Future of an expanding universe

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Current observations suggest that the expansion of the universe 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 once popularly called "Heat Death" is now known as the "Big Chill" or "Big Freeze".[1][2]

If

stellar remnants left behind will disappear, leaving behind only black holes, which themselves eventually disappear as they emit Hawking radiation.[7] Ultimately, if the universe reaches thermodynamic equilibrium, 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.[8]

Cosmology

Infinite expansion does not determine the overall 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 forces or else the universe will end in a Big Crunch.[9]

Observations of the

supernovae.[9] If, as in the concordance model of physical cosmology (Lambda-cold dark matter or ΛCDM), 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

leptons.[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 the Big Crunch, the collapse of the universe into a hot, dense state similar to that after the Big Bang, will supervene.[14][15]

Timeline

For the past, including the Primordial Era, see Chronology of the universe.
See also: Timeline of the far future

The Stelliferous Era

See also: Graphical timeline of the Stelliferous Era
From the present to about 1014 (100 trillion) years after the Big Bang
The image is from the European Space Agency. It is listed as the LH 95 star forming region of the Large Magellanic Cloud. The image was taken using the Hubble Space Telescope.
An image of many stars.

The

gravitational contraction. After the protostar contracts for a while, its core could become hot enough to fuse 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 low

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

Main article: Andromeda–Milky Way collision
4–8 billion years from now (17.8–21.8 billion years after the Big Bang)
This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth's night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull.
An artistic illustration of what it would look like from Earth during the Milky way-Andromeda galaxy collision event.

The Andromeda Galaxy is approximately 2.5 million light years away from our galaxy, the Milky Way galaxy, and they are moving towards each other at approximately 300 kilometers (186 miles) per second. Approximately five billion years from now, or 19 billion years after the Big Bang, the Milky Way and the Andromeda galaxy will collide 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 or not.[19] In 2012, researchers came to the conclusion that the collision is definite after using the Hubble Space Telescope between 2002 and 2010 to track the motion of Andromeda.[20] This results in the formation of Milkdromeda (also known as Milkomeda).

22 billion years in the future is the earliest possible end of the Universe in the Big Rip scenario, assuming a model of dark energy with w = −1.5.[21][22]

False vacuum decay may occur in 20 to 30 billion years if the Higgs field is metastable.[23][24][25]

Coalescence of Local Group and galaxies outside the Local Supercluster are no longer accessible

1011 (100 billion) to 1012 (1 trillion) years

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.[5]

Assuming that

Local Supercluster will pass behind the cosmological 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 more redshifted 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 and background 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.

Luminosities of galaxies begin to diminish

8×1011 (800 billion) years

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 massive red dwarf stars begin to die as white dwarfs.[26]

Local Group and nearest galaxies. The photos of galaxies are not to scale.
A image of the local group of galaxies.

Galaxies outside the Local Supercluster are no longer detectable

2×1012 (2 trillion) years

2×1012 (2 trillion) years from now, all galaxies outside the Local Supercluster will be redshifted 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.[4]

Degenerate Era

From 1014 (100 trillion) to 1040 (10 duodecillion) years

By 1014 (100 trillion) years from now,

degenerate remnants. If protons do not decay, stellar-mass objects will disappear more slowly, making this era last longer
.

Star formation ceases

1012–14 (1–100 trillion) years

By 1014 (100 trillion) years from now,

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 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 two

hydrogen burning as red dwarfs. This process, at least on white dwarfs, could induce Type Ia supernovae.[31]

Planets fall or are flung from orbits by a close encounter with another star

1015 (1 quadrillion) years

Over time, the

stellar remnant.[32]

Stellar remnants escape galaxies or fall into black holes

1019 to 1020 (10 to 100 quintillion) years

Over time, objects in a

stellar 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 their kinetic 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 galactic escape 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 central supermassive black hole.[5][14] It has been suggested that the matter of the fallen remnants will form an accretion disk around it that will create a quasar, as long as enough matter is present there.[35]

Possible ionization of matter

>1023 years from now

In an expanding universe with decreasing density and non-zero cosmological constant, matter density would reach zero, resulting in most matter except black dwarfs, neutron stars, black holes, and planets ionizing and dissipating at thermal equilibrium.[36]

Future with proton decay

The following timeline assumes that protons do decay.

Chance: 1032 (100 nonillion) – 1042 years (1 tredecillion)

The subsequent evolution of the universe depends on the possibility and rate of proton decay. Experimental evidence shows that if the proton is unstable, it has a half-life of at least 1035 years.[37] Some of the Grand 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 (including supersymmetry 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.

Nucleons start to decay

See also: Nucleon

Neutrons bound into nuclei 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 into

leptons
through proton decay.

All nucleons decay

1043 (10 tredecillion) years

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 into photons and leptons. Some models predict the formation of stable positronium 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]

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

If protons decay on higher-order nuclear processes

Chance: 1076 to 10220 years

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 its atoms and molecules via quantum tunneling, and may behave as liquid and become smooth spheres due to diffusion and gravity.[13] Degenerate stellar objects can potentially still experience proton decay, for example via processes involving the Adler–Bell–Jackiw anomaly, virtual black holes, or higher-dimension supersymmetry possibly with a half-life of under 10220 years.[5]

>10145 years from now

2018 estimate of

collapse of a false vacuum; 95% confidence interval is 1065 to 10725 years due in part to uncertainty about the top quark mass.[43]

>10200 years from now

Although protons are stable in standard model physics, a quantum anomaly may exist on the electroweak level, which can cause groups of baryons (protons and neutrons) to annihilate into antileptons via the sphaleron transition.[44] Such baryon/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.

The photon, electron, positron, and neutrino are now the final remnants of the universe as the last of the supermassive black holes evaporate.

Black Hole Era

1043 (10 tredecillion) years to approximately 10100 (1 googol) years, up to 10110 years for the largest supermassive black holes

After 1043 years, black holes will dominate the universe. They will slowly evaporate via Hawking 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 largest black 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 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.[14]

Dark Era and Photon Age

From 10100 years (10
duotrigintillion
years or 1 googol years) and beyond

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, baryons, 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).[47]

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. However, most electrons and positrons will remain unbound.[48] Other low-level annihilation events will also take place, albeit very slowly. The universe now reaches an extremely low-energy state.

Future without proton decay

If protons do not decay, stellar-mass objects will still become black holes, but more slowly. The following timeline assumes that proton decay does not take place.

10161 years from now

2018 estimate of

collapse of a false vacuum; 95% confidence interval is 1065 to 101383 years due in part to uncertainty about the top quark mass.[43][note 1]

Degenerate Era

Matter decays into iron

101100 to 1032000 years from now
Pure (99.97 %+) iron chips, electrolytically refined, as well as a high purity (99.9999 % = 6N) 1 cm3 iron cube for comparison.
All matter will slowly decay into iron which will take from 101100 to 1032000 years.

In 101500 years, cold fusion occurring via quantum tunneling should make the light nuclei in stellar-mass objects fuse into iron-56 nuclei (see isotopes of iron). Fission and alpha particle emission should make heavy nuclei also decay to iron, leaving stellar-mass objects as cold spheres of iron, called iron stars.[13] Before this happens, in some black dwarfs the process is expected to lower their Chandrasekhar limit resulting in a supernova in 101100 years. Non-degenerate silicon has been calculated to tunnel to iron in approximately 1032000 years.[49]

Black Hole Era

Collapse of iron stars to black holes

101030 to 1010105 years from now

Quantum tunneling should also turn large objects into black 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 into neutron stars in around 101076 years.[13]

Dark Era (without proton decay)

1010105 to 1010120 years from now

With black holes having evaporated, all baryonic matter will have now decayed into subatomic particles (electrons, neutrons, protons, and quarks). The universe is now an almost pure vacuum (possibly accompanied with the presence of a

false vacuum). The expansion of the universe slowly causes itself to cool down to absolute zero.[50][51]

Beyond

See also: Ultimate fate of the universe
Beyond 102500 years if proton decay occurs, or 101076 years without proton decay

It is possible that a Big Rip event may occur far off into the future.[52][53] This singularity would take place at a finite scale factor.

If the current

false vacuum, the vacuum may decay into a lower-energy state.[54]

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.[8]

The universe could possibly avoid eternal heat death through random quantum tunneling and quantum fluctuations, given the non-zero probability of producing a new Big Bang in roughly 10101056 years.[55]

Over an infinite amount of time, there could be a spontaneous entropy decrease, by a Poincaré recurrence or through thermal fluctuations (see also fluctuation theorem).[56][57][58]

Massive black dwarfs could also potentially explode into supernovae after up to 1032000 years, assuming protons do not decay.[59]

The possibilities above are based on a simple form of dark 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.

Graphical timeline

Main article: Graphical timeline from Big Bang to Heat Death
See also: Graphical timeline of the universe and Graphical timeline of the Big Bang
Logarithmic scale

See also

Notes

  1. ^ Manuscript was updated after publication; lifetime numbers are taken from the latest revision at https://arxiv.org/abs/1707.08124.

References

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    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☉.

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External links
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