas, over similar periods before, the galactic landscape had remained almost static. Ahead, new stars and nebulae emerge out of the blackness, while others behind fade from sight. Often we imagine that our particle is bound for a close rendezvous with some upcoming star or star cluster. But then we recall the huge distances between everything, and our hopes for a near miss disappear.
The stars themselves are minute targets, almost impossible to hot or graze. But not so the sprawling clouds from which they form. Inevitably, after many millions of years wandering alone down empty corridors of this new galactic home, our particle joins again a great party of gas and dust. A larger than average nebula this, and from its substance larger than average suns are being made. Giant stars are in their embryonic stage here, and when they eventually shine they will be as searchlights amid the candle glow of more common suns.
For billions of years our proton has seen other matter only from afar. For eons it has been merely a wistful spectator to the performances of galaxies and, later, of stars. But now its role is about to change most sensationally.
Within the very heart of a new star’s fusion-powered furnace the proton has taken up station. From every direction other matter in the prestellar nebula is flowing in toward it. It is at the center of one of the biggest infant suns in all space, a 30-million-mile-wide blue supergiant.
Crazily it darts to and fro as the fusion fire of this great new star ignites. Violently it smashes against countless other nuclei, densely packed and fast-moving, all around it. And now we see vividly the fusion process in action.
Under the extreme conditions in a star’s core, lighter nuclei, as they collide, occasionally stick together to make heavier nuclei. The intricacies of such reactions are many, and they need not detain us. The crucial point is this: Lighter nuclei when lumped together weigh slightly less than when they are apart. That excess mass is liberated, during the fusion process, as a vast outpouring of energy. Only a few million tons of matter “burned” in this way each second suffice to power a modest sun for billions of years.
To begin with, and subsequently for most of its life, a star derives its energy by turning hydrogen fuel into helium. No other fusion reaction involving heavier materials comes close to being so rich a source of heat and light. So, we might expect, big stars, having more hydrogen fuel, ought to be able to shine for much longer than smaller stars.
But no! In fact, that is not the case. On the contrary, a supergiant star squanders its core hydrogen reserves extravagantly. In only a few million years the hydrogen at its center is gone. And then the star must resort to burning helium. And then carbon. And then still heavier substances. All the while its core grows hotter and denser, because only under progressively more and more extreme conditions will the heavier elements catch fire.
We see it happening. Five million years after this great star began shining, our proton is no longer a solitary hydrogen nucleus. By incremental stages it has become fused into a silicon nucleus along with twenty-seven other protons and neutrons. The star’s core is now almost full of silicon. And already this almost worthless fusion fuel is being burned, under new, enormously high temperatures and densities within the core, to make iron. To the giant star the start of that process is like a death knell. Iron is the absolute endpoint for a massive star. To fuse iron actually takes more energy than the fusion reaction gives back. So when the core of the supergiant becomes clogged with the nuclei of iron, it is doomed.
A few thousand years later the moment approaches ...
Within less than a second the core shuts down, collapses under its own weight, bounces back because it can collapse no farther, and explodes outward. Then – SUPERNOVA!
In an explosion whose brilliance transiently rivals that of the entire galaxy, the old star blasts itself apart. And now, finally, born in that instant of stellar eruption are substances heavier even than iron. At last the energy is unleashed to create cobalt, nickel, xenon, strontium, platinum, and uranium. Dozens of other elements, too, that in the normal course of stellar evolution can never be made. Only now, in the unshackled fury of a supernova explosion, are they wrought.
And yet most of what spews out from the shattered star are the lighter elements: hydrogen, from the unburned outer layers; helium, from deeper in the old sun, and, in much less amounts, nuclei from carbon to iron.
Why then do we even trouble to mention the creation of heavier elements if they are formed only in minute traces? For this very personal reason: Our own proton has become incorporated into one of these exotic nuclei. As it fled the inner core at one-tenth light-speed, as part of an iron nucleus, it was overtaken by an even faster-moving fireball of energy. In a split second, as the blast wave surged past, it was fused and fused again with other nuclei in an orgy of element-building. Out of the intensely destructive-creative moment it emerged, changed once again – framed now within an atom of gold.
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