Future of an expanding universe

Recent 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.

The future of an expanding universe is bleak. If a cosmological constant accelerates the expansion of the universe, clusters of galaxies will rapidly be driven away from each other, leaving observers in different clusters unable to either reach each other or sense each other's presence in any way. Stars are expected to form normally for at least 10¹⁴ years, but eventually the supply of gas needed for star formation will be exhausted. Once the last star has exhausted its fuel, stars will then cease to shine.^{, §IID, IIE.} The stellar remnants left behind are expected to disappear as their protons decay, leaving behind only black holes which themselves eventually disappear as they emit Hawking radiation.^, §IV. 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.^, §VID.

Cosmology

Indefinite 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.

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. 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. 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.

Future history

In the 1970s, the future of an expanding universe was studied by the astrophysicist Jamal Islam and the physicist Freeman Dyson. More recently, 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.^{, 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 as the Big Crunch, the recontraction of the universe into a hot, dense state similar to that after the Big Bang, will supervene.^{, pp. 190–192;, §VA}

Timeline

For the past, including the Primordial Era, see Timeline of the Big Bang.

The Stelliferous Era, from 1.55 x 10⁸ (155 million) years to 10¹⁴ (100.0137 trillion) years after the Big Bang

See also: Graphical timeline of the Stelliferous Era

The universe is currently 13.7×10⁹ (13.7 billion) years old. This time is in the Stelliferous Era. About 155 million years after the Big Bang, the first star formed. Since then, stars have formed by the collapse of small, dense core regions in large, cold molecular clouds of hydrogen gas. At first, this produces a protostar, which is hot and bright because of energy generated by gravitational contraction. After the protostar contracts for a while, its center will become hot enough to fuse hydrogen and its lifetime as a star will properly begin.^{, pp. 35–39.}

Stars whose mass is very low will eventually exhaust all their fusible hydrogen and then become helium white dwarfs. Stars of low to medium mass will expel some of their mass as a planetary nebula and eventually become a white dwarf; more massive stars will explode in a core-collapse supernova, leaving behind neutron stars or black holes. 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.

The Milky Way Galaxy and the Andromeda Galaxy merge into one galaxy: 3 billion years from now

Main article: Andromeda-Milky Way collision

The Andromeda Galaxy is currently approximately 2.5 million light years away from our galaxy, the Milky Way Galaxy, and the galaxies are moving towards each other at approximately 120 kilometers per second. Approximately three billion years from now, or 17 billion years after the Big Bang, the Milky Way and the Andromeda Galaxy may collide with one another and merge into one large galaxy. Because it is not known precisely how fast the Andromeda Galaxy is moving transverse to us, it is not certain that the collision will happen.

Coalescence of Local Group: 100 billion to 1 trillion years from now

The galaxies in the Local 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.^, §IIIA.

Galaxies outside the Local Supercluster are no longer detectable: 3 trillion years from now

Assuming that dark energy continues to make the universe expand at an accelerating rate, 3×10¹² (3 trillion) years from now, all galaxies outside the Local Supercluster will be red-shifted to such an extent that even gamma rays they emit will have wavelengths longer than the size of the observable universe of the time. Therefore, these galaxies will no longer be detectable in any way.

The Degenerate Era, from 10¹⁴ (100 trillion) to 10⁴⁰ years from now

By 10¹⁴ (100 trillion) years from now, star formation will end, leaving all stellar objects in the form of degenerate remnants. This period, known as the Degenerate Era, will last until the degenerate remnants finally decay.^{, § III–IV.}

Star formation ceases: 10¹⁴ (100 trillion) years

It is estimated that in at least 10¹⁴ (100 trillion) years, star formation will end.^, §IID. The least massive stars take the longest to exhaust their hydrogen fuel (see stellar evolution). Thus, the longest living stars in the universe are low-mass red dwarfs, with a mass of about 0.08 solar masses, which have a lifetime of 10¹⁴ (100 trillion) years. ^§IIA. Coincidentally, this is comparable to the length of time over which star formation takes place. ^§IID. 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 become white dwarfs. The only objects remaining with more than planetary mass will be brown dwarfs, with mass less than 0.08 solar masses, and degenerate remnants: white dwarfs, produced by stars with initial masses between about 0.08 and 8 solar masses, and neutron stars and black holes, produced by stars with initial masses over 8 solar masses. Most of the mass of this collection, approximately 90%, will be in the form of white dwarfs. ^§IIE. 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 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. ^§IIIC; If the combined mass is not above the Chandrasekhar limit but is larger than the minimum mass to fuse carbon (about 0.9 solar masses), a carbon star could be produced, with a lifetime of around 10⁶ (1 million) years.^, p. 91 Also, if two helium white dwarfs with a combined mass of at least 0.3 solar masses collide, a helium star may be produced, with a lifetime of a few hundred million years.^, p. 91 Finally, if brown dwarfs collide with each other, a red dwarf star may be produced which can survive for over 10¹⁴ (100 trillion) years. ^§IIIC.

The last stars in the Universe burn out and die: 200 Trillion (2x10^14) Years from now

The last stars in the Universe, the smallest Red dwarfs will burn out and die in 200 trillion years, about 100 trillion years after stellar formation ceases. The Universe will become extremely dark in 200 trillion years when the last star burns out. Even though all of the stars will burn out in 200 trillion years there can still be occasional light in the Universe. One of the ways that light can exist in the Universe beyond 200 trillion years in the future is if two white dwarfs with a combined mass of more than about 1.4 solar masses happen to merge, the resulting object undergoes runaway thermonuclear fusion. The result is a Type Ia supernova. Very, very rarely, the darkness of the Degenerate Age is dispelled for a few weeks while a supernova explodes. Another way that light can exist in the Universe beyond 200 trillion years is if brown dwarfs collide with each other. Occasionally, brown dwarfs collide with each other and form a new red dwarf star which can survive for upto 100 trillion years. When brown dwarfs collide with each other to form a new red dwarf star they can supply light for upto 100 trillion years whereas two white dwarfs colliding with each other supply a burst of light for only a few weeks. An advanced civilization 200 trillion years in the future would probably be able to manipulate the obits of stars and planets. This advanced civilization would probably be able to push brown dwarfs into each other for another 10^40 years. Thus the Universe could be filled with light from sextillions of stars for another 10^40 years until protons decay

Planets fall or are flung from orbits by a close encounter with another star: 10¹⁵ years from now

Over time, the orbits of planets will decay due to gravitational radiation, or planets will be ejected from their local systems by gravitational perturbations caused by encounters with another stellar remnant.^{, §IIIF, Table I.}

Dynamical relaxation of the galaxy: 10¹⁸ years from now

This proceeds mainly through distant stellar encouters. The same scattering effect happens to the white dwarfs and black dwarfs and their remnants within galaxies, leaving mostly scattered stellar debris and supermassive black holes.

White dwarfs and Black dwarfs fall or are flung from orbits: 10¹⁹years from now

The combined effect of dynamical relaxation and close encounters is to produce a collapse of the central regions of the galaxy into a black hole, together with an evaporation of stars from the outer regions. The evaporated stars achieve escape velocity and become detached from the galaxy after a time of the order of 10^19 yr. We do not know what fraction of the mass of the galaxy ultimately collapses and what fraction escapes. The fraction escaping probably lies between 90% and 99%.

Over time, brown dwarfs and stellar remnants will pass close to each other. When this happens, the trajectories of the objects involved in the close encounter change silghtly. After a large number of encounters, lighter objects tend to gain kinetic energy while the heavier objects lose it. Objects which gain enough energy will 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 are ejected from the galaxy, leaving a small fraction (approximately 1%) which fall into the central supermassive black hole.

The supermassive black holes are all that remains of galaxies once all protons decay, but even these giants are not immortal.

White dwarfs and Black dwarfs orbits decay by gravitational radiation: 10²⁰years from now

Any gravitationally bound system of objects orbiting around each other will decay by this mechanism of radiation drag in 10²⁰ years. This process is expected to take about 10²⁰ years.^{, §IIIA;, pp. 85–87}

The Milky Way Galaxy and similar galaxies decay: 10²⁴ years from now

In 10²⁴ years the Milky Way Galaxy and similar galaxies will have decayed. (27) This is ashows that dynamical relaxation dominates gravitational radiation in the evolution of galaxies.

Protons and neutrons (nucleons) start to decay: >10³² years

The subsequent evolution of the universe depends on the existence and rate of proton decay. Experimental evidence shows that if the proton is unstable, it has a half-life of at least 10³² years. If a Grand Unified Theory is correct, then there are theoretical reasons to believe that the half-life of the proton is under 10⁴¹ years.^, §IVA. If not, the proton is still expected to decay, for example via processes involving virtual black holes, with a half-life of under 10²⁰⁰ years.^, §IVF

In the event that the proton did not decay at all, stellar-mass objects would still disappear, but more slowly. In 10¹⁵⁰⁰ years, cold fusion occurring via quantum tunnelling would make the nuclei in ordinary matter fuse into iron-56 nuclei (see isotopes of iron.) Quantum tunnelling should then turn large objects into black holes. Depending on the assumptions made, the time this takes to happen can be calculated as from $10^{10^{26}}$ years to $10^{10^{76}}$ years. (To calculate the value of such numbers, see tetration.)

The rest of this timeline assumes that the proton half-life is approximately 10³⁷ years.^, §IVA. Shorter or longer proton half-lives will accelerate or retard the process.

Half of all protons and neutrons (nucleons) decay: 10³⁷ years

Given the above assumption on the half-life of the proton, one-half of all baryonic matter has now been converted into gamma radiation and leptons through proton decay.

All protons and neutrons (nucleons) decay: 10⁴⁰ years

Given our assumption on the half-life of the proton and neutrons (nucleons)^, §IVA will have undergone roughly 1,000 half-lives by the time the universe is 10⁴⁰ years old. To put this into perspective, there are an estimated 10⁸⁰ protons currently in the universe. This means that the number of nucleons will be slashed in half 1,000 times by the time the universe is 10⁴⁰ years old. Hence, there will be roughly ½^1,000 (approximately 10^–301) as many nucleons remaining as there are today; that is, zero nucleons remaining in the universe at the end of the Degenerate Age. Effectively, all baryonic matter has been changed into photons and leptons.

The Black Hole Era, from 10⁴⁰ years to 1.7x 10¹⁰⁶ years from now

Black hole estimated lifetimes
Mass	Lifetime
Mass of the Moon	1.1×1044 years
Mass of the Earth	6×1049 years
1 M☉	2.2×1066 years
10 M☉	2.2×1069 years
100 M☉	2.2×1072 years
1,000 M☉	2.2×1075 years
10,000 M☉	2.2×1078 years
100,000 M☉	2.2×1081 years
106 (1 million) M☉	2.2×1084 years
107 (10 million) M☉	2.2×1087 years
108 (100 million) M☉	2.2×1090 years
109 (1 billion) M☉	2.2×1093 years
1010 (10 billion) M☉	2.2×1096 years
1011 (100 billion) M☉	2.2×1099 years
1012 (1 trillion) M☉	2.2×10102 years
1013 (10 trillion) M☉	2.2×10105 years
2×1013 (20 trillion) M☉	1.7×10106 years

Matter is liquid at zero temperature: 10⁶⁵ years from now

Even the most rigid materials cannot preserve their shapes or their chemical structures for very long times. On a time scale of 10^65 years, every piece of rock behaves like a liquid, flowing into a spherical shape under the influence of gravity. Its atoms and molecules will be ceaselessly diffusing around like the molecules in a drop of water.

Evaporation of Black Holes: 2.2 x 10⁶⁶ to 1.7 x 10¹⁰⁶ years from now

Black holes now dominate the Universe. They will slowly evaporate via Hawking radiation. A black hole with a mass of around 1 solar mass will vanish in around 2.2×10^66 years. As the lifetime of a black hole is proportional to the cube of its mass, larger black holes take longer to decay. The largest supermassive black holes, with a mass around 2×10^13 (20 trillion) solar masses, will vanish in around 1.7×10^106 years. Over most of a black hole's lifetime, the radiation emitted is predicted to be mostly in the form of neutrinos, with approximately 17% of the radiated energy in photons and 2% in gravitons.[16]

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 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.^{, pp. 148–150.}

Black holes will continue to form for another 10^40 years. Black holes will be able to form as long as there is baryonic matter in the Universe. As time goes on, Black holes will continue to get larger as they suck up more matter. Even though all of the protons in the Universe have decayed in the Black Hole Era, there can still be light in the Universe when a black hole ends its life. This is because, as the mass of a black hole decreases, the amount of radiation it emits increases. During the last few seconds of its life, an evaporating black hole emits a burst of light, X-rays, and gamma rays. This means that during the Black Hole Era the Universe will occasionally be filled with some light when a black hole ends its life. So the Universe will contain light for another 1.7x10^106 years. After 1.7 x 10^106 years all of the light in the Universe will be permanently gone as the last supermassive black hole ends its life. The Universe will then become permanently dark and devoid of all matter. The Universe will remain dark and devoid of all matter forever, because in 1.7x 10^106 years, there will be nothing left to create matter or light in the Universe. At this moment in time the Universe will enter its final Era which is called the Dark Era. Once the Universe enters the Dark Era it will remain in that Era forever.

The Dark Era from 1.7x 10¹⁰⁶ years to 10^(10^10^10^10^1.1) Years From Now

The lowly photon is now king of the Universe as the last of the supermassive black holes evaporate.

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. It will be cold, and dark, and there is no known process which will ever change things.

By this era, with only very diffuse matter remaining, activity in the universe will have tailed off dramatically, 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.^, §VF3. Other low-level annihilation events will also take place, albeit very slowly.

The Universe now reaches an extremely low-energy state. What happens after this is speculative. It's 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.^, §VE. Finally, the Universe may settle into this state forever, achieving true heat death.

All Matter Decays into Iron:10¹⁵⁰⁰years from now

In matter at zero temperature, nuclear as well as chemical reactions will continue to occur. Elements heavier than iron will decay to iron by varoius processes such as fission and alpha emission. Elements lighter than iron will combine by nuclear fusion reactions, building gradually up to iron. Consider for example the fusion reaction in which two nuclei of atomic weight 1/2 A, charge 1/2 Z combine to form a nucleus (A,Z). The Coulomb repulsion of the two nuclei is effectively screened by electrons until they come within a distance. On the time scale of 10¹⁵⁰⁰ years, ordinary matter is radioactive and is constantly generating nuclear energy.

Collapse of iron star to neutron star: 10^(10^76) years from now

After 10¹⁵⁰⁰ years has elapsed, most of the matter in the universe is in the form of ordinary low-mass stars that have settled down into white dwarf configurations and become cold spheres of pure iron.

But an iron star is still not in its state of lowest energy. It could release a huge amount of energy if it could collapse into a neutron star configuration. To collapse, it has only to penetrate a barrier of finite height and thickness.

It is an interesting question, whether there is an unsymmetrical mode of collapse passing over a lower saddle point than the symmetric mode.

We do not know whether every collapse of an iron star into a neutron star will produce a supernova explosion. At the very least, it will produce a huge outburst of energy in the form of neutrinos and a modest burst of energy in the form of x rays and visible light.

The universe will still be producing occasional fireworks after times as long as 10^(10^76).

Collapse of ordinary matter to black holes: Alternative Theory

The long lifetime of iron stars is only correct if they do not collapse with a shorter lifetime into black holes. For collapse of any piece of bulk matter into a black hole, the same formulae apply as for collapse into a neutron star. If small black holes are possible, a small part of a star can collapse by itself into a black hole.

Once a small black hole has been formed, it will in a short time swallow the rest of the star. This will happen in about 10^(10^26) to 10^(10^32 years)

New Stelliferous Era: from 10^(10^10^10^10^1.1)to 10^(10^10^10^10^1.1) + 100 Trillion Years

In 10^(10^10^10^10^1.1) years the Universe will return back to the way it currently is now. This is according to the Poincare recurrence time of the Universe. Thus a New Stelliferous Era will begin and last for another 100 trillion years. After another 100 trillion years has passed a New Degenerate Era will begin. Thus the Universe will go around in a never ending cycle for all time.

10^(10^10^10^2.08) years; scale of an estimate Poincare recurrence time for a black hole containing the mass within the presently visible region of our universe

10(10^10^10^10^1.1) years—scale of an estimated Poincaré recurrence time for the quantum state of a hypothetical box containing a black hole with the estimated mass of the entire universe, observable or not, assuming a certain inflationary model with an inflaton whose mass is 10−6 Planck masses.[8] This time assumes a statistical model subject to Poincaré recurrence. A much simplified way of thinking about this time is in a model where our universe's history repeats itself arbitrarily many times due to properties of statistical mechanics, this is the time scale when it will first be somewhat similar (for a reasonable choice of "similar") to its current state again.

Alternative futures of the universe

10⁽¹⁰²⁶⁾ years—low estimate for the time until all matter collapses into black holes, assuming no proton decay.

10⁽¹⁰⁷⁶⁾ years—high estimate for the time until all matter collapses into neutron stars or black holes, again assuming no proton decay.

10^(10^10^10^2.08) years; scale of an estimate Poincare recurrence time for a black hole containing the mass within the presently visible region of our universe

10^{(10^10^10^10^1.1)} years—scale of an estimated Poincaré recurrence time for the quantum state of a hypothetical box containing a black hole with the estimated mass of the entire universe, observable or not, assuming a certain inflationary model with an inflaton whose mass is 10−6 Planck masses.[8] This time assumes a statistical model subject to Poincaré recurrence. A much simplified way of thinking about this time is in a model where our universe's history repeats itself arbitrarily many times due to properties of statistical mechanics, this is the time scale when it will first be somewhat similar (for a reasonable choice of "similar") to its current state again.

Graphical timeline

See also: Graphical timeline from Big Bang to Heat Death, Graphical timeline of our universe, and Timeline of the Big Bang

See also

• Heat death of the universe
• 1 E19 s and more
• Second law of thermodynamics
• Big Rip
• Big Crunch
• Big Bounce
• Big Bang
• Cyclic model
• Dyson's eternal intelligence
• Final anthropic principle
• Ultimate fate of the Universe
• Graphical timeline of the Stelliferous Era
• Graphical timeline from Big Bang to Heat Death. This timeline uses the loglog scale for comparison with the graphical timeline included in this article.
• Graphical timeline of our universe. This timeline uses the more intuitive linear time, for comparison with this article.
• Timeline of the Big Bang
• Graphical timeline of the Big Bang
• The Last Question, a short story by Isaac Asimov which considers the inevitable oncome of heat death in the universe and how it may be reversed.