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Current observations suggest that theexpansion of theuniverse will continue forever. The prevailing theory is that the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario popularly called "Heat Death" is also known as the "Big Chill" or "Big Freeze". Some of the other popular theories include the Big Rip, Big Crunch, and the Big Bounce.[1][2]
Ifdark energy—represented by thecosmological constant, aconstant energy density filling space homogeneously,[3] orscalar fields, such asquintessence ormoduli,dynamic quantities whose energy density can vary in time and space—accelerates the expansion of the universe, then the space between clusters ofgalaxies will grow at an increasing rate.Redshift will stretch ancient ambient photons (including gamma rays) to undetectably long wavelengths and low energies.[4]Stars are expected to form normally for 1012 to 1014 (1–100 trillion) years, but eventually the supply of gas needed forstar formation will be exhausted. As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker.[5][6] According to theories that predictproton decay, thestellar remnants left behind will disappear, leaving behind onlyblack holes, which themselves eventually disappear as they emitHawking radiation.[7] Ultimately, if the universe reachesthermodynamic equilibrium, a state in which the temperature approaches a uniform value, no furtherwork will be possible, resulting in a final heat death of the universe.[8]
Infinite expansion does not constrain theoverall spatial curvature of the universe. It can be open (with negative spatial curvature), flat, or closed (positive spatial curvature), although if it is closed, sufficientdark energy must be present to counteract the gravitational forces or else the universe will end in aBig Crunch.[9]
Observations of theCosmic microwave background by theWilkinson Microwave Anisotropy Probe and thePlanck mission suggest that the universe is spatially flat and has a significant amount ofdark energy.[10][11] In this case, the universe might continue to expand at an accelerating rate. The acceleration of the universe's expansion has also been confirmed by observations of distantsupernovae.[9] If, as in theconcordance model ofphysical cosmology (Lambda-cold dark matter or ΛCDM), dark energy is in the form of acosmological constant, the expansion will eventually become exponential, with the size of the universe doubling at a constant rate.[citation needed]
If the theory ofinflation is correct, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang; but inflation ended, indicating an equation of state much more complicated than those assumed so far for present-day dark energy. It is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict.[citation needed]
In the 1970s, the future of an expanding universe was studied by the astrophysicistJamal Islam[12] and the physicistFreeman Dyson.[13] Then, in their 1999 bookThe Five Ages of the Universe, the astrophysicistsFred Adams andGregory Laughlin divided the past and future history of an expanding universe into five eras. The first, thePrimordial Era, is the time in the past just after theBig Bang whenstars had not yet formed. The second, theStelliferous Era, includes the present day and all of the stars andgalaxies now seen. It is the time during which stars form fromcollapsing clouds of gas. In the subsequentDegenerate Era, the stars will have burnt out, leaving all stellar-mass objects asstellar remnants—white dwarfs,neutron stars, andblack holes. In theBlack Hole Era, white dwarfs, neutron stars, and other smallerastronomical objects have been destroyed byproton decay, leaving only black holes. Finally, in theDark Era, even black holes have disappeared, leaving only a dilute gas ofphotons andleptons.[14]
This future history and the timeline below assume the continued expansion of the universe. If space in the universe begins to contract, subsequent events in the timeline may not occur because theBig Crunch, the collapse of the universe into a hot, dense state similar to that after the Big Bang, will prevail.[14][15]
Theobservable universe is currently 1.38×1010 (13.8 billion) years old.[16] This time lies within the Stelliferous Era. About 155 million years after theBig Bang, the first star formed. Since then, stars have formed by the collapse of small, dense core regions in large, coldmolecular clouds ofhydrogen gas. At first, this produces aprotostar, which is hot and bright because of energy generated bygravitational contraction. After the protostar contracts for a while, its core could become hot enough tofuse hydrogen, if it exceeds critical mass, a process called 'stellar ignition' occurs, and its lifetime as a star will properly begin.[14]
Stars of very lowmass will eventually exhaust all their fusiblehydrogen and then becomeheliumwhite dwarfs.[17] Stars of low to medium mass, such as our ownsun, will expel some of their mass as aplanetary nebula and eventually becomewhite dwarfs; more massive stars will explode in acore-collapse supernova, leaving behindneutron stars orblack holes.[18] In any case, although some of the star's matter may be returned to theinterstellar medium, adegenerate remnant will be left behind whose mass is not returned to the interstellar medium. Therefore, the supply of gas available forstar formation is steadily being exhausted.[citation needed]
TheAndromeda Galaxy is approximately 2.5 million light years away from our galaxy, theMilky Way galaxy, and they are moving towards each other at approximately 300 kilometres (186 miles) per second. Approximately five billion years from now, or 19 billion years after theBig Bang, the Milky Way and the Andromeda galaxy willcollide with one another and merge into one large galaxy based on current evidence. Up until 2012, there was no way to confirm whether the possible collision was going to happen.[19] In 2012, researchers came to the conclusion that the collision is definite after using theHubble Space Telescope between 2002 and 2010 to track the motion of Andromeda.[20] This results in the formation ofMilkdromeda (also known asMilkomeda).[citation needed]
22 billion years in the future is the earliest possible end of the Universe in theBig Rip scenario, assuming a model ofdark energy withw = −1.5.[21][22]
False vacuum decay may occur in 20 to 30 billion years if theHiggs field is metastable.[23][24][25]
Thegalaxies in theLocal Group, the cluster of galaxies which includes the Milky Way and the Andromeda Galaxy, are gravitationally bound to each other. It is expected that between 1011 (100 billion) and 1012 (1 trillion) years from now, their orbits will decay and the entire Local Group will merge into one large galaxy.[5]
Assuming thatdark energy continues to make the universe expand at an accelerating rate, in about 150 billion years all galaxies outside theLocal Supercluster will pass behind thecosmological horizon. It will then be impossible for events in the Local Supercluster to affect other galaxies. Similarly, it will be impossible for events after 150 billion years, as seen by observers in distant galaxies, to affect events in the Local Supercluster.[4] However, an observer in the Local Supercluster will continue to see distant galaxies, but events they observe will become exponentially moreredshifted as the galaxy approaches the horizon until time in the distant galaxy seems to stop. The observer in the Local Supercluster never observes events after 150 billion years in their local time, and eventually all light andbackground radiation lying outside the Local Supercluster will appear to blink out as light becomes so redshifted that its wavelength has become longer than the physical diameter of the horizon.
Technically, it will take an infinitely long time for all causal interaction between the Local Supercluster and this light to cease. However, due to the redshifting explained above, the light will not necessarily be observed for an infinite amount of time, and after 150 billion years, no new causal interaction will be observed.
Therefore, after 150 billion years, intergalactic transportation and communication beyond the Local Supercluster becomes causally impossible.
8×1011 (800 billion) years from now, the luminosities of the different galaxies, approximately similar until then to the current ones thanks to the increasing luminosity of the remaining stars as they age, will start to decrease, as the less massivered dwarf stars begin to die aswhite dwarfs.[26]
2×1012 (2 trillion) years from now, all galaxies outside theLocal Supercluster will be redshifted to such an extent that evengamma rays they emit will have wavelengths longer than the size of theobservable universe of the time. Therefore, these galaxies will no longer be detectable in any way.[4]
By 1014 (100 trillion) years from now,star formation will end,[5] leaving all stellar objects in the form ofdegenerate remnants. Ifprotons do not decay, stellar-mass objects will disappear more slowly, making this eralast longer.
By 1014 (100 trillion) years from now,star formation will end. This period, known as the "Degenerate Era", will last until the degenerate remnants finally decay.[27] The least-massive stars take the longest to exhaust their hydrogen fuel (seestellar evolution). Thus, the longest living stars in the universe are low-massred dwarfs, with a mass of about 0.08solar masses (M☉), which have a lifetime of over 1013 (10 trillion) years.[28] Coincidentally, this is comparable to the length of time over which star formation takes place.[5] Once star formation ends and the least-massive red dwarfs exhaust their fuel,nuclear fusion will cease. The low-mass red dwarfs will cool and becomeblack dwarfs.[17] The only objects remaining with more thanplanetary mass will bebrown dwarfs (with mass less than 0.08 M☉), anddegenerate remnants:white dwarfs, produced by stars with initial masses between about 0.08 and 8 solar masses, andneutron stars andblack holes, produced by stars with initial masses over 8 M☉. Most of the mass of this collection, approximately 90%, will be in the form of white dwarfs.[6] In the absence of any energy source, all of these formerly luminous bodies will cool and become faint.
The universe will become extremely dark after the last stars burn out. Even so, there can still be occasional light in the universe. One of the ways the universe can be illuminated is if twocarbon–oxygen white dwarfs with a combined mass of more than theChandrasekhar limit of about 1.4 solar masses happen to merge. The resulting object will then undergo runaway thermonuclear fusion, producing aType Ia supernova and dispelling the darkness of the Degenerate Era for a few weeks.Neutron stars could alsocollide, forming even brighter supernovae and dispelling up to 6 solar masses of degenerate gas into the interstellar medium. The resulting matter from thesesupernovae could potentially create new stars.[29][30] If the combined mass is not above the Chandrasekhar limit but is larger than the minimum mass tofuse carbon (about 0.9 M☉), acarbon star could be produced, with a lifetime of around 106 (1 million) years.[14] Also, if two helium white dwarfs with a combined mass of at least 0.3 M☉ collide, ahelium star may be produced, with a lifetime of a few hundred million years.[14] Finally, brown dwarfs could form new stars by colliding with each other to formred dwarf stars, which can survive for 1013 (10 trillion) years,[28][29] or by accreting gas at very slow rates from the remaininginterstellar medium until they have enough mass to starthydrogen burning as red dwarfs. This process, at least on white dwarfs, could induce Type Ia supernovae.[31]
Over time, theorbits of planets will decay due togravitational radiation, or planets will beejected from their local systems bygravitational perturbations caused by encounters with anotherstellar remnant.[32]
Over time, objects in agalaxy exchangekinetic energy in a process calleddynamical relaxation, making their velocity distribution approach theMaxwell–Boltzmann distribution.[33] Dynamical relaxation can proceed either by close encounters of two stars or by less violent but more frequent distant encounters.[34] In the case of a close encounter, twobrown dwarfs orstellar remnants will pass close to each other. When this happens, the trajectories of the objects involved in the close encounter change slightly, in such a way that theirkinetic energies are more nearly equal than before. After a large number of encounters, then, lighter objects tend to gain speed while the heavier objects lose it.[14]
Because of dynamical relaxation, some objects will gain just enough energy to reach galacticescape velocity and depart the galaxy, leaving behind a smaller, denser galaxy. Since encounters are more frequent in this denser galaxy, the process then accelerates. The result is that most objects (90% to 99%) are ejected from the galaxy, leaving a small fraction (maybe 1% to 10%) which fall into the centralsupermassive black hole.[5][14] It has been suggested that the matter of the fallen remnants will form anaccretion disk around it that will create aquasar, as long as enough matter is present there.[35]
In an expanding universe with decreasing density and non-zerocosmological constant, matter density would reach zero, resulting in most matter exceptblack dwarfs,neutron stars,black holes, andplanets ionizing and dissipating atthermal equilibrium.[36]
The following timeline assumes that protons do decay.
The subsequent evolution of the universe depends on the possibility and rate ofproton decay. Experimental evidence shows that if theproton is unstable, it has ahalf-life of at least 1035 years.[37] Some of theGrand Unified theories (GUTs) predict long-term proton instability between 1032 and 1038 years, with the upper bound on standard (non-supersymmetry) proton decay at 1.4×1036 years and an overall upper limit maximum for any proton decay (includingsupersymmetry models) at 6×1042 years.[38][39] Recent research showing proton lifetime (if unstable) at or exceeding 1036–1037 year range rules out simpler GUTs and most non-supersymmetry models.
Neutrons bound intonuclei are also suspected to decay with a half-life comparable to that of protons. Planets (substellar objects) would decay in a simple cascade process from heavier elements to hydrogen and finally to photons and leptons while radiating energy.[40]
If the proton does not decay at all, then stellar objects would still disappear, but more slowly. See§ Future without proton decay below.
Shorter or longer proton half-lives will accelerate or decelerate the process. This means that after 1040 years (the maximum proton half-life used by Adams & Laughlin (1997)), one-half of all baryonic matter will have been converted intogamma rayphotons andleptons through proton decay.
Given our assumed half-life of the proton,nucleons (protons and bound neutrons) will have undergone roughly 1,000 half-lives by the time the universe is 1043 years old. This means that there will be roughly 0.51,000 (approximately 10−301) as many nucleons; as there are an estimated 1080 protons currently in the universe,[41] none will remain at the end of the Degenerate Age. Effectively, all baryonic matter will have been changed intophotons andleptons. Some models predict the formation of stablepositronium atoms with diameters greater than the observable universe's current diameter (roughly 6×1034 metres)[42] in 1098 years, and that these will in turn decay to gamma radiation in 10176 years.[5][6]
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If the proton does not decay according to the theories described above, then the Degenerate Era will last longer, and will overlap or surpass the Black Hole Era. On a time scale of 1065 years solid matter is theorized to potentially rearrange itsatoms andmolecules viaquantum tunneling, and may behave as liquid and become smoothspheres due to diffusion and gravity.[13] Degenerate stellar objects can potentially still experience proton decay, for example via processes involving theAdler–Bell–Jackiw anomaly,virtual black holes, or higher-dimensionsupersymmetry possibly with a half-life of under 10220 years.[5]
2018 estimate ofStandard Model lifetime beforecollapse of a false vacuum; 95% confidence interval is 1065 to 10725 years due in part to uncertainty about the topquark mass.[43]
Although protons are stable in standard model physics, aquantum anomaly may exist on theelectroweak level, which can cause groups of baryons (protons and neutrons) to annihilate into antileptons via thesphaleron transition.[44] Suchbaryon/lepton violations have a number of 3 and can only occur in multiples or groups of three baryons, which can restrict or prohibit such events. No experimental evidence of sphalerons has yet been observed at low energy levels, though they are believed to occur regularly at high energies and temperatures.
After 1043 years, black holes will dominate the universe. They will slowly evaporate viaHawking radiation.[5] A black hole with a mass of around 1 M☉ will vanish in around 2×1064 years. As the lifetime of a black hole is proportional to the cube of its mass, more massive black holes take longer to decay. A supermassive black hole with a mass of 1011 (100 billion)M☉ will evaporate in around 2×1093 years.[45]
The largestblack holes in the universe are predicted to continue to grow. Larger black holes of up to 1014 (100 trillion)M☉ may form during the collapse of superclusters of galaxies. Even these would evaporate over a timescale of 10109[46] to 10110 years.
Hawking radiation has athermal spectrum. During most of a black hole's lifetime, the radiation has a low temperature and is mainly in the form of massless particles such asphotons and hypotheticalgravitons. As the black hole's mass decreases, its temperature increases, becoming comparable to theSun's by the time the black hole mass has decreased to 1019 kilograms. The hole then provides a temporary source of light during the general darkness of the Black Hole Era. During the last stages of its evaporation, a black hole will emit not only massless particles, but also heavier particles, such aselectrons,positrons,protons, andantiprotons.[14]
After all the black holes have evaporated (and after all the ordinary matter made of protons has disintegrated, if protons are unstable), the universe will be nearly empty.Photons,leptons,baryons,neutrinos,electrons, andpositrons will fly from place to place, hardly ever encountering each other.Gravitationally, theuniverse will be dominated bydark matter,electrons, andpositrons (notprotons).[47]
By this era, with only very diffuse matter remaining, activity in the universe will eventually tail off dramatically (compared with previous eras), with very low energy levels and very large time scales, with events taking a very long time to happen if they ever happen at all. Electrons and positrons drifting through space will encounter one another and occasionally formpositronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate. However, most electrons and positrons will remain unbound.[48] Other low-level annihilation events will also take place, albeit extremely slowly. The universe now gradually tends towards itslowest energy state.
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If protons do not decay, stellar-mass objects will still becomeblack holes, although even more slowly. The following timeline that assumesproton decay does not take place.
2018 estimate ofStandard Model lifetime beforecollapse of a false vacuum; 95% confidence interval is 1065 to 101383 years due in part to uncertainty about the topquark mass.[43][note 1]
In 101500 years,pycnonuclear fusion occurring viaquantum tunnelling should make the lightnuclei in stellar-mass objects fuse intoiron-56 nuclei (seeisotopes of iron).Fission andalpha particle emission should make heavy nuclei also decay to iron, leaving stellar-mass objects as cold spheres of iron, callediron stars.[13] Before this happens, however, in someblack dwarfs the process is expected to lower theirChandrasekhar limit resulting in asupernova in 101100 years. Non-degenerate silicon has been calculated to tunnel to iron in approximately 1032000 years.[49]
Quantum tunneling should also turn large objects intoblack holes, which (on these timescales) will instantaneously evaporate into subatomic particles. Depending on the assumptions made, the time this takes to happen can be calculated as from 101026 years to 101076 years. Quantum tunneling may also make iron stars collapse intoneutron stars in around 101076 years.[13]
With black holes having evaporated, nearly all baryonic matter will have decayed into subatomic particles (electrons, neutrons, protons, and quarks). The universe is now an almost pure vacuum (possibly accompanied with the presence of afalse vacuum). The expansion of the universe slowly causes itself to cool down toabsolute zero. The universe now reaches an even lower energy state than the earlier one mentioned.[50][51]
Whatever event happens beyond this era is highly speculative. It is possible that aBig Rip event may occur far off into the future.[52][53] This singularity would take place at a finite scale factor.
If the currentvacuum state is afalse vacuum, the vacuum may decay into an even lower-energy state.[54]
Presumably, extreme low-energy states imply that localized quantum events become major macroscopic phenomena rather than negligible microscopic events because even the smallest perturbations make the biggest difference in this era, so there is no telling what will or might happen to space or time. It is perceived that the laws of "macro-physics" will break down, and the laws of quantum physics will prevail.[8]
The universe could possibly avoid eternal heat death through randomquantum tunneling andquantum fluctuations, given the non-zero probability of producing a new Big Bang creating a new universe in roughly 10101056 years.[55]
Over an infinite amount of time, there could also be a spontaneousentropy decrease, by aPoincaré recurrence or throughthermal fluctuations (see alsofluctuation theorem).[56][57][58]
The possibilities above are based on a simple form ofdark energy. However, the physics of dark energy are still a very speculative area of research, and the actual form of dark energy could be much more complex.
If protons decay:
If protons don't decay:
Since we have assumed a maximum scale of gravitational binding – for instance, superclusters of galaxies – black hole formation eventually comes to an end in our model, with masses of up to 1014M☉ ... the timescale for black holes to radiate away all their energy ranges ... to 10109 years for black holes of up to 1014M☉.
| Background | |
|---|---|
| History of cosmological theories | |
| Past universe | |
| Present universe | |
| Future universe | |
| Components | |
| Structure formation | |
| Experiments | |
