27 What Lies Ahead?
205
trigger new star formation in each galaxy. The galaxies may also pass through
each other without any spectacular effect.
27.2 One Hundred Billion Billion (∼10 20 ) Years
At the level of galaxies, the stars collide with each other with a low probability
as just noted. That makes the stars evaporate on average in a time scale of 10
19
years. The stars may also leave a galaxy as a result of collisions so the galaxies
may start to lose stars. There are two ways this can be halted. The presence
of the dark matter may delay this process somewhat. It may also happen that
brown dwarfs, which do not shine like our sun, have all their nuclear energy
unspent, and when two brown dwarfs collide, they may merge and form a new
sun-like star. This may replenish some of the star content of the galaxies.
27.3 Far Future: Beyond the Next Million Trillion
Trillion Years ∼10 30 Years
As the universe expands under the influence of the cosmological constant and
the resulting accelerated expansion, all galaxies fly farther apart, and in each
galaxy stars vanish. Most become black holes, making the universe cold, dark,
and depressing. There can also be interesting wrinkles in what happens in the
far future, if for example the baryon number is not conserved. The fact that it
is not conserved is something that seems guaranteed by Sakharov’s argument
about the origin of matter in the universe. As a typical example, we may assume
that the proton is unstable and it decays to positron and a π
0 . The π
0 decays
in an instant to two photons. The proton may also decay part of the time to
neutrinos and a π
+ . The lower bounds on the lifetimes for these processes is
of the order of 10
33 years. Similarly, neutron–anti-neutron oscillation inside a
nucleus has a lifetime of the same order. This means that eventually when the
universe is 10
33 years old, these decay processes leading to the disappearance of
the proton and a bound neutron start to be effective. Note that in our universe
there are about 10
78 total of protons and neutrons right now. At that time, all
the baryons that made the stars and planets will start to evaporate to electrons,
positrons, photons, and some neutrinos. That will leave a dust of these particles
as the end product in the universe. If the electric charge conservation was not
an exact law, then an electron or positron could decay to a neutrino and a
photon or a majoron, in which case, the end product will all be photons with
205
trigger new star formation in each galaxy. The galaxies may also pass through
each other without any spectacular effect.
27.2 One Hundred Billion Billion (∼10 20 ) Years
At the level of galaxies, the stars collide with each other with a low probability
as just noted. That makes the stars evaporate on average in a time scale of 10
19
years. The stars may also leave a galaxy as a result of collisions so the galaxies
may start to lose stars. There are two ways this can be halted. The presence
of the dark matter may delay this process somewhat. It may also happen that
brown dwarfs, which do not shine like our sun, have all their nuclear energy
unspent, and when two brown dwarfs collide, they may merge and form a new
sun-like star. This may replenish some of the star content of the galaxies.
27.3 Far Future: Beyond the Next Million Trillion
Trillion Years ∼10 30 Years
As the universe expands under the influence of the cosmological constant and
the resulting accelerated expansion, all galaxies fly farther apart, and in each
galaxy stars vanish. Most become black holes, making the universe cold, dark,
and depressing. There can also be interesting wrinkles in what happens in the
far future, if for example the baryon number is not conserved. The fact that it
is not conserved is something that seems guaranteed by Sakharov’s argument
about the origin of matter in the universe. As a typical example, we may assume
that the proton is unstable and it decays to positron and a π
0 . The π
0 decays
in an instant to two photons. The proton may also decay part of the time to
neutrinos and a π
+ . The lower bounds on the lifetimes for these processes is
of the order of 10
33 years. Similarly, neutron–anti-neutron oscillation inside a
nucleus has a lifetime of the same order. This means that eventually when the
universe is 10
33 years old, these decay processes leading to the disappearance of
the proton and a bound neutron start to be effective. Note that in our universe
there are about 10
78 total of protons and neutrons right now. At that time, all
the baryons that made the stars and planets will start to evaporate to electrons,
positrons, photons, and some neutrinos. That will leave a dust of these particles
as the end product in the universe. If the electric charge conservation was not
an exact law, then an electron or positron could decay to a neutrino and a
photon or a majoron, in which case, the end product will all be photons with