A renowned British astronomer takes us on a voyage to the farthest galaxies and the end of the universe.

Consider the Year 5,000,000,000 A.D.

“On August 25, the Voyager 2 space-probe will flash past Neptune, revolutionizing what we know of this faraway world. Yet for Voyager, this will be neither the beginning nor the end of its fabulous cosmic journey.

“The very atoms of which Voyager is made were forged in the cores of massive stars that exploded billions of years ago. For billions more years those atoms lay dormant in the earth’s crust, before intelligent beings fashioned them into a spacecraft to sail free once more in the void between worlds. But Voyager’s exploits will not end with its spectacular flybys of the outer planets. Nothing can stop it now from leaving our solar system forever. And who knows what strange encounters it may have then?”

Millions of microscopic craters pepper its metal frame. A ragged hole, teeming with stars, gapes in its high-gain antenna. Otherwise, in the year 5 billion A.D., spaceship Voyager is intact.

Five billion years: In the previous such era human intelligence had synthesized from stardust. In the last one-millionth of that term, man had progressed from building its first cities to engineering lasers and genes. Now what the human race had become would not even recognize the tiny, battered spaceship as its handiwork.

Man has gone, or changed indescribably. Yet the galaxy — the familiar Milky Way — remains much the same. From still plentiful gas and dust in the spiral arms, the mightiest of suns continue to coalesce, streak through their bright careers, and erupt sensationally at the end as supernovas. Together with more subtle ways of shedding stellar matter, these titanic outbursts replenish the galaxy for future generations of stars.

At the opposite extreme, red dwarfs, many as ancient as the galaxy itself, glow feebly. What is another five billion years to these stellar Methuselahs? Their modest hydrogen stocks are good for 50 billion — perhaps 500 billion — years at the miserly rate they are used.

But it is not so with Voyager’s native star. Somewhere, lost in the fairy dust of a spiral arm, the old sun is dying. Already it has doubled in size and brightness. On earth the climate has altered radically, the polar caps melted. Exotic new life-forms have emerged to suit the changed environment, and yet even these thermophilic species are doomed in the traumas to come.

Three billion years more elapse. Like a feather in a whirlpool, Voyager is swept around the galaxy a dozen more times. Now, in 8 billion A.D., the sun is swollen grotesquely, has become a red giant, 150 times its original size. Solar luminosity is up 2,000 times, temperature down 1,800 degrees centigrade to a ruddy 4,000 degrees.

And already the sun has cannibalized two of its attendant planets. First to go was Mercury, its old orbit engulfed by Sol’s distending surface layers. Then Venus succumbed. Now earth is the new hell-world. Oceans boiled dry, its scorched mountains and plains are hot enough to melt lead. And it is utterly, permanently, scoured of life. Only the noisome clouds of gas that vent ceaselessly from its surface proffer any movement.

So, it ends as it began. For a second time in its history the earth is lifeless, searingly hot. A great star illuminates its daytime sky, but it’s not the Sol of old — compact, energy rich, and benign. The monstrous florid sun that now blazes down sprawls across half the sky.

What has happened since the spacecraft left home? Why has the aging sun changed so profoundly?

First, around 5 billion A.D., hydrogen in the sun’s core started to sputter and run out. This was what had kept the sun shining more or less steadily for eons preceding. Now, choked with helium ashes, its useful fuel gone, the old solar-fusion power plant was condemned to shut down. With no outward radiation pressure left to buoy it up, the sun’s core began to sag under its own weight. And in the process, some of its stored gravitational energy was set free. Exactly half this energy helped supply radiation from the surface; the other half drove a stellar central-heating system that catastrophically altered the sun’s appearance. Heat migrating outward from the collapsing core caused the sun’s entire atmospheric envelope to mushroom. And with that, the galaxy gained a new red giant.

Continuing to shrivel under its own gravity, the sun’s central region became hotter and hotter. At the fringes of the core, where hydrogen was still abundant, the temperature climbed to ten million degrees. And at this point a hydrogen-burning shell flared up around the core. Like a super-scale brush fire, it spread outward through the surrounding layers of the sun. Energy released by this mobile fusion reactor now contributed to the light that escaped from the solar surface.

Such is the sun’s condition eight billion years after Voyager’s launch. The cosmic clock ticks on…

Steadily the sun’s sleeping helium core grows hotter. Gravitational contraction pushes the temperature up to 100 million degrees and sets the scene for — helium flash! Suddenly the core is alive again with fusion reactions. Helium nuclei smash together to form the heavier elements carbon and oxygen. And as the helium burns, so its carbon-oxygen ashes are dumped upon the shrunken core.

But helium is a poorer source of energy than hydrogen. Also, at the much higher temperature needed to fuse it, the reactions occur faster and the core helium supply is exhausted after only a few million years. Again, the sun’s central reactor shuts down. A second shell, this time kindled by helium, fires up at the perimeter of the hot carbon-oxygen core and follows in the path of the still outward-moving hydrogen-burning shell.

Then comes an uncertain phase in the development of Sol. By one mechanism or another, the sun starts to lose matter at a fairly high rate. Over a few thousand — at most a few million — years, it sheds virtually all of its dilated hydrogen-helium atmosphere. The outer layers may escape piecemeal by way of a gusty stellar wind. Or they may be shot out en masse as a bright expanding shell of gas. Whatever the means of ejection, all that eventually remains of the sun is a white dwarf — a hot, dense, planet-size ball of inert matter.

Voyager travels on. By 20 billion A.D. its old home star is a shadowy crystalline sphere of super-thick carbon. Earth, still bound by gravitational allegiance to the spent solar dwarf, is dark, dead, frigid as space. The star system that once nurtured the human race has expired.

20 billion A.D.: Now the cosmos has doubled in size since Voyager set sail. It is bigger — because space-time has expanded — more rarefied. somewhat cooler. And yet in character, the universe is little changed from the days when some large-brained biped took its first halting steps starward.

And the Milky Way? That, too, looks familiar enough. Still hugging the galaxy’s plane are the warm, glowing nurseries of protostars. Elsewhere an occasional supernova bursts, showering several suns’ worth of fertile matter back into the galactic melting pot. Stellar reincarnation goes on.

Another 20 billion years pass, and another, and another.

For 100 billion years the universe has endured, Voyager for four-fifths of that time. And now there are palpable signs that the galaxies are aging.

In the Milky Way, the star-making machinery has ground almost to a halt. What unconsolidated gas remains is too thinly spread to ever claw itself together again by gravity.

Slowly the galaxy changes hues. The white and blue of its once-majestic spiral arms fade as. one by one, the most massive stars disappear. Eventually even sun-like stars become rare, leaving only red dwarfs to light up the Milky Way with their dull ruby glow.

It is the same throughout the rest of the universe. And though maybe a few renegade, late-developing spirals remain tinged with youthful blue, before long these, too, slip into senescence.

Voyager turns one trillion. And now the golden epoch of stars is nearly over. Even the lightest, most venerable of the red dwarfs are coming to the end of their hydrogen reserves. So the soft glow of the galaxy’s spiral arms, halo, and core steadily fades. Sometime between 1 trillion and 100 trillion A.D. the last of the stars in the universe goes out. Every star has consumed its usable fusion energy stock and subsided under self-gravity to become a cold, dark cinder.

Most common in this gloomy far-off age are the relics of small stars. These, the white dwarfs — earth-size or smaller — that have chilled and darkened to velvet blackness. In 100 trillion A.D. non-luminous white dwarfs, or “black dwarfs,” populate every corner of the galaxy.

Scarcer are the dense remains of heavier suns — the neutron stars. Scarcer yet, though in numbers much greater than when Voyager began its journey, are the dark, sinister carcasses of the heaviest stars of all-black holes.

And so Voyager enters the second great era of the mature universe. The first ended when the last of the stars went out.

Now, in this dreary ensuing phase, planets are being freed from their primeval moorings by close encounters between stars every seven trillion years or so.

For the most part, the liberation of planets proceeds in the dark. Slowly, painfully slowly, each star in the galaxy is stripped of its worlds. Possibly 100 close encounters are needed to shatter a solar system entirely. In the more rarefied, peripheral reaches of the galaxy, the process may be delayed — there are fewer chances for stars to meet. But in Deep Time, all the worlds throughout the cosmos slip their gravitational anchors. At some point between 1,000 trillion and 100,000 trillion A.D., the second great era of the future universe draws to a close. Now every world is unbound. Like their decayed parent suns, they steal along private lonely trails through galaxies cold and dark.

And so the third far-off era of the universe begins. The galaxies themselves are poised to evaporate.

Envision a star as if it were a molecule in a liquid. Struck hard enough through collision with its neighbors, a molecule may escape from the liquid’s surface. So, in similar style, may stars break free.

Just as stars can come close enough to eject planets, so they may, on still rarer occasions, approach so close as to hurl themselves free from the gravitational pull of the galaxy. During such a near stellar miss, one star gains kinetic energy at the other’s expense. And if the boosted sun gathers enough speed from the encounter, it will simply break the shackles of its galactic orbit altogether.

The gravitational “slingshot effect” — that ’is what the scientists who built Voyager would have called it. Indeed, Voyager itself once used the slingshot to tour Sol’s outer planets. As the spacecraft encountered Jupiter in July 1979, it purposely followed a path that would utilize the Jovian gravity field to hurl it on, at increased speed, toward Saturn. What kinetic energy Voyager gained was at Jupiter’s expense, so that the fifth planet was slowed minutely in its orbit and made to fall imperceptibly toward the sun.

In Deep Time, dead stars in all the galaxies experience the slingshot effect over and over again. Multiple close encounters banish star after dead star into intergalactic space. By 1 million trillion A.D. nine-tenths of the mass of every galaxy has vaporized in this way — planets, dust, and gas, as well as stars — while the remaining one-tenth, robbed of kinetic energy by the same near collisions, has been drawn by the galactic gravity embrace into an increasingly dense core.

Perhaps there had always been a super-massive black hole at the Milky Way’s heart. If not, it makes little difference now. By the close of this era, all major galaxies in space will have acquired such dark, fearfully dense cores. And into one of these great bottomless pits of gravity every unevaporated star remnant is about to plunge.

Half-familiar is the scene — a strange, distorted echo of the infant cosmos. For were galactic black holes not once active before? As quasars — the brilliant cores of youthful star cities? Those laser-bright beacons of the early universe were born of matter straying too close to the black heart of galaxies. And now it seems they are primed, ready to burn again in this far-flung epoch.

Steadily, inexorably, what matter is left in the Milky Way joins a swirling maelstrom around the central core. A trifling fleck deep within this frenzied whirlpool is all that remains of earth — and even that is about to be incinerated. Of the other solar planets, most were cast out of the galaxy trillions of years ago. The black dwarf of Sol, like earth, is ensnared in the hot vortex. Hot vortex — and growing hotter. And brighter. Glowing now, the Milky Way’s core bursts into brilliant candescence. All around the universe the galaxies are putting on their final show — going quasar once more.

Perilously close, Voyager skirts to the outer edge of the black hole’s domain. Dead ahead — a straggling neutron star, itself already doomed. White light from the blazing necklace of the black hole dances off Voyager’s interstellar record, a message that now will never be played — less than half the original spacecraft remains intact.

And now Voyager is upon the neutron star, banking sharply around its precipitate gravity field. Again, the slingshot effect. And suddenly Voyager is reprieved, hurtling outward, away from the central danger zone.

From afar the spacecraft’s pitted golden disk, like a glazed eye, stares at the Milky Way as the old galaxy devours itself. Light rays, some testimony to the demise of man’s home planet, rebound from “The Sounds of Earth” — then spear farther into Deep Time, drawing out yet another subtle cosmic thread.

So Voyager departs the galaxy within which it was forged. And travels on into a greater void.

This is space in the year 1027 — an ever-widening ocean in which 90 percent of all cosmic matter wanders free in the form of extinct stars, planets, lesser debris, and individual sub-atomic particles. Well beyond comprehension now is the scale of the universe and the extent to which its contents have been diluted. On average, one sun’s worth of matter enjoys a privy estate of 1028 cubic light-years. One hundred suns in a bubble of space as big as the entire twentieth-century cosmos!

The remaining ten percent of matter is locked up in a billion or so super-massive black holes, caliginous fossils of the old galactic clusters. Slippery and sheer are the gravitational walls of these abysmal space-time pits. And yet so far apart are the black holes that their influence upon one another is as nothing.

Galaxies without stars, stars without planets, and everywhere the ruin of gravitational collapse: What further devastation can the universe suffer?

Now, in 1021 A.D., space is chilled to absolute zero. All conventional ways of generating heat have gone. And the minuscule contribution made by decaying protons is at last becoming apparent.

How, then, does the disintegration of a proton heat a dead star? The proton simply delivers itself spontaneously of a shower of energy-rich positrons, photons, and neutrinos. Only the neutrinos — light-speed wraiths — escape directly to the stellar surface.

In the claustrophobic interior of a black dwarf, a newborn positron soon confronts its opposite number, an electron. And when that happens, because they are particle and anti-particle, they annihilate each other. Their sole legacy — two identical high-energy photons that, as they are promptly absorbed, serve to heat minutely the surrounding stellar matter.

So, by proton decay, is the surface temperature of a black dwarf raised — to about one degree above absolute zero. And cool as that may seem, it is toasty indeed compared with the wintry chill of the cosmic background.

Once all the protons and neutrons are gone, at an agonizingly slow rate, there will be no more stars. As their baryonic skeletons crumple, every black dwarf and neutron star in space will vanish in a puff of photons and neutrinos.

Spring in the year 100 million trillion trillion: Super-massive black holes are the only islands, unutterably small and lonely, in a cosmos awash with electrons, positrons, photons, and neutrinos. All “heavy” matter has eroded into these tiny, insubstantial grains. As a whole, the universe has enlarged 10,000 million trillion times since the age of man. And now the average distance between each electron and positron is greater than the diameter of the old Milky Way.

These electrons and positrons, whence did they come? Not from the old black dwarfs and neutron stars. As the protons inside these objects broke apart, the resulting high-density gruel of particles and anti-particles annihilated itself almost totally. No, the pelagic electrons-positrons that remain came largely from the decay of protons already floating free in the cosmic void — from archaic interstellar and intergalactic gas.

And now, in nature and scale, the universe has become altogether alien. Even that most mundane event, one sub-atomic particle bumping into another, has all but ceased — the separation distances are too great. Everything is desperately dull and banal.

Ten times more the universe ages, ten times more – 1035 A.D., 1040, 1050, 1060. Now the entire age of bright stars seems less than the wink of an eye. More incomprehensibly brief is it in relation to this future cosmos than the quantum gravity era, spanning just 10-43 of a second, seemed to humans.

1065 A.D., 1070 and then — change!

In roughly the year 100 billion trillion trillion trillion trillion trillion, a positron that came from Voyager finds a mate. With its companion electron it forms a single atom of positronium. Yet never was there an atom such as this. The orbit of the two particles around each other is as wide as the entire twentieth-century universe! Unbelievable as that may seem, this vacuous piece of matter is still millions of times smaller compared with its own huge cosmos than a humble hydrogen atom (less than 10-8 of an inch across) was in relation to the universe when humans lived.

Everywhere the same remote marriage of electrons and positrons is taking place. At last their energy of motion has fallen below the attractive energy invested in their bound state. An ethereal sea of positronium rapidly fills all of space.

But even that is not the end. As soon as an electron and positron conjoin, they begin spiraling in toward each other. When finally they meet, their first kiss proves deadly. Particle and anti-particle, the couple realize too late their fatal difference. In an instant they extinguish one another. And then, in fine theatrical style, their photonic spirits race apart as a pair of empyreal gamma rays.

Not that this self-destruction of the positronium sea is a quick affair. It takes about 1045 times as long as the age of the universe when the positronium first formed. So only by 10116 A.D. have the majority of positrons and electrons contrived to annihilate each other. And well before that, a far more striking event has taken place.

The scene on the universe’s 10,000 trillion-trillion-trillion-trillion-trillion-trillion-trillion-trillionth (10100th) birthday: All protons and neutrons have decayed. Most of space is replete with an incredibly dilute gas of positronium, steadily annihilating itself. Every stellar-size black hole has popped. And those exploding now are the biggest ones of all, harboring the crushed matter of entire galaxies. It is a spectacular finale. The universe ends with a fireworks display!

Except that the universe never really ends. It goes on, growing bigger, colder, darker, and sparser. Or does it?

A date unknown: The denudation of the last black hole belongs to a forgotten past. Even the final decay of the positronium sea took place lost eons ago. Now there is only blackness, ultimate cold, space without end.

And some positrons.

In company with some other electrons and their anti-particles, they survived the breakup of the positronium sea. And now these last specks of matter, together with greater legions of photons and neutrinos, are all that remain in the universe.

How else could it have ended? With the cosmos rebuilding itself? That was mere armchair theory, a physicist’s pipe dream. It never came to anything. Nor will it ever. For what the human mind could not grasp, mercifully, is the utter desolation of this place. Were just two particles to come within a trillion trillion light-years of each other now, it would be a staggering juxtaposition.

And remember, space-time is still growing. The average gap between particles is widening, not narrowing. So that as time goes on, the prospect for some impromptu local gathering taking place becomes ever more bleak.

Is this, then, the true future universe: space, huge and burgeoning, the home of solitary particles in eternal isolation? All of it cooling, thinning forever?

Will the cosmos in time fully unwind? Will the energy of all particles in the universe at last be smeared over a random thermal distribution? If so, this would be the “heat death” — the supreme blah — that nineteenth-century physicists first contemplated.

At the moment of heat death, entropy would peak at its maximum possible value — one. Chaos would rule unchallenged.

And the alternative? That would happen only if photons born of the decay of black holes and of the positronium sea stayed clear of collision with other particles. Then might the photons avoid ever sharing their high-grade energy and thus coming into thermal balance with the rest of the cosmos. In this case, the universe would drift closer to the dismal heat-death state, but never actually reach it.

But what difference does it make — 100-percent chaos or 99.999-percent chaos? Either way, in time the universe ends up dark, dreary, hopelessly washed-out.

In time. In Deep Time. At the far end of Deep Time, for that is where the cosmos has arrived. And paradoxically, it seems that time’s condition now is no more certain than it was at genesis.

In the beginning there was the problem of time’s source: If time had a moment of birth, what was there before that? In the prenatal state of genesis, how could anything have happened, preparations been made, outside of time?

And now this new riddle. In the far-future universe, change has ground virtually to a halt. With particles so widely spaced, even the dispatch and receipt of a solitary photon becomes almost a divine event. And without change, what significance has time?

Perhaps time’s axis needs new labels. Did it ever really make good sense to think of the Big Bang as being at Time Zero?

Moving back toward the Big Bang, events were crowded much closer together. Within the “first second” there was more change — a higher density of events — than in all the subsequent history of the universe put together. And edging back ever nearer to the point of creation, events were squeezed still more tightly into thinner and thinner micro-slices of “conventional” time.

Take, then, a bold step. Redraw the map of time. Rather than the Big Bang at t = 0, mark it at t = – infinity. Then, looking back, it becomes genesis, not Armageddon, that lies an infinite period away. And the remote future of the cosmos? Put that at Time Zero!

The result — a fresh view of the scheme of things. Rather like those tongue-in-cheek antipodean charts with south at the top and Australia, lording it, big and prominent, just above center.

And, too, the new temporal plan may offer more than just novel perspective. With the Big Bang set at t minus infinity, time stretches out farthest in the direction of most activity. Which is promising. Also, it removes the stubborn problem of “What came first?” since before any given event there would always be an infinite number of others. The quest for a genesis spark would be doomed to frustration.

As to the future universe, the new scheme literally has no time for it. As events become fewer and further apart, time loses the framework against which it was previously measured. Beyond a certain point, with every particle effectively cut off from every other, time would cease to have any meaning at all-the point of Time Zero.

Is it wild surmise? Will time really come to an end at some stage in the future cosmos? Or will it live on, ethereally aloof from the dispersed wreckage of matter, energy, and space? Again, it turns on the true nature of time. And that, as ever, remains elusive.

But one thing does seem clear. Even if time itself survives indefinitely, its direction — the way its “arrow” points — must come increasingly into doubt. Why? Because time’s forward direction is taken to be that in which order dissolves into chaos, in which entropy increases. When all that remains is total chaos — chaos changing into equivalent chaos — there is no longer anything by which to distinguish past from future.

And so, to the end of everything. Entropy has peaked. Confusion abounds. Time is — uncertain.

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