Ultimate fate of the Universe

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“”I had a dream, which was not all a dream. The bright sun was extinguish'd, and the stars Did wander darkling in the eternal space, Rayless, and pathless, and the icy earth Swung blind and blackening in the moonless air

—Lord Byron

The ultimate fate of the Universe, based on scientific observations and not theological or mythological belief, has been a topic of scientific research for decades. This article deals with the current scientific consensus and includes some speculation on what would happen with life in any of the proposed scenarios for it.

Role of cosmological parameters and shape

The critical density of the Universe, expressed by the Greek letter Ω, has a very important role to determine what will happen to the Universe, as its shape depends on that value: a value less than 1 means a Universe of negative curvature ("Open Universe"), that will expand forever while if Ω was larger than 1 the Universe would have a positive curvature ("Closed Universe"), that would expand to a point before halting its expansion and imploding on itself (see further). The case of Ω being exactly 1 is the one of a Universe of zero curvature ("Flat Universe"), that will keep expanding forever but with said expansion slowing with time until it stopped after an infinite time. Current measurements are coincident with a flat Universe, thus with exactly the critical density and that in theory would keep expanding forever, but slower and slower.

This simplistic view was changed in 1998 with the discovery of the acceleration of the Universe's expansion. Most models suggest that "dark energy", a negative pressure field that opposes gravity and whose most simplistic model is a cosmological constant (a value of vacuum's energy density) represented by the also Greek letter Λ, is the thing to blame. Said acceleration means that even if Ω was larger than 1, unless Λ was small enough the Universe would not collapse again and as we'll see below has deep implications for the future of the Universe assuming it does not change with time.

Before we go on, note how what follows assumes that the laws of physics will not change in the future or at least not too much which especially in the case of the most ludicrously high times described further down is even questionable.

Big Freeze: everything fading away

Based on an indefinite expansion of the Universe as stated above, the most likely scenario for its end is known as "Big Freeze", what considers that its expansion will continue indefinitely. However while that looks like immortality, it is actually much closer to a very slow death as the Universe will end up reaching heat death: a state of maximum entropy with similar temperatures (thus with no further possibilities to do any work) everywhere and too cold to support any kind of life. As we'll see in the next sections, the amounts of time needed for the processes that will take place in that future and will lead to that end are far beyond our grasp but compared to the eternity that awaits are nothing.

After the work of Fred Adams and Gregory Laughlin in 1997 the history of the Universe has been divided into five "ages": the "Primordial Era", the "Stelliferous Era", the "Degenerate Era", the "Black Hole Era" and the "Dark Era". The first one is far away in the past corresponding to the times between the Big Bang and the formation of the first stars, a few hundred million years after it.

Stelliferous Era

The evolution of the universe and its horizons in comoving distances, assuming dark energy keeps accelerating its expansion

The "Stelliferous Era" began a few hundred million years after the Big Bang, when the first stars formed, and we're within it. As the name implies, it is an epoch in which stars are abundant objects and it's expected to last until 10¹⁴ years into the future, when standard star formation will have ceased, having exhausted the available gas, and all of them up to their remnants have ceased to shine. However many things will happen before lights go out.

The current large-scale structure of the Universe is Turtles all the way down with one turtle flying elsewhere an extensive web-like network of filaments and walls of galaxies, with its nodes corresponding to galaxy clusters and especially superclusters, surrounding large (sometimes very large), empty, extensions. Under the accelerated expansion of the Universe caused by dark energy it's expected this structure will freeze its evolution and be torn apart stopping the growth of said galactic clusters and superclusters leaving isolated galaxies and clusters of galaxies that will have become "island universes" completely disconnected one from each other and separated by ever-expanding voids stopping structure formation. This is expected to culminate 150 billion years in the future, when all galaxies outside the Local Supercluster (or whatever remains of it in those distant epochs) will be so redshifted that they will be unobservable. Other research paints an even bleaker picture, in which our Local Group will be entirely isolated within that time frame with no galaxies outside it visible, as the cosmological horizon (ie, the radius of the observable Universe), currently at more than 46 billion light-years from us, shrinks to "just" 41 million light-years, long before stars stop shining.

At smaller scales, it's expected that galactic clusters and superclusters will change their shape from the current, flattened, ones to almost spherical ones within a similar timeframe (around 100 billion years from now) losing their current structure and as noted above stopping their growth, and in the case of superclusters of galaxies mass segregation causing them to have their most massive galaxy clusters being concentrated on their central regions while the lightest ones are sent to the periphery, to finally merge ceasing to exist as distinct clusters of galaxies. However most of the current galaxies and groups of them will end up alone, not in said groupings. In clusters of galaxies, besides the continuation of processes that are currently going on as galaxy harassment and ram-pressure stripping, mass segregation at smaller scales will cause the lightest galaxies to be expelled to the outside of them, up to leaving said clusters due to gravitational interactions with others and accelerating that way said process, while the heaviest ones go to its core and given enough time those will merge to form large systems until there are no more galaxies to fuse with. Meanwhile in spiral and irregular galaxies remaining around, the trickle of infalling cosmic cold gas thought to fuel star formation will very likely be shut off too due the isolation mentioned above caused by dark energy, with star formation slowing down considerably.

Finally when we turn to the stars themselves while they return to the interstellar medium some of the gas they're made of in the course of their evolution not only more or less of the stars' mass is finally locked away on a compact, inert, remnant (white dwarf (planet-sized, left behind by low and medium-mass stars), neutron star, (a city-sized corpse of high-mass ones), or black hole for the most massive of them) but also that the returned gas comes enriched (or polluted, as you prefer) with elements made in the nuclear reactions that took place in the stars' innards during their lives, means the hydrogen reserves to form new stars are being exhausted while helium, metallicity, and the number of stellar corpses are increasing with time. Star formation is expected to go on for hundreds of billions of years, and may last for up to a trillion (10¹²) years or even more. However stars of those very late times will be different than the current ones because of the mentioned ever-growing abundances of helium and heavier elements, with them having considerably shorter lifetimes than current stars and the most massive of them being lighter than modern ones. Conversely, that will cause low-mass stars to be still more likely to form than high-mass ones and even the minimum mass for an object to be able to fuse hydrogen may be lowered, the final consequence being the formation of "frozen stars" with temperatures of just a couple hundred K that would radiate in the infrared shining much less than even the smallest stars of today, thus lasting even more, and presumably before such kind of stars others hotter and more luminous than these but much colder and fainter than present-day ones (i.e., just hot enough to shine with a very faint dull red) could also form in such distant days.

The luminosity of the Milky Way galaxy (or rather the remnant of its future merger with Andromeda) through the Stelliferous era

As time goes on, there'll be a last star massive enough to go supernova, a last Sun-like star, and finally only the lowest-mass stars (red dwarfs) will remain after having literally buried all others. As their expected lifetimes (up to trillions of years) are considerably longer than the current age of the Universe, their evolution has been studied using computer modelling. The modelling shows how the least massive and longest lived of such stars do not expand into red giants, but instead become more luminous and hotter as they age, transforming into "blue dwarfs", that exist for billions of years before dying as white dwarfs. Thanks to that increase in luminosity, future galaxies will be about as luminous as modern ones (and after having red dwarf colors for a time, they'll become bluer in a sort of apparent rejuvenation mimicking the evolution of their stars) for several hundred billion years before, as more and more red dwarfs die as white dwarfs, beginning to fade away until the Universe finally goes dark at least in visible light ending this era.

An interesting caveat of the processes outlined above is that to the rejoice of certain people the pillars that support the Big Bang theory (cosmic microwave background, existence of distant galaxies and expansion of the Universe, and the primordial abundances of hydrogen and helium) will go away leaving hypothetical aliens that could exist by those late epochs with no ways to know how everything began. It is also worth of special consideration in what regards to the cosmic microwave background how in just some hundreds of billions of years from now, when there're still many stars around, after having been redshifted into oblivion, it will be replaced by the equivalent of Hawking radiation emitted by the cosmological horizon, at an uniform temperature of 10^-29K, which as noted further down will have quite unpleasant consequences in the still more distant future of the Universe.

What about Earth and the Sun?. The former will almost certainly be destroyed -like poor Mercury and Venus- around 7.59 billion years from now by the latter going red giant after it exhausts its central hydrogen supply roughly 4.8 billion years from now (stellar evolution takes its time), absorbing it, but not before all of them having been burned to Hell and back (it seems Venus cannot catch a break), and leaving as all proof of its existence just a tiny increase of the Sun's metallicity. Meanwhile the latter after 120 million years of fusing on its core the helium result of hydrogen burning, some instabilities, and an ejection of its outer layers as a planetary nebula will become a white dwarf with half of its current mass that will fade away as a black dwarf. The outer Solar System planets are expected to survive the ordeal and orbit the dead Sun.

Degenerate Era

The Universe from the Degenerate Era onwards, as it would appear to human eyes (Artist's impression).

The "Degenerate Era" will be an era of moral degeneration and depravity begin 10¹⁴ years in the future, but its end depends on whether protons are unstable or not. If protons decay, depending on the latter's unknown half-life, may last from around 10³⁰ years, to 10⁴⁰ years or up to 10²⁰⁰ years, far beyond the evaporation of black holes (see further). This Universe will be almost entirely dark to an organ such as the human eye, but not so in other wavelengths, plus will generally be very cold, as in almost absolute zero, and especially lonely for the reasons noted above. The protagonists of those distant times will be the cooled corpses left behind by stars: black dwarfs (white dwarfs that emit no radiation at all having finally cooled to the temperature of their surroundings and crystallized (read: become solid),) neutron stars, and black holes (the first two are composed of degenerate matter, giving this epoch its name). Plus a veritable number of brown dwarfs, even more of what can be considered debris — planets, asteroids, comets, you name it —, and finally some gas that has managed not to have been incorporated into stars or dispersed away. All that stuff will keep orbiting around the centers of their dead galaxies. As time goes by, very close approaches between (dead) stars passing too close will be more likely to happen. They, if taking place too close, may even dislodge planetary systems that would have survived to those distant epochs, estimations suggesting that by around 10¹⁵ years most stars will have lost their planets.

Stellar collisions will also happen, their outcomes depending of the nature of the colliding bodies. Two colliding brown dwarfs may produce a red dwarf (plus planets around it if an accretion disk managed to form around,) that will shine for around 10¹³ years. This will be a channel to form new stars, which is estimated will form relatively many of them and will be able to let a Milky Way-sized galaxy have around 100 hydrogen-fusing stars for a long time, as long as there're brown dwarfs around to give birth new stars as the old ones die away. If the collisions are between black dwarfs, more exotic objects such as helium-burning or carbon-burning stars, whose lifetimes will be considerably shorter, may be born. Another channel to form stars will be accretion with time of the very little remaining interstellar gas by said brown dwarfs. Far more energetic — and given the general darkness of the epoch far more spectacular than at present — events will also happen, among others Type Ia supernovae when either two black dwarfs massive enough collide or sufficient gas is accreted from the interstellar medium by a lone one, plus Gamma-ray bursts when two neutron stars are the objects that crash.

Time will keep ticking relentlessly and as time passes by those gravitational interactions — the so-called "dynamical relaxation" plus the already mentioned mass segregation — will cause the heaviest objects to sink to the galactic center while the lightest ones are sent away, even expelled from the galaxy ("galactic evaporation"). The timeframe for this to happen is estimated to be 10²⁰ years onwards, and after it ends it will be supplemented by orbit decay due to emission of gravitational radiation, but on a considerably longer time scale (around 10²⁴ years). On galaxy clusters and superclusters that still exist in that epoch, a similar process will occur at supergalactic scale with the most massive galaxies or whatever remains of them (read: supermassive black holes) falling to the center — to eventually merging, forming even more massive ones — and the lightest being sent to the periphery of the cluster.

The final product of the processes outlined above will be that around 10³⁰ years, all that remain of galaxies and galaxy clusters will be supermassive (or more) black holes plus a lot of flotsam and jetsam roaming across the endless darkness and if the Universe is still pressing the pedal to the metal way more alone than anything we can imagine, but not without a final display of fireworks as the mentioned objects that have fallen to the center of the galaxy will produce an accretion disk around it feeding a quasar, which will last as long as matter is present, just around 10⁹ years.

Assuming protons are unstable, their decay will be the next event of significance to happen in those distant epochs. While the time is dependent on the proton's unknown half-life, the results will be the same: a continuous decrease of the mass of a given object until it vanishes away plus decomposition of atoms until said object is just a lump of frozen hydrogen. This will also release a tiny trickle of energy heating said object, but just very little (a proton decay-powered black dwarf will emit just around 400 Watts). For black dwarfs and neutron stars, the temperature that they will attain thanks to proton decay will be from 0.06K to 1K and in the case of neutron stars 100K, in both cases quite hot compared to the tremendously cold Cosmos of that epoch. As their protons go away and they lose mass, they'll expand losing first their condition of degenerate objects, the black dwarf becoming then a frozen ball of hydrogen with the mass and diameter of Jupiter, and later their conditions of stars — when the object formerly known as a black dwarf is transparent to its radiation — to finally disappear. Neutron stars will suffer a similar fate, but their expansion may be explosive, destroying them. As a final caveat, remember that the given lifetime of a proton is a half-life, meaning that is the time needed for half of them to decay, plus that time again for half of the remaining ones to decay and so on. If a proton lasts 10³⁷ years, for example, by the year 10⁴⁰ they'll be no more, with the decay products being photons and leptons (electrons and positrons).

Were protons eternal, the Degenerate Era would last a whole lot more with Freeman Dyson among others having studied in depth what would happen in that case. First of all, with an Universe under runaway acceleration as ours where temperature is expected to reach an absolute minimum (10^-29K) while density is always decreasing, it's expected that matter would end up ionized, disintegrating anything material except stellar remnants and planets that still existed in such era leaving behind just stray atoms and subatomic particles, event that could also happen if protons lasted enough. If it either did not happen or occurred much further away in time, quantum tunneling would cause matter to behave in scales of 10⁶⁵ years like a liquid, meaning everything would be roughly spherical with its chemical structure changed as atoms moved around (and those jokes about physicists and spherical cows in a vacuum would no longer be that). Much later the same process would begin to transform all normal matter into iron, and while Dyson gave originally a rough estimate of 10¹⁵⁰⁰ years for said event to be complete and considered it would be mostly harmless, finer calculations show how for at least the most massive black dwarfs this would take from 10¹¹⁰⁰ years to an impressive 10³²⁰⁰⁰ years and would end with such stellar corpses exploding as supernovae, with presumably everything else surviving to having become composed of iron. Much later on said quantum tunneling would cause the collapse of either anything with (more or less) mass into black holes after a nothing short of mind-blowing 10¹⁰²⁶ years or normal stars into either neutron stars or black holes after an still more impressive 10¹⁰⁷⁶ years, in both cases the holes decaying instantly in comparison with those time scales and the Dark Era (see further down) beginning. What would not go black hole, thus surviving for all eternity, is unclear, and Dyson suggests either nothing with mass (so the Universe would end up made of just photons), stuff less massive than the Planck mass (ie, very small iron scraps in those faraway ages), things smaller than a low-mass asteroid, or finally those less massive than the Chandrasekhar limit.

For those who may be concerned, the ultimate fate of the dead Sun and whatever remains of the Solar System is unclear but most likely the latter will be disrupted by those close stellar approaches and its component bodies, like the black dwarf Sun, will be expelled of the galaxy to die of proton decay (if that was going to happen), assuming they did not collide with another rogue body before.

Finally, some models suggest dark matter may go down the drain too if composed of particles as WIMPs. If that happens, it will occur via either annihilation among themselves or capture by astrophysical objects with the dark matter haloes that surround the galaxies being depleted after 10²⁵ years (i.e.: the times of galactic evaporation). Said dark matter annihilation will keep black dwarfs (relatively) warm, at 60K (which in contrast with the background temperature will be far from cold). Other subatomic particles that may form dark matter as Axions will probably disappear in a timeframe similar to that of proton decay. This also means that galaxies will lose mass, thus their gravitational grip on the objects that form them will be weaker, that helping in the dynamical processes outlined above.

Black hole Era

After protons and neutrons are gone for good, the Universe will be filled by an extremely thin plasma of electrons and positrons plus photons, neutrinos, dark matter if it does not decay the way described above... and black holes, the last vestiges of a Universe once filled with stars and galaxies.

Things will begin to be quite boring, as absolutely nothing will happen for countless eons except the very occasional fall of a particle into a black hole or the collision between an electron and a positron, producing a pair of gamma photons. Those two types of particle may form according to some models atoms of positronium, with initial diameters larger than even the current observable Universe, whose orbits will decay with time but very, very slowly until their final annihilation. But entropy does not forbid and black holes will be its next victims. Emission of Hawking radiation will cause them to begin to lose mass and, once their evaporation is advanced, they'll shine like extremely hot and small fireflies in the darkness of this era, to finally disappear in a burst of radiation. The time needed for a black hole to vanish goes from 2×10⁶⁶ years for one with the mass of the Sun to 1×10⁹⁸ years for a hole with the mass of a large galaxy, and finally 1.7×10¹⁰⁶ years for one with the mass of a galactic supercluster. Since during the very latest stages of black hole evaporation things go quantum and extremely hot, the ultimate fate of the hole is not known and a tiny, dark matter-like, remnant could be left.

Dark Era

After the last of the supermassive black holes is history, the Universe will enter into the "Dark Era" for probably all eternity. It will be an unimaginably dark, cold (at just those 10^-29K, and especially nearly empty place where electrons, positrons, photons, neutrinos, and (maybe) dark matter will roam free, only very rarely encountering each other, assuming they're not carried away by a Universe in perhaps runaway acceleration. The positronium atoms mentioned above are expected to decay in timescales of around 1.7×10¹⁰⁶ years, if they manage to form at all due to such accelerated expansion of the Universe.

The Universe will be in the clutches of heat death, in an extremely low energy state, meaning very, very little energy will remain, with things taking a very long time to happen if they ever happen at all. It's highly speculative what will happen next (maybe a Big Rip in around 10²⁵⁰⁰ years (see further)), maybe a vacuum metastability event in around 10¹⁰¹²⁰ years (see further too), maybe just plain heat death... who knows?), as in those extreme conditions it's thought quantum effects will prevail and our understanding of what happens then is unknown, although the Poincaré recurrence theorem suggests that after an absurdly long time the universe would essentially reset itself back to a pre-Big Bang state.. Even Boltzmann brains could pop up if you can wait 10¹⁰⁵⁰ years and a new Big Bang could take place if you can wait 10¹⁰¹⁰⁵⁶ years. Eternity is awful long time, especially towards the end, and given enough time anything could happen. Over an infinite amount of time, there could also be a spontaneous entropy decrease by a Poincaré recurrence or through thermal fluctuations.

Life in an ever-expanding Universe

Of course there've been speculations about the kind of life that could develop in or adapt to live in an ever-expanding Universe. It's clear they'll have very hard times in store with a number of crises to face, most notably the isolation in space of their galaxies, the death of all stars (in other words, no more sources of abundant energy to tap into), their galaxies evaporating away, and especially proton decay for when it ends there'll be little more than a bunch of subatomic particles to use as a basic survival kit. For the stelliferous and degenerate eras it's clear (Loeb suggests Universe's peak habitability will be reached 10¹³ years in the future assuming habitability of planets orbiting low-mass stars is not suppressed, and Krauss and Starkman suggest that if protons did not decay life similar to ours could exist up to 10⁵⁰ years in the future) that even after the lights have gone out life like us (ideally, advanced civilizations, who should have things easier) could exist even if their existence would be rather dire, always on the search for energy and attempting to use on the most efficient way their scarce resources. It's even conceivable that a civilization advanced enough could change things such as stellar orbits around a galaxy's center, forming stellar clusters to manage for their survival, or possibly even controlling a gas cloud to form stars from it, even if both would take a lot of time (but at least time would be the only resource they'd have a virtually endless supply of). In the end, nature could well have ended up "technologized" so the limit between what's natural and what's artificial had disappeared.

Things will be considerably worse once protons decay and not just because matter will have dissolved into oblivion. Black holes, at least before they become the last non-subatomic objects in existence, may be a source of energy if civilization(s) are advanced enough to take advantage of Hawking radiation besides extracting energy from their rotation in one way or another. but once they have disappeared whatever exists will be pretty much SOL. Freeman Dyson has suggested that beings composed of electrons, their energy coming from the electron-positron annihilation, could endure essentially forever even in the Dark era 'verse by combining periods of activity with longer and longer ones of hibernation, playing with their subjective time and an always declining metabolic rate. Unfortunately those ideas were proposed much before it was known the Universe is pressing the pedal to the metal, and with that in mind things would be much worse for them. Not only they'd have to face a severe dearth of resources caused by the runaway expansion of the Universe but also with or without the latter the sort of "alarm clocks" that they used to awake would sooner or later fail because of quantum effects, killing the being. If that's not enough Dyson assumed the temperature of the Universe would forever be decreasing so they could be in thermal equilibrium with it, but it's currently known it will reach a minimum as stated above (10^-29K) meaning no more thermal equilibrium and no more life. The only way to survive would be if those beings practiced a sort of reversible computing, forever reshuffling their memories and with no communications of any kind with the outside. It's certainly questionable to consider that "life". Nonetheless, other research suggests that in an ever-expanding Universe with no cosmological constant (ie, one where dark energy dissipated away) information processing (read, life) could continue indefinitely by tapping into for example the temperature gradients caused by gravitational waves, and even such luck could exist for universes under an accelerated expansion not brought by the dreaded cosmological constant, so life could not end FUBAR'ed.

Last but not least, assuming the theory of eternal inflation was true, it has been proposed a doomed civilization could send a message or even some sort of Ark-like capsule that contained its knowledge, cultural legacy, etc. up to the possibility of rebuilding its builders to any of the new Universes popping up in such scenario. However it has been shown it's far more likely they'd be either swallowed by a black hole, which would also be formed in such kind of multiverse, or drowned in countless fake messages and capsules formed by natural quantum processes in the same way as the already mentioned Boltzmann brains with the only solution being to build a whole lot of them.

Big Crunch: All together again

Representation of a "big crunch". Toasty!

The Gnab Gib "Big Crunch" is the opposite of the "Big Freeze", in which the Universe instead of expanding forever will halt its expansion, begin to contract, and finally implode on itself. This scenario was favored in the past, but observations show that unless dark energy gives us a prank of cosmological size it will not happen. Don't worry, this will be considerably shorter than the previous section (but it will be much hotter and denser).

If the Universe was closed its contraction would not be instantaneous; putting on the brakes would take a considerable amount of time, meaning more time between expansion, slowdown, and the final implosion: as the expansion and contraction are symmetrical, the latter would require the same time as the former. The lower its density and closer to the critical one, the more events than are expected to happen in an open or flat Universe as depicted above would happen. Hypothetical observers who were still around because of the delay caused by the speed of light would see the Universe was beginning its implosion and how redshifts decreased and became blueshifts everywhere, first the closest and last the galaxies farthest away. As time passed by, the temperature of the cosmic microwave background would increase instead of decreasing, as had happened before during its expansion, and by the time the Universe had a size similar to the current one it would again have our current overall temperature of almost 3 Kelvin. However it would be a more aged Universe than now, with more dead stars and fewer shining ones.

Billions of years later, the cosmic (no more microwave, now infrared) background radiation would have reached room temperature, meaning that a planet such as ours would be unable to radiate excess heat, thus maintaining its equilibrium. Global warming would be something really global, not just a thing limited to a single planet. Meanwhile superclusters would begin to merge being followed by galaxy clusters and finally the galaxies themselves leaving the Universe as a big hyper-galaxy, where everything would be bathed in a temperature of a few hundred K (thank the cosmic infrared background radiation) and rising, but at least stellar collisions and encounters would be rare for now.

Now the real fun begins. As the Universe kept contracting, stellar encounters first and collisions later would be more and more frequent, causing havoc among their planetary systems — but it would be trivial to worry about that since the increase in energy of the background radiation, now a mix of what was in the past the harmless cosmic microwave background plus all that energy emitted by stars, active galactic nuclei, etc during the many previous billions of years that would also suffer an ever-increasing blueshift, would cause the night sky to glow a dull red, later yellow, still later on white... you get the picture. The Universe would basically become a huge furnace, roasting all those life forms that had managed to survive the previous ordeal first and causing the stars to be unable to get rid of their internal heat, cooking them until they exploded later, beginning with the coldest ones (M-type stars as red dwarfs), following the hottest ones (Wolf-Rayet stars), and ending with stellar remnants when temperatures were hot enough to fuse heavy elements as helium or carbon, with the background radiation having arrived to the Gamma ray range after its excursion up through the electromagnetic spectrum. All that would remain would be a hot, dense plasma where any structure left from before would have disappeared and that as time passed by would become hotter and denser, and things would almost happen symmetrically to what had happened during the Big Bang: temperatures and densities would be so high that atoms would decompose followed by subatomic particles, leaving just quarks.

But we've said the final implosion would almost be symmetrical to the initial expansion. That's because during the latter there'd be a whole lot of black holes that were not present during the former. They, after surviving the ordeals experienced by everything else and having a good time sucking hot plasma, would begin an orgy of mergers while space's curvature and temperature kept increasing until there was basically just a single hypermassive black hole with the mass of the entire Universe: the Big Crunch singularity. And then, at the Big Crunch, the black hole would consume the universe itself.

Game over. Just as the Universe and space-time had begun with the Big Bang they'd cease to exist in the Big Crunch and it would be meaningless to ask what would happen next. But for those who may be concerned, just like the gravitational singularity of null size and infinite temperature in the center of a black hole tends to be considered a failure of general relativity that would disappear in a quantum gravity theory, the same would happen in the Big Crunch, meaning that there's no way to know what would really happen after. It's even possible the Universe would "reboot" in a new Big Bang with things starting again, maybe rewinding its entropy and/or with new physical laws, but with everything from the past Universe having been sent to oblivion.

Living in a collapsing Universe

Yes, there've been speculations too about the fate of living beings in a collapsing Universe. It's clear that, unless the Universe needed a really long time to reverse expansion, enough to have protons and/or black holes fizzling out, life would be not very different from the one in an expanding Universe. When the cosmic radiation's energy screamed upward they'd have to change some things, and once everything was just searing hot plasma with black holes coming in hot they'd have to change a lot of things. Quoting Freeman Dyson in such regard:

No matter how deep we burrow into the earth to shield ourselves from the ever-increasing fury of the blue-shifted background radiation, we can only postpone by a few million years our miserable end.

In this scenario the problem is not the lack of energy as in the Big Freeze but its excess instead and how to get rid of the surplus. More energy means physical processes going faster, thus the information-processing ability would increase too meaning that for those beings adapted to live in the Big Crunch the final collapse could be infinitely far away thanks to their subjective time accelerating more and more, even if the outside was just at seconds or much less.

John Barrow and Frank Tipler have studied what exactly would happen in the final moments of the collapsing Universe, finding that as it's very likely the collapse would be far from symmetrical and the Universe would oscillate. A sort of super-being existing there would have to act very fast to take advantage of it and have all of its parts connected, but at least said oscillations would give the required energy to drive thought processes. To improve this, some models suggest said oscillations would be infinite, so for that... thing its subjective time would be infinite (if the implosion was symmetrical, thoughts would be limited due to limitations caused by the speed of light — the maximum speed of any physical process). With that plus so much computing power, Tipler has suggested it could even simulate a whole lot of imaginary worlds, not just being able to ponder about its existence as well as the Universe surrounding it.

Now the bad news. Not only are those ideas based on physical models that could be unrealistic but also quantum effects during the last part of the collapse could limit the number of thoughts of that superbeing and it remains to be seen if it would be able to shed heat fast enough to be able to operate in a Universe that's increasing its temperature very fast. In the two latter cases the end would come sooner or later.

Ripping everything apart: Big Rip

The "Big Rip" is another way everything could end and it's nastier than the two previous endings of above, at least for meatware beings such as us. When the Big Freeze came we'd be long gone. The Big Crunch would kill us before the black holes came, even if it was a slow death being roasted as the cosmic background radiation heated us. The Big Rip would not give us that luxury.

In this scenario everything from galactic superclusters down to space-time itself and everything in between would be ripped apart at infinite distances by the runaway expansion of the Universe at a given time, this coming courtesy of dark energy or rather a nasty form of it named Phantom energy, which in the equation of the state of dark energy appears when $w$, a ratio between dark energy's pressure and its density, is less than -1. If it was equal to -1 or higher there'd be no Big Rip.

The paper that proposed this takes $w$ to be equal to -1.5. In their scenario, the Big Rip would take place just 22 billion years in the future (i.e., when the Universe still had long to live in other alternative endings). One billion years before the end, galaxy clusters would be ripped apart followed by the Milky Way when the Rip was 60 million years away. Three months before the finale the Solar System (or whatever remained of it, of course, since this event would happen after the Sun's death) would be unbounded, with the Earth, had it survived so long, exploding at 30 minutes before the end. Finally atoms themselves would be annihilated when the Big Rip singularity was just 10^-19 seconds away, followed by the very space-time itself transforming the Universe into an unusual form of singularity (as usual, "singularity" probably means our physics break down there, us being unable to fathom what would come next if anything).

Note also that subsequent studies have presented variants of this scenario: "pseudorips" where the runaway acceleration of the Universe would reach a maximum, and then decrease (and what structures would be destroyed depended of the moment when that happened), "little rips", where the runaway expansion of the Universe would disrupt everything but the space time (this just at infinity), even if later on than in a Big Rip, and "the little sibling of the Big Rip", similar to the former and that would also see the "Big Rip" happening in an infinite time.. Finally, a related, highly speculative idea known as the "Big Bad Trip", is that phantom energy if accreted into a wormhole (not a black hole), if they existed at all, could cause it to grow so large that would swallow the entire Universe before a Big Rip could take place even if it's anyone's guess where and how the latter would "regurgitate" the former.

The best measurements, from ESA's "Planck" satellite, show $w$ to be equal to -1.03 +/- 0.03, meaning the Big Rip will occur farther away in time — if it happens, as $w$ could well turn out to be -1 (note measurement errors and that previous analysis of the same data found a value of $w$ of -1.006 +/- 0.045..) Note also that it's to date unknown how will evolve dark energy in the future, and as commented in the last note were $w$ to change funny things could happen-

Life in the Big Rip (or rather, after it)

Despite this, some speculations about the Rip suggest it could mark the beginning of a new cosmic inflation phase, as conditions (speed of that runaway expansion) would become similar to those during said epoch so a new Universe (or more) could be born from the ashes of the old.

Other authors have suggested that just before the Big Rip, wormholes (if they existed at all, that is) would grow a lot and an advanced civilization could use them to escape from the dying Universe, or at least to send information through them; other authors, however, have claimed in such scenario nothing more complex than a proton could be able to transverse such wormholes.

Big Slurp: ???

Last but not least the nastiest way our Universe could end: a "Big Slurp", also known as "vacuum metastability event". Compared to it, certain bad trips and other stuff that comes from Fundies are just a joke.

In this scenario, the vacuum that to us seems stable would actually be unstable and without warning a bubble of true vacuum would appear elsewhere, expanding at the speed of light and devouring everything on its path until the observable Universe first and the entire Universe (much) later had been wiped out that way. Within the bubble, everything from physical constants to physical laws would change... just to have unstable space there that would collapse into a singularity in microseconds or less. Game over, man. Game over.

Coleman & DeLuccia's paper where this was outlined has this gem that says it all:

The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.

The part that makes this nightmarish is that in theory this could happen anytime, the next second, 10¹⁰⁰⁰⁰ years in the future... whatever and anywhere, in any part of the Universe, and that, as the bubble would come crashing at the speed of light we'd be unable to see it coming; the only positive side of this is that we'd be instantly destroyed without feeling anything. The good news is that measurements of the Higgs boson and the top quark masses suggest this would not happen for many billions of years, more exactly at least 10¹³⁹ years according to other calculations.In addition to that, there could be new still undiscovered physics that stabilized the vacuum, especially considering how the Standard Model of Particle Physics is seen as an incomplete one, and there still remains the possibility of our Universe being stable. It has also been suggested the most destructive form of false vacuum decay -the Coleman & DeLuccia's collapse into a singularity- could actually be impossible.

As per the Big Rip scenario above, theoretical research has presented milder variations of this scenario in which businesses would go more or less as usual, letting stars and galaxies and even life continue existing even with things somewhat changed, while in others we're almost as FUBAR'ed. Yet another proposal is that black hole decay could induce it but the possibility of true vacuum bubbles wiping the Universe out would depend of both how numerous they are as well as the rate of expansion of the latter; later proposals suggest either the true vacuum bubble would implode into a black hole instead of the reversal, that such process is actually suppressed around fast rotating black holes.

Note also that survival of the Universe as noted above depends of both how fast it would expand and the bubbles' nucleation rate, meaning that in the current scenario of an accelerated expansion of the Universe it could even outrun them with such bubbles being unable to percolate and merge; "you" would eat true vacuum sooner or later when one of them formed in your area of space but until then things would go as in the Big Freeze scenario of above with no danger of one coming in hot from beyond the cosmological horizon.

Of course there've been fears (wild speculations, really) that our particle accelerators, especially the Large Hadron Collider, could bring this on. Never mind that even our most advanced instruments are toys compared with the energies the Universe can summon with things as cosmic rays, with risk assessments having been published that point to the continued existence of the Moon (and other celestial bodies for the case) for several billions of years despite it being having been continuously pummeled by lots of cosmic rays with far more energies than those our particle accelerators can summon (nor it has been transformed into strange matter or has been sucked into a black hole either) as proof of no danger existing when one smashes atoms at relativistic speeds.

Living beings during this scenario and after it

Unless Coleman & DeLuccia were wrong and something alive could come into being in the new, post-Big Slurp, Universe of course (which, were physical laws and everything else different, would be truly alien). Or unless the false vacuum had already decayed just after the Big Bang, producing cosmic inflation as commented above.

But wait, there's more!

In the depths of ArXiv you can find still more ideas about the way everything will end courtesy of the arcane world of theoretical physics, such as time itself ending someday and this happening when Earth is still around. However other physicists have found a way to resolve the paradox that brings said idea and show the calculations that reach to that fate being wrong, and as noted in the article even if that event could happen requires the eternal inflation model to be correct and even not all universes born on it would be affected.

Stay tuned for more …

A song for Time’s last sigh

“”This is our last song, but what a song

—Ouroborus

See also

• Expanding universe
• Big Bang
• Alternative cosmology
• Existential risk for the end of the human race
• List of predictions of the end of the world — the many theological guesses that have all been wrong thus far
• The ultimate level of Super Mario Galaxy, which takes place in the fantasical center of the universe and is called "Fate of the Universe". It is as accurate as the theological guesses.

External link(s)

• In a nutshell: Three ways to destroy the Universe.