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Observations suggest that the expansion of the universe will continue forever. If so, the Universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze.[1]
If dark energy—represented by the cosmological constant, a constant energy density filling space homogeneously,[2] or scalar fields, such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space—accelerates the expansion of the Universe, the space between clusters of galaxies will grow at an increasing rate. Redshift will stretch ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies.[3] Stars are expected to form normally for 1012 to 1014 (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. And as existing stars run out of fuel and cease to shine, the Universe will slowly and inexorably grow darker, one star at a time.[4] §IID, [5] According to theories that predict proton decay, the stellar remnants left behind will disappear, leaving behind only black holes, which themselves eventually disappear as they emit Hawking radiation.[6] Ultimately, if the Universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe.[7]
Observations of the cosmic background radiation by the Wilkinson Microwave Anisotropy Probe suggest that the Universe is spatially flat and has a significant amount of dark energy.[9] In this case, the Universe should continue to expand at an accelerating rate. The acceleration of the Universe's expansion has also been confirmed by observations of distant supernovae.[8] If, as in the concordance model of physical cosmology (Lambda-cold dark matter or ΛCDM), the dark energy is in the form of a cosmological constant, the expansion will eventually become exponential, with the size of the Universe doubling at a constant rate.
If the theory of inflation is true, 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]
This future history and the timeline below assume the continued expansion of the Universe. If the Universe begins to recontract, subsequent events in the timeline may not occur because the Big Crunch, the recontraction of the Universe into a hot, dense state similar to that after the Big Bang, will supervene.[12], pp. 190–192;[13]
Stars of very low mass will eventually exhaust all their fusible hydrogen and then become helium white dwarfs.[15] Stars of low to medium mass will expel some of their mass as a planetary nebula and eventually become white dwarfs; more massive stars will explode in a core-collapse supernova, leaving behind neutron stars or black holes.[16] In any case, although some of the star's matter may be returned to the interstellar medium, a degenerate remnant will be left behind whose mass is not returned to the interstellar medium. Therefore, the supply of gas available for star formation is steadily being exhausted.
Assuming that dark energy continues to make the Universe expand at an accelerating rate, in about 150 billion years all galaxies outside the local group will pass behind the cosmological horizon. It will then be impossible for events in the local group 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 group.[3] However, an observer in the local group will continue to see distant galaxies, but events they observe will become exponentially more time dilated (and red shifted[3]) as the galaxy approaches the horizon until time in the distant galaxy seems to stop. The observer in the local group never actually sees the distant galaxy pass beyond the horizon and never observes events after 150 billion years in their local time. Therefore, after 150 billion years intergalactic transportation and communication becomes causally impossible.
The universe will become extremely dark after the last star burns out. Even so, there can still be occasional light in the Universe. One of the ways the Universe can be illuminated is if two carbon–oxygen white dwarfs with a combined mass of more than the Chandrasekhar limit of about 1.4 solar masses happen to merge. The resulting object will then undergo runaway thermonuclear fusion, producing a Type Ia supernova and dispelling the darkness of the Degenerate Era for a few weeks.[21][22] If the combined mass is not above the Chandrasekhar limit but is larger than the minimum mass to fuse carbon (about 0.9 M☉), a carbon star could be produced, with a lifetime of around 106 (1 million) years.[12], p. 91 Also, if two helium white dwarfs with a combined mass of at least 0.3 M☉ collide, a helium star may be produced, with a lifetime of a few hundred million years.[12], p. 91 Finally, if brown dwarfs collide with each other, a red dwarf star may be produced which can survive for 1013 (10 trillion) years.[20][21]
Because of dynamical relaxation, some objects will gain enough energy to reach galactic escape velocity and depart the galaxy, leaving behind a smaller, denser galaxy. Since encounters are more frequent in the denser galaxy, the process then accelerates. The end 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 central supermassive black hole.[4], §IIIAD;[12], pp. 85–87
If dark energy—represented by the cosmological constant, a constant energy density filling space homogeneously,[2] or scalar fields, such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space—accelerates the expansion of the Universe, the space between clusters of galaxies will grow at an increasing rate. Redshift will stretch ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies.[3] Stars are expected to form normally for 1012 to 1014 (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. And as existing stars run out of fuel and cease to shine, the Universe will slowly and inexorably grow darker, one star at a time.[4] §IID, [5] According to theories that predict proton decay, the stellar remnants left behind will disappear, leaving behind only black holes, which themselves eventually disappear as they emit Hawking radiation.[6] Ultimately, if the Universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe.[7]
Cosmology
Infinite expansion does not determine the spatial curvature of the Universe. It can be open (with negative spatial curvature), flat, or closed (positive spatial curvature), although if it is closed, sufficient dark energy must be present to counteract the gravitational attraction of matter and other forces tending to contract the Universe. Open and flat universes will expand forever even in the absence of dark energy.[8]Observations of the cosmic background radiation by the Wilkinson Microwave Anisotropy Probe suggest that the Universe is spatially flat and has a significant amount of dark energy.[9] In this case, the Universe should continue to expand at an accelerating rate. The acceleration of the Universe's expansion has also been confirmed by observations of distant supernovae.[8] If, as in the concordance model of physical cosmology (Lambda-cold dark matter or ΛCDM), the dark energy is in the form of a cosmological constant, the expansion will eventually become exponential, with the size of the Universe doubling at a constant rate.
If the theory of inflation is true, 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]
Future history
In the 1970s, the future of an expanding universe was studied by the astrophysicist Jamal Islam[10] and the physicist Freeman Dyson.[11] Then, in their 1999 book The Five Ages of the Universe, the astrophysicists Fred Adams and Gregory Laughlin have divided the past and future history of an expanding universe into five eras. The first, the Primordial Era, is the time in the past just after the Big Bang when stars had not yet formed. The second, the Stelliferous Era, includes the present day and all of the stars and galaxies we see. It is the time during which stars form from collapsing clouds of gas. In the subsequent Degenerate Era, the stars will have burnt out, leaving all stellar-mass objects as stellar remnants—white dwarfs, neutron stars, and black holes. In the Black Hole Era, white dwarfs, neutron stars, and other smaller astronomical objects have been destroyed by proton decay, leaving only black holes. Finally, in the Dark Era, even black holes have disappeared, leaving only a dilute gas of photons and leptons.[12], pp. xxiv–xxviii.This future history and the timeline below assume the continued expansion of the Universe. If the Universe begins to recontract, subsequent events in the timeline may not occur because the Big Crunch, the recontraction of the Universe into a hot, dense state similar to that after the Big Bang, will supervene.[12], pp. 190–192;[13]
Timeline
Stelliferous Era
- From 106 (1 million) years to 1014 (100 trillion) years after the Big Bang
Stars of very low mass will eventually exhaust all their fusible hydrogen and then become helium white dwarfs.[15] Stars of low to medium mass will expel some of their mass as a planetary nebula and eventually become white dwarfs; more massive stars will explode in a core-collapse supernova, leaving behind neutron stars or black holes.[16] In any case, although some of the star's matter may be returned to the interstellar medium, a degenerate remnant will be left behind whose mass is not returned to the interstellar medium. Therefore, the supply of gas available for star formation is steadily being exhausted.
Milky Way Galaxy and the Andromeda Galaxy merge into one
- 5 billion years from now (18.7 billion years after the Big Bang)
Coalescence of Local Group and galaxies outside the Local Group are no longer accessible
- 1011 (100 billion) to 1012 (1 trillion) years
Assuming that dark energy continues to make the Universe expand at an accelerating rate, in about 150 billion years all galaxies outside the local group will pass behind the cosmological horizon. It will then be impossible for events in the local group 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 group.[3] However, an observer in the local group will continue to see distant galaxies, but events they observe will become exponentially more time dilated (and red shifted[3]) as the galaxy approaches the horizon until time in the distant galaxy seems to stop. The observer in the local group never actually sees the distant galaxy pass beyond the horizon and never observes events after 150 billion years in their local time. Therefore, after 150 billion years intergalactic transportation and communication becomes causally impossible.
Galaxies outside the Local Supercluster are no longer detectable
- 2×1012 (2 trillion) years
Degenerate Era
- From 1014 (100 trillion) to 1040 years
Star formation ceases
- 1014 (100 trillion) years
The universe will become extremely dark after the last star burns out. Even so, there can still be occasional light in the Universe. One of the ways the Universe can be illuminated is if two carbon–oxygen white dwarfs with a combined mass of more than the Chandrasekhar limit of about 1.4 solar masses happen to merge. The resulting object will then undergo runaway thermonuclear fusion, producing a Type Ia supernova and dispelling the darkness of the Degenerate Era for a few weeks.[21][22] If the combined mass is not above the Chandrasekhar limit but is larger than the minimum mass to fuse carbon (about 0.9 M☉), a carbon star could be produced, with a lifetime of around 106 (1 million) years.[12], p. 91 Also, if two helium white dwarfs with a combined mass of at least 0.3 M☉ collide, a helium star may be produced, with a lifetime of a few hundred million years.[12], p. 91 Finally, if brown dwarfs collide with each other, a red dwarf star may be produced which can survive for 1013 (10 trillion) years.[20][21]
Planets fall or are flung from orbits by a close encounter with another star
- 1015 (1 quadrillion) years
Stellar remnants escape galaxies or fall into black holes
- 1019 to 1020 (10 to 100 quintillion) years
Because of dynamical relaxation, some objects will gain enough energy to reach galactic escape velocity and depart the galaxy, leaving behind a smaller, denser galaxy. Since encounters are more frequent in the denser galaxy, the process then accelerates. The end 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 central supermassive black hole.[4], §IIIAD;[12], pp. 85–87
The supermassive black holes are all that remains of galaxies once all protons decay, but even these giants are not immortal.
Nucleons start to decay
- >1034 years
Experimental evidence shows that if the proton is unstable, it has a half-life of at least 1034 years.[26] If any of the Grand Unified theories are correct, then there are theoretical reasons to believe that the half-life of the proton is under 1041 years.[27] Neutrons bound into nuclei are also expected to decay with a half-life comparable to the proton's.[27]
In the event that the proton does not decay at all, stellar-mass objects would still disappear, but more slowly. See Future without proton decay below.
The rest of this timeline assumes that the proton half-life is approximately 1037 years.[27] Shorter or longer proton half-lives will accelerate or decelerate the process. This means that after 1037 years, one-half of all baryonic matter will have been converted into gamma ray photons and leptons through proton decay.
Hawking radiation has a thermal 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 as photons and hypothetical gravitons. As the black hole's mass decreases, its temperature increases, becoming comparable to the Sun'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 as electrons, positrons, protons and antiprotons.[12], pp. 148–150.
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, neutrinos, electrons, and positrons will fly from place to place, hardly ever encountering each other. Gravitationally, the Universe will be dominated by dark matter, electrons, and positrons (not protons).[30]
By this era, with only very diffuse matter remaining, activity in the Universe will have tailed off dramatically (compared with previous eras), with very low energy levels and very large time scales. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate.[31] Other low-level annihilation events will also take place, albeit very slowly. The Universe now reaches an extremely low-energy state.
Presumably, extreme low-energy states imply that localized quantum events become major macroscopic phenomena rather than negligible microscopic events because the smallest perturbations make the biggest difference in this era, so there is no telling what may 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.[7]
The universe could possibly avoid eternal heat death through quantum fluctuations, which could produce a new Big Bang in roughly
years.[33]
Over an infinite time there could be a spontaneous entropy decrease, by a Poincaré recurrence or through thermal fluctuations (see also fluctuation theorem).[34][35][36][37]
years to 
years. Quantum tunnelling may also make iron stars collapse into neutron stars in around years.[11]
In the event that the proton does not decay at all, stellar-mass objects would still disappear, but more slowly. See Future without proton decay below.
The rest of this timeline assumes that the proton half-life is approximately 1037 years.[27] Shorter or longer proton half-lives will accelerate or decelerate the process. This means that after 1037 years, one-half of all baryonic matter will have been converted into gamma ray photons and leptons through proton decay.
All nucleons decay
- 1040 years
Black Hole Era
- 1040 years to 10100 years
Hawking radiation has a thermal 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 as photons and hypothetical gravitons. As the black hole's mass decreases, its temperature increases, becoming comparable to the Sun'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 as electrons, positrons, protons and antiprotons.[12], pp. 148–150.
If protons do not decay as described above
In the event the proton does not decay as described above, the Degenerate Era will last longer, and will overlap the Black Hole Era. In a timescale of approximately 1065 years, apparently rigid objects such as rocks will be able to rearrange their atoms and molecules via quantum tunnelling, behaving as a liquid does, but more slowly.[11] However, the proton is still expected to decay, for example via processes involving virtual black holes, or other higher-order processes, with a half-life of under 10200 years.[4], §IVF For example, under the Standard Model, groups of 2 or more nucleons are theoretically unstable because chiral anomaly allows processes that change baryon number by a multiple of 3.Dark Era and Photon Age
- From 10100 years
The lonely photon is now king of the universe as the last of the supermassive black holes evaporates.
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, neutrinos, electrons, and positrons will fly from place to place, hardly ever encountering each other. Gravitationally, the Universe will be dominated by dark matter, electrons, and positrons (not protons).[30]
By this era, with only very diffuse matter remaining, activity in the Universe will have tailed off dramatically (compared with previous eras), with very low energy levels and very large time scales. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate.[31] Other low-level annihilation events will also take place, albeit very slowly. The Universe now reaches an extremely low-energy state.
Beyond
What happens after this is speculative. It is possible that a Big Rip event may occur far off into the future. Also, the Universe may enter a second inflationary epoch, or, assuming that the current vacuum state is a false vacuum, the vacuum may decay into a lower-energy state.[32]Presumably, extreme low-energy states imply that localized quantum events become major macroscopic phenomena rather than negligible microscopic events because the smallest perturbations make the biggest difference in this era, so there is no telling what may 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.[7]
The universe could possibly avoid eternal heat death through quantum fluctuations, which could produce a new Big Bang in roughly
years.[33]Over an infinite time there could be a spontaneous entropy decrease, by a Poincaré recurrence or through thermal fluctuations (see also fluctuation theorem).[34][35][36][37]
Future without proton decay
If the proton does not decay, stellar-mass objects will still become black holes, but more slowly. The following timeline assumes that proton decay does not take place.Matter decays into iron
- 101500 years from now
Collapse of iron star to black hole
- to years from now
years to 
years. Quantum tunnelling may also make iron stars collapse into neutron stars in around years.[11]Graphical timeline
