The Future of the Universe
The future of the Universe is prescribed by the quantity of matter that it contains and by the existence of an anti-gravitational force that was detected in recent studies about the expansion of the Universe. The luminous matter is no more than about 1% of the matter that is necessary for the refraining of the expansion and subsequent contraction of the Universe.
The Dark Matter
It is known, for the observed movements of the galaxies, that the quantity of dark matter is, at least, 10 times higher than the quantity of luminous matter. The dark matter may assume two possible forms:
Brown dwarf Gliese 229 B: small bright sphere to the right (HST - NASA)
The hypothesis of the Open Universe had a boost since it was observed in 1998 a mysterious acceleration of the expansion of the Universe. The Universe is 13,7 billion years old (about 1010). It's estimated that, in an open, ever-expanding and infinite Universe (with a negative curvature, saddle-shaped), in about 1014 years the stellar activity will come to an end, since the available fuels existing in the interstellar space will become exhausted.
In 1018 years, the galaxies will start to collapse, given the emission of energy under the form of gravitational waves and the escape into the intergalactic space of the stars that, as a consequence of the close encounters with other stars and corresponding gravitational interactions, will have gradually gained more velocity, or in other words, more energy. This escape will reduce the galaxy to the less energetic and slower stars, which will provoke its contraction, encouraging new approximations and accelerations of stars that, again, will be able to eject themselves from the galaxies. Given the small dimensions of the galactic remainders resulting from that, the stars that resist until the end against such emigrations will finally be attracted by the huge black holes that inhabit the galactic centres, being consumed by them and, this way, feeding their growth.
If the protons don't decay, after 101600 years the white dwarfs will collapse to form neutron stars and in an unimaginable time scale (between (1014)26 and (1010)76 years) they will coalesce until they are transformed into black holes. These, in turn, will end up evaporating and giving place to a featureless Universe of particles and radiation.
In the case the Universe was finite and closed (with a positive curvature, sphere-shaped) the galactic clusters would end up getting close to each other and merging.
Some hundred million light years later, the galaxies themselves would merge and a single hipergalaxy would finally arise.
After some million years, the stars would be so close that the entire sky would be as bright as the Sun.
When the temperature of the space exceeded the temperature of the stars, these would explode. Many black holes would be constantly forming from the collapsing matter.
About 100 000 years before the Big Crunch the formation of black holes would happen at a gorgeous rhythm.
1000 years before the end, the black holes themselves would coalesce and, at the moment of the Big Crunch, everything would disappear in the interior of a singularity with infinite temperature and density. The 4 fundamental forces would become united again and the conditions for a new Big Bang would be created.
The 5th Force
The recent observations strongly suggest that the expansion of the Universe is accelerating and, thus, the Universe shall be open. The " dark energy " that originates this acceleration is still completely involved in mystery and nothing is known about its nature. It is only supposed that it shall be much dimmer than the gravity, due to the obvious hardness to be detected. It is also supposed that the vacuum rather than mass is empowering the action of this force, since the Universe only started to accelerate in recent times, when the Universe would already be empty enough to allow the appearance of this force. In the composition of the whole Universe, the dark energy shall represent about 73% of its content, the unknown type of dark matter represents about 23% and the ordinary matter only 4%.
Diagram showing the ingredients that make up the universe, according to the actual conception of it. (The differences between the proportions mentioned in this text are a symptom of the uncertainty that still exists concerning to this theme)