In our universe, every galaxy is racing away from all the others.
Each one inexorably flung into the deepest regions of creation,
traveling on an uncontrollable journey through spacetime that
will not end until the embers of all of their stars have faded
into darkness.
It is generally thought that the gravitational attraction of all
the matter in the universe, both seen and in the form of dark matter,
is not enough to overcome the forces of acceleration driving the
expansion of the cosmos.
This means that the future of creation is intricately connected to
the second law of thermodynamics - the decay of the universe has
begun.
If we accept the implications of the current observations of universal
expansion, then the future of the universe is rather bleak. The
inexorable tide of inflation will ultimately land the cosmos on the
shores of desolation and heat death.
The Stelliferous Era
About 155 million years after the Big Bang, the first star formed
and ever since stars have forged from coalescing gas where the
pressure of gravity ignited the star's fusion engine.
This epoch in the lifetime of our universe is dominated by the
processes that cause the stars to shine. Most of the radiation
during this time is due to the stellar fusion of lighter elements
into heavier ones. This is the age of the stars.
We currently reside during this period and it known as the Stelliferous
Era.
Of all the stars in the universe, only 20% of them are the size our
our sun and larger. This includes blue giants, red giants and all
main sequence stars like the Sun. The remaining 80% belong to very
low mass stars. Most of the stars in our universe are red dwarfs.
The lifetime of a red dwarf star is staggering. Because they are
such low mass, then can burn their hydrogen fuel into helium more
efficiently. These stars shine for over 10 trillion years.
Since the higher mass stars live nowhere near this long, they will
disappear first - some as supernovae, and others, like our Sun, as
a Planetary Nebula. Long after they are gone, all that remains
will be red dwarfs, white dwarfs, brown dwarfs, neutron stars and
black holes.
Also during the Stelliferous Era, almost 100 trillion years from
now, all galaxies in our local group will have redshifted so far
that even gamma rays will have wavelengths longer than the observable
universe.
The galaxies will have disappeared.
The Degenerate Era
As the red dwarfs slowly burn their nuclear fuel over untold
millennia, over cosmic timescales so vast as to be meaningless to
our brains, our galaxy, the Milky Way, will die. Stars will be
flung out from random gravitational encounters and the central black
hole will also devour them until the galaxy itself no longer exists.
The Milky Way is dead.
Eventually, in 100 trillion years, the last stars will have died,
all that remains are their corpses: white dwarfs, neutron stars and
black holes. Once the last of the red dwarfs exhaust their fuel,
all nuclear fusion in the universe will have ceased. The red dwarfs
have cooled into white dwarfs and they, along with neutron stars
and black holes will be the only objects with mass still shining
in the heavens.
We have arrived in the Degenerate Era.
Once the last star burns out, the universe will be a very dark
place, but the feeble embers of the remaining white dwarfs will
still produce some light. At this point in the life of the universe,
they are the brightest objects anywhere.
The Black Hole Era
After the embers of the white dwarfs, brown dwarfs and neutron stars
have faded, some 10^40 years hence, and all protons and neutrons
have decayed into positrons and neutrinos, black holes will dominate
the universe.
Here, the primary source of radiation throughout the universe comes
from Hawking radiation.
Black holes cannot live forever, they are also slowly evaporating.
Small fluctuations in spacetime cause particle-antiparticle pairs
to appear close to the event horizon of a black hole. Due to the
strong gravity so close to the black hole, one of the pair is
captured while the other escapes into space. This feeble signal
will be the only source of warmth throughout the cosmos.
The universe is colder and darker than it has ever been, the average
temperature only 5 Kelvin.
The Dark Era
The lifetime of a supermassive black hole of 100 billion solar
masses is about 10^99 years. At this point in the lifetime of the
universe, all that remains is diffuse matter. All activity in the
universe has all but stopped and is in an extremely low-energy
state. What happens here - during this so-called Dark Era - is
unclear.
The universe may stay in this state forever, this heat death signaling
all that remains of a once active and vibrant universe.