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 1012 to 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.
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.
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
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.
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.
Assuming that dark energy continues to make the universe expand at an accelerating rate, 2×1012 (2 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.
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.
It is estimated that in 1014 (100 trillion) years or less, 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 order 1013 (10 trillion) years. 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 1013 (10 trillion) years. §IIIC.
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 are ejected from the galaxy, leaving a small fraction (perhaps 1% to 10%) which fall into the central supermassive black hole., §IIIAD;, pp. 85–87
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 Neutrons bound into nuclei are also expected to decay with a half-life comparable to the proton's., §IVA
In the event that the proton did 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., §IVA. Shorter or longer proton half-lives will accelerate or retard 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.
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 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 ½1,000 (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 will have been changed into photons and leptons.
After 1040 years, black holes will dominate the universe. They will slowly evaporate via Hawking radiation., §IVG. A black hole with a mass of around 1 solar mass will vanish in around 2×1066 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) solar masses will evaporate in around 2×1099 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 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.
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.
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., §VID.
With 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.
In 101500 years, cold fusion occurring via quantum tunnelling should make the light nuclei in ordinary matter 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.
Quantum tunnelling should also turn large objects into black holes. Depending on the assumptions made, the time this takes to happen can be calculated as from years to years. (To calculate the value of such numbers, see tetration.) Quantum tunnelling may also make iron stars collapse into neutron stars in around years.
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