Ultimate fate of the Universe
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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 (for history and what's behind what follows check Ultimate fate of the Universe at the other Wiki).
 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 of 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 will 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), is the thing to blame. Said acceleration means that even if Ω was larger than 1, 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.[notes 1]
 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". Basically, the Universe will be in 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 "Stelliferous Era" began a few hundred million years after the Big Bang, when the first stars formed,[notes 2] 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. However many things will happen before lights go out.[notes 3]
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 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" 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.
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. As for the galaxies in them, those will merge to form large systems until there are no more galaxies to fuse with.[notes 4] or may even be expelled of their clusters due to gravitational interactions with others. 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 an inert remnant (white dwarf, neutron star, or black hole) 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, meaning the gas reserves to form new stars are being exhausted with time. Star formation is expected to go on for hundreds of billions of years, and may last for up to a billion (1012) years or even more. However stars of those very late times will be different than the current ones because of the ever-growing abundances of metals,[notes 5] with them having shorter lifetimes 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.
As time goes on, only the lowest-mass stars (red dwarfs) will remain. 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 those 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 (and the latter cool to "black dwarfs", that emit no radiation at all), beginning to fade away until finally the Universe goes dark 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, 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.
 Degenerate era
The "Degenerate Era" will
be an era of moral degeneration and depravity begin 1014 years in the future, but its end depends whether protons are unstable or not. In the first case it may last until around 1030 years, but factoring proton decay if it occurs will end in 1040 years or up to 10200 years, far beyond the evaporation of black holes (see further)[notes 6], but in the second case it will last a whole lotta more[notes 7]. This Universe will be almost entirely dark for an organ such as the human eye, but not so in other wavelengths.
The protagonists of those distant times will be the cooled corpses left behind by stars: white dwarfs -now as black dwarfs-, neutron stars, and black holes (the first two are composed of degenerate matter, giving that the name to this epoch. Plus a veritable number of brown dwarfs, even more of what we can be considered as debris -planets, asteroids, comets... name it-, and finally some gas that has managed not to have been incorporated into stars or dispersed away. All that will keep orbiting around the centers of their dead galaxies. and 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 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), that will shine for around 1013 years. This will be a channel to form new stars, that is estimated will be able to generate around 100 stars shining in a Milky Way-sized galaxy for a long time, while there're brown dwarfs around. If the collisions are between white 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 too happen, among others Type Ia supernovae when two white dwarfs massive enough collide 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"- will cause the heaviest objects to sink to the galactic center while the lightest ones are sent away, even expelled of the galaxy ("galactic evaporation"). The time frame 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 considerable longer time frame (around 1024 years)[notes 8]. On galaxy clusters that still exist in that epoch, a similar process will occur at supergalactic scale with the most massive galaxies or whatever remains of them falling to the center and the lightest being sent to the periphery of the cluster.
The final product of the processes outlined above will be that around by the 1030 year all that remains of galaxies will be supermassive black holes plus a lot of flotsam and jetsam roaming across the endless darkness, 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, that 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[notes 9]. 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-time, 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 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).
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 halo that surrounds the galaxy being depleted after 1025 years (ie: 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 being cold). Other subatomic particles that may form dark matter as Axions will probably disappear in a timeframe similar to that of proton decay
 Black hole era
After protons and neutrons are gone for good[notes 10], 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 an Universe once filled with stars and galaxies.
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[notes 11]. But time 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 for one with the mass of the Sun to 1×1098 years for one with the mass of a large galaxy. Note that since during the very latest stages of black hole evaporation things go quantum and very hot, the ultimate fate of the hole it's not known and it could even left a tiny, dark matter-like, remnant.
 Dark era
After the last of the supermassive black holes is history, the Universe will enter into the "Dark era" for all eternity. It will be an unimaginable dark, cold (at just 10-29K), and empty place where electrons, positrons, photons, neutrinos, and (perhaps) dark matter will roam free only very hardly ever encountering each other, assuming they're not carried away by an Universe in perhaps runaway acceleration. The positronium atoms mentioned above are expected to decay in timescales larger than 10100 years, if they managed to form at all.
The Universe will be in the clutches of heat death, in an extremely low energy state with things taking a very long time to happen if they ever happened at all. Note that it's entirely unknown what will happen next (maybe a Big Rip (see further)), maybe a vacuum metastability event (see further too), maybe just plain heath death... who knows), as in those extreme conditions it's thought quantum effects will prevail and our understanding of what happens by then is unknown. Even a new Big Bang could take place if you can wait 10101056 years. Eternity is a very long time especially at the end[notes 12], and all may be possible.
 Life in an ever-expanding Universe
Of course there've been especulations about the kind of life that could develop or adapt to live in an ever-expanding Universe. It's clear they'll have very hard times in front with a number of crisis 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), 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 Degenerate era it's clear (Krauss and Starkman suggest that if protons did not decay life similar to ours could exist up to 1050 years in the future) even after lights have gone out life as us (much better said advanced civilizations who should have things easier) could exist even if their existence would be quite dire always on the search for energy[notes 13]. It's even conceivable that a civilization advanced enough could change things as stellar orbits around a galaxy's center forming stellar clusters to manage for their survival or even could control 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 on virtually endless supply). At the end, nature could have well ended "technologized" so the limit between what's natural and what's articial had disappeared.
Things will be considerably worse once protons decay and not just because matter will have dissolved into oblivion[notes 14]. Black holes, as well as much before they become the last non-subatomic objects in existence, may be a source of energy while they exist using their Hawking radiation when it's advanced enough, extracting energy from their rotation in one way, or in other. but once they have disappeared whatever existed will be pretty much SOL. Freeman Dyson 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 the sort of "clocks" that they used to awake would sooner or later fall because of quantum effects, killing the being. If that's not enough Dyson assumed the temperature of the Universe would be decreasing forever so they could be in thermal equilibrium with it, but it's currently know 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 kept reshuffling their memories forever, with no communications of any kind with the outside, and it's questionable at all to call that "life"[notes 15].
 Big Crunch: All together again
The "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. 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[notes 16]. 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 the brakes would take a considerable amount of time, meaning than the more time that passed between expansion, slowdown, and the final implosion (as the expansion and contraction are simmetrical, the latter would require the same time than the former) the more events that are expected to happen in an open or flat Universe as depicted above would happen[notes 17]. Hypothetical observers that were still around because of the delay caused by the speed of light would take a time to see the Universe was beginning its implosion and how redshifts decreased and became blueshifts everywhere, first the closest and latest the galaxies farthest aways. 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 have again the almost 3K it now has. However it would be a more evolved Universe than now, with more dead stars and less shining ones.
Billions of years later, the cosmic (no more microwave, now infrared) background radiation would have reached room temperature, meaning that a planet as ours would have impossible to radiate excess heat maintaining its equilibrium. Global warming would be something really global, not just a thing limited to a single planet[notes 18]. 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 at a temperature of a few hundred K (thank the cosmic infrared background radiation) and raising, 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 would be more and more frequent, causing havoc among their planetary systems -but it would be trivial to worry about that since the increase of energy of the background radiation would cause the night sky to glow dull red, later yellow, white... you get the picture. The Universe would basically be 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 baking them until they exploded later. 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 explosion. 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 experimented 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.
Game over. Just as the Universe and the space-time had began to exist in the Big Bang they'd cease to exist in the Big Crunch and would be meaningless to ask what would happen next. But for those who may be concerned note that, 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 of 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 a lot of things.
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 in the outside was just at seconds or much less
John Barrow and Frank Tipler have studied what would exactly 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 those ideas are 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 remains to be seen if it would be able to get rid fast enough of heat on an Universe increasing its temperature very fast too in order to be able to operate. 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 Universe's endings above, at least for meatware beings as us. When the Big Freeze comes 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[notes 19]
In this scenario all from galactic superclusters down to the 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, that in the equation of state of dark energy appears when , 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 to be equal to -1.5. In their scenario, the Big Rip would take place just 22 billion years in the future (ie: when the Universe still had much to live). 1 billion years before the end, galaxy clusters would be ripped apart followed by the Milky Way when the Rip was 60 million years away. 3 months before the finale the Solar System would be unbounded (or whatever remained of it, of course), with the Earth, had it survived the Sun's death, exploding at 30 minutes before the end. Finally atoms themselves would be annihilated when the Big Rip was just 10-19 seconds away.
The most recent measurements show to be equal to -1.006 +/- 0.045, meaning the Big Rip will occur farther away in time -if it what was going to happen, as could well turn out to be -1 (note measurement errors)-.
 Life in the Big Rip
 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 even if somehow we could defend of it, the only positive thing being 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. In addition to that, there could be new still undiscovered physics that stabilized the vacuum and still remains the possibility of our Universe being stable[notes 21]
Of course there've been fears that our particle accelerators, especially the Large Hadron Collider, could bring this. Never mind that even our most advanced instruments are toys compared with the energies the Universe can summon with things as cosmic rays.
 Living beings during this scenario
Unless Coleman & DeLuccia were wrong and something could form in the new, post-big Slurp, Universe of course. Or unless the false vacuum had already decayed just after the Big Bang producing cosmic inflation as commented above.
 See also
- Expanding universe
- Big Bang
- Alternative cosmology
- List of predictions of the end of the world — the many theological guesses that have all been wrong thus far
- ↑ It's not known how will evolve dark energy in the future: if it will not change, will dissipate (and the Universe will keep expanding but not in runaway fashion), or will become positive (attractive), causing a collapse of the Universe. However the latter possibility is deemed unlikely.
- ↑ More exactly, 700 million years after the Big Bang
- ↑ We assume here that dark energy will not change and the Universe's expansion will keep its acceleration, albeit with no Big Rip.
- ↑ For the Local Group, where our galaxy — the Milky Way — lives, it's expected that its two largest galaxies, it and Andromeda will merge within 4-5 billion years, followed by the gobble-up of all other galaxies in the Group between 100 billion and 1 trillion years in the future.
- ↑ In astronomy, anything that is not hydrogen or helium.
- ↑ Remember here the meaning of powers of ten: from, for example from 1014 to 1015 there're 10 times 1014. Times are, as stated at first, so long that seem like eternity. But they're not eternity.
- ↑ In this scenario, quantum tunneling would transform normal matter into iron within 101500 years and after a nothing short of mind blowing 101076 years into black holes, that within that time frame would decay instantly.
- ↑ On binary systems that have been able to survive to those ages, this will too be another channel to form stars
- ↑ Remember that neutrons are unstable outside atomic bodies, with half-lifes of less than 15 minutes
- ↑ Assuming the shortest half-lifes given above
- ↑ Note that if the Universe kept accelerating into those ages, positrons and electrons would likely not form those atoms
- ↑ Quite far more than some who offer "eternal life" think
- ↑ Even if meatware or equivalent was replaced by far more efficient cyberware until we basically had sentient robots and computers. But what's more important: our current form of inefficient fluid sacks or our technological and cultural legacy?
- ↑ Assuming, of course, proton decay takes place before black holes are history
- ↑ But who knows?. Since those things will happen in a future so far away maybe they'll not waste their time and resources just to delay a doom that cannot be averted and will simply learn how to design and produce a new Universe and will use wormholes to move there or at least information to re-build themselves on that place
- ↑ Remember that the density of matter, both normal and dark, is less than one third of the minimal density needed to have a close Universe.
- ↑ For dramatism, we'll assume the Universe contracts when there're still plenty of stars around (funnier than when there'll be just stellar corpses everywhere, and much more funny than when there'll be just subatomic particles whizzing by)
- ↑ Never mind that by that epoch Earth would have been burned to a crisp by the red giant Sun long ago
- ↑ And it would come complete with an equivalent of the Rapture, except that it would affect everyone, and they'd be send to the skies, not to Heaven
- ↑ Again, remember that "singularity" means that our theories fail somewhere. Maybe things would be different there and maybe even something capable to know joy could exist after all
- ↑ It's even conceivable the "Big Slurp" had actually happened in the past, corresponding to the cosmic inflation epoch
- ↑ A sum of the Universe's matter (both normal and dark) density Ωm — plus the dark energy density (ΩΛ) now that it's known to exist
- ↑ 2.0 2.1 Planck 2015 results. XIII. Cosmological parameters. Note how the value of matter's density is less than one third of the critical density.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 A dying universe: the long-term fate and evolution of astrophysical objects
- ↑ 4.0 4.1 4.2 Life, the Universe, and Nothing: Life and Death in an Ever-expanding Universe
- ↑ 5.0 5.1 Future Evolution of Cosmic Structure in an Accelerating Universe
- ↑ Future evolution of bound superclusters in an accelerating Universe
- ↑ The green valley is a red herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies
- ↑ 8.0 8.1 Red Dwarfs and the End of the Main Sequence
- ↑ The return to an static universe and the end of cosmology. Note that as the only way to know the actual past history of the Universe would be records that managed to survive to those late ages, it has jokingly been suggested that cosmology may become a sort of religion. Also note that those beings could fathom the origin and the fate of their island Universe, but that's all.
- ↑ Brown Dwarf Accretion: Nonconventional Star Formation over Very Long Timescales
- ↑ 11.0 11.1 11.2 Deep Time, David J. Darling, New York: Delacorte Press, 1989, ISBN 978-0-38529-757-8.
- ↑ Gravitational demise of cold degenerate stars
- ↑ Spontaneous Inflation and the Origin of the Arrow of Time
- ↑ Spontaneous entropy decrease and its statistical formula
- ↑ 15.0 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 Davies, Paul (January 9, 1997). The Last Three Minutes: Conjectures About The Ultimate Fate Of The Universe. Basic Books. ISBN 978-0-465-03851-0.
- ↑ Time without end: Physics and biology in an open universe
- ↑ Phantom Energy: Dark Energy with w<-1 Causes a Cosmic Doomsday
- ↑ Gravitational effects on and of vacuum decay. A freely, seemingly legal, PDF version can also be found on the Net.
- ↑ "Will our universe end in a 'big slurp'? Higgs-like particle suggests it might".
- ↑ Viewpoint: Are We on the Brink of the Higgs Abyss?
- ↑ As this one