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This article deals with the current scientific consensus about the evolution of the Universe from the Big Bang to the present epoch plus a peek to its possible futures as revealed by science, as well as scientific speculations about the kind of life that could exist in those futures, and hopes to be a starter for readers who want to know more about this fascinating subject. Expect some of the ideas depicted below to change more or less as Science Marches On; however the basic picture is robustly backed by both theory and observation and highly unlikely to change. If you want to know about the history behind these ideas, The Other Wiki article is a good place to start.

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(Almost) the beginning of everything: the Big Bang.

Yes, almost. The Big Bang theory actually says nothing about the way the Universe came to be (or why it even banged), the Time Zero itself, and in order to arrive there we need a quantum theory of gravity -the Holy Grail of physicists-note . At best we can say that 13.8 billion years ago there was something very hot and dense, and extrapolating our theories to that moment we obtain that the Universe would have been by then a singularity of infinite density and temperature.

Theories about how the Universe came to be range (from an scientific perspective, that is) from it simply appearing and considering to ask what existed before the Big Bang as meaningless, as there was no space or time where anything could exist in tonote , as that is somewhat uncomfortable for scientists, other ideas as a quantum fluctuation appearing in the nothingnessnote , perhaps the carcass of a very old Universe note , our Universe being just one among a myriad of others, it having existed forever as a Cosmic Egg before it banged as a "quantum Hell" infinitely hot and devoid of space-time, or in string theory two branes colliding and producing that way a Big Bang.

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Before we go on, it's time to shoot down an important misconception about the Big Bang: the Big Bang was not an explosion as we understand it, with everything going violently outward from a single point located in the middle of nowhere. Everything, energy, space, and time appeared there so the correct way to describe it is that the Big Bang happened everywhere, in all points of the Universe at the same time, not just in the corner of the Universe we can observenote . Just as what may have happened at Time Zero itself it's very MindScrew-y, yes, and you can bet cosmologists do not think very much about it.

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The Planck Epoch: from 0 to 10-43 seconds.

We can say something about this epoch of the Universe's history but not very much, as that aforementioned theory of quantum gravity is a must. The Universe of this epoch would not only have been very dense and hot, with a temperature of 1032K but also very smooth even if quantum effects began to give it some structure. And it's also thought the four fundamental forces of naturenote  were one.

The Great Unification Epoch: from 10-43 to 10-36 seconds.

As the Universe expanded and cooled, gravity separated from the other three forces at the beginning of this epoch. In this extremely hot mess physical parameters as mass or charge were meaningless until the strong interaction followed gravity and left the other two remaining forces together. That break up would have funny consequences for the Universe as we'll see next.

The Electroweak Epoch: from 10-36 to 10-12 seconds.

The mentioned breakup of fundamental forces is thought to have caused an extremely fast expansion of the Universe ("cosmic inflation"), whose lasting is often considered a distinct epoch: the inflationary epoch. In a tiny fraction of second after the Big Bang, from 10-36 to 10-33 or 10-32 seconds, it expanded at least 1026 times (likely much more)note  cooling a lot (but still being extremely hot by our standards; we're talking from going from 1027K to a chilly 1022K -some inflationary models suggests it would have cooled down to absolute zero instead, ie: heat death-)

Cosmic inflation was developed to explain four properties of the Universe:

1) The Universe is, as far away we can see and everywhere, very homogeneous: matter distribution is pretty much the same everywhere as is the temperature of the cosmic microwave background photons even in two parts of the Universe that never were in contact. The only way to explain that is that the Universe had expanded very fast in the past.

2) The curvature of the Universe is, as far we can detect, null (what is known as a "flat universe"). Theory shows that large-scale structure formation can only form in a flat universe that has just a given critical density; had it a density too high it would have collapsed back, and had it one too low structure (read: galaxies) would not have formed. Inflation solves this by smearing the original inhomogeneities, dating from the Big Bang itself, over areas considerably larger than the observable Universe.

3) Most Great Unification Theories predict that... weird things as magnetic monopoles would have been formed in great numbers just after the Big Bang. However none of them have been detected to date. Assuming those theories are correct, cosmic inflation would have also scattered them all over a very large space, so we'd be very lucky to find onenote .

4) As mentioned above, pre-inflation inhomogenities were replaced with quantum fluctuations, that would have been expanded to galactic sizes and that, as we'll see further, would have acted as seeds for structure formation.

The exact details (for example what particle drove it out (some scientists think on the famous Higgs boson or an undetected one related to it), or the exact time when it ended) remain to be worked out, and while observations of the cosmic microwave background by space missions as "WMAP" or "Planck" give strong support to the simplest inflationary models there're still plenty of models to choose from and not all cosmologists support either the cosmic inflation model or the never-ending inflation already mentionednote 

Once inflation ended, subatomic particles existing during the Great Unification Epoch were thinly scattered over the entire Universe. However the energy stored by the inflationary field was dumped out heating the Universe again to temperatures similar to those before the inflationary era and forming subatomic particles as quarks and gluons. Dark matter may have been formed by this epoch, however others put its creation earlier on.

The Quark Epoch: from 10-12 to 10-6 seconds.

At 10-12 seconds, the two remaining forces that were together (electromagnetism and weak interaction) parted ways forever and this, with the 'verse having cooled to an even chillier 1015K. Particles got mass thanks to the Higgs field, caused by the Higgs boson, and matter and anti-matter particles (quarks, antiquarks, and gluons) were formed at 10-11 seconds. Things were still too hot to have something larger formed.

The Hadron Epoch: from 10-6 to 1 second.

The fun continues. The Universe keeps cooling and expanding, allowing first quarks and antiquarks to combine with others of their kind forming hadrons and antihadronsnote  and later to annihilate among themselves releasing a lot of energy and leaving a small residue of matter that forms all the visible things of the Universenote .

The Lepton Epoch: from 1 second to 10 seconds.

Temperatures have descended to 109K (ie: as hot as the nuclei of some high-mass, evolved, stars) and leptons (electrons and positrons), that formed in pairs before and could live in equilibrium, also annihilate among themselves releasing gamma-ray photons and leaving just a small residue of electrons. At 10 seconds, temperature is so low that no more electron-positron pairs can be formed and matter begins to rule the Universe (and will keep so for some billions of years). Universe's expansion is controlled by gravity, and due to that cooling down of the Universe neutrinos decouple from matter leaving a cosmic neutrino backgroundnote 

The Photon Epoch: from 1 second to 370.000 years.

Things begin to be boring, as matter and radiation are coupled together while dark matter begins to build structures independently of the former. The Universe becomes a very hot, opaque, fog and temperature keeps its descent. From 3 minutes to 20 minutes after the Big Bang, another fusion dance takes place as nuclear fusion ensues leaving behind an Universe mainly composed of hydrogen (75%) with some helium (25%) and just traces of lithium (and even less of heavier elements), as confirmed by observations.

Nothing of interest will happen for a long period of time; the Universe keeps expanding and cooling, but it's still so hot that is opaque to radiation as photons collide with matter and this veil will not be lifted until 370,000 years after the Big Bang, when the temperature is of "just" 3,000K. Then matter and energy will part ways and atoms will finally form, with the photons emitted by then forming the cosmic background radiation -as the Universe cools down and expands that background will shift to infrared radiation, and later (the current epoch) to radio waves-, and those quantum fluctuations mentioned that were formed on the cosmic inflation and acted as seeds for structure formation leaving their imprint on itnote -. Note also that, while the Universe is no longer opaque to radiation, it would be dark for an organ as the human eye, and this would not change until the first stars formed much later.

The "Dark Ages" of the Universe: from 370,000 to 700 million years.

Meanwhile large-scale structure keeps taking shape in this darkness and it's conceivable that, were enough heavy elements around, some million years (between 10 and 17) after the Big Bang the cosmic radiation background would have warmed whatever could have formed by them to temperatures enough to have liquid water, life perhaps appearing for the first time. Nonetheless, when the scaffolding formed by dark matter is completed 100 million years after the Big Bang is finished normal matter will fall towards it and collapse. It was thought stars would begin to form -in large numbers, at least- by those epochs; however recent research shows stars did not begin to shine in numbers until 700 million years after the Big Bang, much later than what was thought.

Structure formation: from 700 million years to 7.8 billion years.

Now the Universe is dominated by stars that re-ionize again hydrogen and helium. These were times of huge stars where the dearth of heavy elements as well as other conditions hardly ever present today allowed them to be much more massive than the current ones. Said monstruous stars, that depending on their masses died either collapsing directly to form black holes or in powerful explosions (hypernovae/gamma-ray bursts), had a very deep impact on their galaxies, enriching them with heavy elements that they created on their nuclear furnaces thus paving the way for planets and eventually life to appearnote .

However those were also hard times for any life that could have appeared in those epoch, as galaxies not only were making stars galore but also were smaller (thus -much- more abundant) and much closer than now, collisions and mergers between them being very frequent and with it even more star formation as gas, something more than plentiful by then, was still more stirred. Said mergers not only began to build larger and larger galaxies, but also black holes that had been formed by the death of the most massive stars coalesced forming massive ones that tended to be located in galactic centers -and, of course, galactic collisions and fusions meant said black holes would merge among themselves-. In addition to that insatiable desire to gorge everything, matter that fell to those black holes formed accretion disks around them feeding quasarsnote  with a luminosity of up to hundreds of galaxies, often accompanied by massive starbursts as large amounts of gas were funneled into the small space of a galactic center due to said galaxy interactions and mergers and compressed until starbirth begannote . This assemblage also continued on larger scales, with galaxy groups merging to form larger clusters of galaxies and so on. However as time went on much fewer but considerably larger galaxies remained and together with the assemblage of large-scale structure the Universe began to take a look that would be quite familiar to us, with this likely having happened 6 billion years after the Big Bang.

This epoch as well as the one that follows are well described by the Lambda-CDM model, the standard cosmological model that to date is supported by a significant number of observations even if it's not without some issues.

The Dark energy era: from 7.8 billion years to now (13.8 billion years).

Once upon a time there was an Universe dominated by matter, both visible and dark one -far more abundant-, that seemed that it would be expanding forever at a similar speed as the density of matter was not enough to cause it to stop, much less to contract it again, where structure formation would keep going quite far into the future, and where even if its expansion finally carried its galaxies so far away one from each other that they would be for all purposes isolated process would take a lot of time.

Wrong. 7.8 billion years after the Big Bangnote  dark energy that had been lurking in the shadows during those eons made its apparition. Its effects were at first not visible (everything in this adolescent Universe took its time), but as time went on they were so causing the Universe to expand faster and faster. Meanwhile, when the Universe was around 9.1 billion years old, somewhere in the spiral arm of a typical large spiral galaxy, an unremarkable star was formed and on its third planet appeared carbon compounds able to replicate themselves, that as time went on evolved into beings able to ponder about the Universe, how it had began... and how it would end.

The future of the Universe.

The ultimate fate of the Universe depends of its total densitynote , and there're just three outcomes:

  • If the total density is less than 1, we've an open Universenote  that will keep expanding forever.
  • If the total density is exactly 1, we've a flat Universe that will keep expanding forever too, but in theory will stop its expansion after an infinite timenote 
  • If the total density is more than 1, we've a closed Universenote , whose expansion will stop to contract and collapse again into a singularity or whatever.

That was the status of things until the discovery of dark energy in the late 90's, whose density is also added to the sum. With it provocating a repulsive force that is accelerating the Universe's expansion even if the total density of the Universe was larger than 1 it could overtake gravitation and cause it to keep expanding forever, and conversely, if it was to change in the future it could bring a collapse of the Universe if it became atractive (and if it dissipated, the Universe's expansion would continue, albeit at constant speed)note 

Observations show that our Universe is, as commented above, flat as far as we can measure it, and its density is -also within error range- exactly the critical onenote , suggesting that it will expand forever and if dark energy does not change faster and faster. As we'll see on the next sections that will have deep implications for its future.

Big Freeze: everything fades into oblivion.

Based on an indefinite expansion of the Universe as stated above (flat or open Universe), 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 nothingnote 

After the seminal work of Fred Adams and Gregory Laughlinnote , 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". We'll follow that classification:

Primordial Era.

The "Primordial Era" is the epoch already discussed above between the Big Bang and the formation of the first stars, a few hundred million years after it.

Stelliferous Era.

The "Stelliferous Era" began when the first stars formed and of course 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 after exhausting the available gas. However many things will happen before lights go outnote 

The current large-scale structure of the Universe is a web-like network of filaments and walls of galaxies, with its nodes corresponding to galaxy clusters and especially superclusters, surrounding large (sometimes very large, up to billions of l. y.), empty, extensions named "voids". Under the accelerated expansion of the Universe caused by dark energy it's expected this structure will be torn apart by the expanding voids 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 extensions of empty space stopping structure formation. This is expected to culminate 150 billion years in the future, when all galaxies outside the Local Superclusternote  (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 Groupnote  will be entirely isolated within that time frame with no galaxies visible outside it.

Going down the scale, it's expected that the winds of dark energy will stop cluster and supercluster growth as mentioned above and that already mentioned isolation will cause them to change their shape from the current, flattened, ones to almost spherical ones with a well-defined edge 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 or may even be expelled from their clusters due to gravitational interactions with other galaxiesnote . A physical process called "dynamical relaxation", that will be discussed further, could even begin to take place at full scale in some clusters with the most massive galaxies concentrated in the center while the lightest ones end up in the cluster's periphery. And in spiral and irregular galaxies remaining around in those times, 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 as their reserves of gas are steadily consumed.

Finally when we turn to the galaxies and the stars themselves while the latter 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 ends locked away on an inert remnant (white dwarf, neutron star, or black hole) but also 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 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 metalsnote  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 just in the infrared and that would be dimmer than even the dimmest stars of this day (and even more longer-lived).

As time goes on, there will be a last star massive enough to go supernova, a last Sirius-like star, a last Sun-like star, and finally only the lowest-mass stars (red dwarfs) will remain. As their expected lifetimes (up to 1013 years, and perhaps even more) 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"note , that exist for billions of years before dying as white dwarfs. Thanks to that increase in luminosity, even if their stars die away, future galaxies will be about as luminous as modern ones (and after having for a long time orange-red colors of red dwarf 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), begin to fade away until finally the Universe goes dark ending the Stelliferous eranote .

Degenerate era.

The "Degenerate Era" will begin 1014 years in the future, but its end depends whether protons are unstable or not. In the first case, depending on the unknown proton's half-lifenote , may last from around 1030 years, to 1040 yearsnote  or up to an even more mind-blowing 10200 years, far beyond the evaporation of black holes (see further), but in the second case it will last a whole lotta more, so long that You Cannot Grasp the True Form does not even to begin to describe itnote . 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 dwarfsnote , 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 galaxiesnote , 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 after 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 deep into the Degenerate Era as long as there're enough 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 bornnote . 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 Ia supernova 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 already mentioned "dynamical relaxation"- 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 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)note . 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 year 1030 AD 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 in a sort of remembrance of those long-past early days, that will last as long as matter is present, just around a billion years (very little in those timescales).

Some models suggest dark matter may go down the drain if composed of certain types of hypothetical particles. If that happens, it will occur via either annihilation among themselves or capture by astrophysical objects with the dark matter halo that surrounds the galaxies being depleted after around 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 cold), and as the galaxy loses mass that way its gravitational grip into the bodies that form it will be weaker, translating that into the processes outlined above being fostered.

Assuming protons are unstable, their decay will be the next event of significance to happen in those distant epochsnote . While the time is dependent on the unknown proton's 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 hydrogennote . This will also release a tiny trickle of energy heating said object, and "tiny" means here tinynote . For black dwarfs and neutron stars, the temperature that they will attain thanks to proton decay will be up to 1K and in the case of neutron stars 100Knote . 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 own radiation- to finally disappear. Neutron stars will suffer a similar fate, but their expansion may be explosive destroying themnote . The products left by the death of protons will be photons and leptonsnote .

Black hole era.

Assuming the shorter proton lifetimes given above, after both them 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 an Universe once filled with stars and galaxies.

Things begin to be really boring, as nothing will happen for countless eons except the very occasional fall of a particle into a black hole here and there or the collision between an electron and a positron producing a pair of gamma photons. Electrons and positrons may form according to some models atoms of what is known as "positronium", composed of an electron and a positron with initial diameters larger than even the current observable Universe, whose orbits will decay with time but very, very slowly in a dance of death until their final embrace and annihilationnote . 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 2x1066 years for one with the mass of the Sun to 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 is unknown and it could even leave 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 the winds of dark energy in 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 that extreme environment it's thought quantum effects will prevail and our understanding of what will happen is very poor. Forever is a very long time especially at its end, and everything may be possiblenote .

Big Crunch: Everything 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, dying not in ice but in fire instead. This scenario was favored in the past, but observations show that unless dark energy gives us a prank of cosmological size it will not happennote . Don't worry, this will be considerably shorter than the previous section (but things 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 happennote . Hypothetical observers that were still around because of the delay caused by the speed of light would take a long 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 awaynote . 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 find it impossible to radiate excess heat to maintain its equilibrium. Global warming would be something really global, not just a thing limited to a single planetnote . 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 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 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.

The endnote . 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.

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 came we would (very likely) be long gone. The Big Crunch would kill us before the black holes came, even if it was a slow death being roasted to Hell and back as the cosmic background radiation endlessly increased its temperature. The Big Rip would not give us that luxury.

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 on its equation of state 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 2003 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 (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 (or whatever remained of it, of course) would be unbounded, 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 data show w to be very close to -1, meaning the Big Rip will occur farther away in time -if it what was going to happen, as due to measurement errors w could well turn out to be exactly -1; other research suggests a similar result-.

Big Slurp: ???.

Last but not least the nastiest way our Universe could endnote : a "Big Slurp", also known as "vacuum metastability event" or "quantum vacuum collapse". You'll see soon why is that.

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 overnote .

The 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 are that measurements of the Higgs boson and the top quarknote  masses suggest this would not happen for many billions of years and in addition to that, there could be new still undiscovered physics that stabilized the vacuum and our 'verse could be stable after allnote .

Of course there've been fears that our particle accelerators, especially the Large Hadron Collider, could bring this after messing too much with subatomic particles, especially the Higgs boson. Never mind that even our most advanced instruments are just toys compared with the energies the Universe can summon with things as cosmic rays or especially during the Big Bang itself.

Life in the distant future of the Universe.

Naturally there've been especulations about the kind of life that could develop in any of the scenarios depicted above (well, not all of them anyway). Here are some of them Don't be very optimistic

Life in the always expanding Universe.

It's clear that civilizations in an ever expanding 'verse will have to face very hard times with a number of crisis in store, 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. Those aliens will certainly be more than just starfishy.

For the Degenerate era it's clear (some studies 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 that should have things easier) could existnote . 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 in order 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, the limit between what's natural and what's articial could blur until it disappeared.

Things will be considerably worse once protons decay and not just because matter will have dissolved into oblivion. Black holes, as well as much before they become the last non-subatomic objects in existence (if protons are gone for good before black holes go away), may be a source of energy while they exist using their Hawking radiation when it's advanced enough or extracting energy from their rotation but once they have disappeared whatever existed will be pretty much SOL. It was suggested in the late 70's 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 will 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 fail because of quantum effects, killing the being. If that's not bad enough it was thought the temperature of the Universe would be decreasing forever so they could be in thermal equilibrium with it, but it's currently thought 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 were forever reshuffling their memories, with no communications of any kind with the outside, and it's questionable at all to call that "life"note 

Life in the 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, not just moving first to refrigerated environments and to refrigerated environments very deep into planets later, 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.

It has been studied what would exactly happen in the final moments of the collapsing Universe, being found that as it's very likely the collapse would be far from symmetrical and the Universe would oscillate. Any 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, it has been 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 laternote .

Life in the Big Rip.

Are you kidding?. Unless blowing up the Universe that way meant repeating the early Universe's inflationary phase, so a new Universe would be born like a phoenix from the ashes of the old one, or unless someone managed (yes, again) to construct a new Universe and move there life is pretty much screwed.

Living beings in a quantum vacuum collapse scenario.

Coleman and de Luccia: [...] 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.
'Nuff said. That is, unless both authorsnote  were wrong and something alive, even if it was unable to know joy, could form in the new, post-big Slurp, Universe -or "singularity" meant an Universe rebooting again- and unless too the false vacuum had already decayed just after the Big Bang producing cosmic inflation as commented above. No, unless you've done that before it happens, you will not be able to move to another Universe if the Slurp comes without warning.
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