The Stelliferous Era :
From the present to about (100 trillion) years after the Big Bang s. LH 95 star forming region of the Large Magellanic Cloud. The image was taken using the Hubble Space Telescope. Source: European Space Agency (ESA/Hubble) The
observable universe is currently 1.38 (13.8 billion) years old. This time lies within 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 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. Stars of low to medium mass, such as our own
sun, 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. 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 :
4–8 billion years from now (17.8–21.8 billion years after the Big Bang) 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 kilometres (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. 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. 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
= −1.5.
False vacuum decay may occur in 20 to 30 billion years if the
Higgs field is metastable.
Coalescence of Local Group and galaxies outside the Local Supercluster are no longer accessible :
(100 billion) to (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 (100 billion) and (1 trillion) years from now, their orbits will decay and the entire Local Group will merge into one large galaxy. of
galaxies Galaxies outside the Local Supercluster are no longer detectable :
(2 trillion) years 2 (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.
Degenerate Era :
From (100 trillion) to (10 duodecillion) years By (100 trillion) years from now,
star formation will end, 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 over (10 trillion) years. Coincidentally, this is comparable to the length of time over which star formation takes place. If the combined mass is not above the Chandrasekhar limit but is larger than the minimum mass to
fuse carbon (about ), a
carbon star could be produced, with a lifetime of around (1 million) years.
Planets fall or are flung from orbits by a close encounter with another star :
(1 quadrillion) years 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.
Stellar remnants escape galaxies or fall into black holes :
to (10 to 100 quintillion) years Over time, objects in a
galaxy exchange
kinetic energy in a process called
dynamical relaxation, making their velocity distribution approach the
Maxwell–Boltzmann distribution. Dynamical relaxation can proceed either by close encounters of two stars or by less violent but more frequent distant encounters. In the case of a close encounter, two
brown dwarfs or
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. 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.
Possible ionization of matter :
> 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. == Future with proton decay ==