Finally, there remains one last question: how will the universe end? We know now that it started very small and very hot and that it is today almost cold and gigantic, like a balloon which would inflated disproportionately. Well, luckily the universe can not possibly burst like a balloon, but like him it can deflate!
What will happen in a few billion years? Will the universe continue to swell or will it deflate, reverse to contract and return to the beginning?
Since no one can come from the future to explain what will happen, astronomers have made calculations to try to answer this question. They also tried to measure the universe to know its shape. Unfortunately there are many answers and nobody knows today which is the good one!
If the universe contains a lot of matter for example, under the effect of the gravitation the expansion can slow down and then stop and the universe could go back, become again very small before the stars and the galaxies are absorbed by the Black holes and disappear definitely as shown in the animation on the right.
But do not worry, it will certainly not do it for tens or even hundreds of billions of years. Currently no clusters of galaxies have been observed to slow down, but it is a very very slow phenomenon that can not be detected during a human life.
On the other hand, if the universe contains very little material, the expansion will continue for a very long time, perhaps indefinitely. In this case, the universe will gradually cool down, all the stars will go out and die and only black holes will remain (see Life and Death of the Stars), the temperature of which will be lower than that of the first one. ‘space.
After billions of billions of years, the black holes will capture all the stars that remain in the universe and will become more and more massive and voluminous. Finally, when the black holes are warmer than the space, a little above -273.15 ° C, they will evaporate and disappear in an explosion of light.
When the universe reaches the absolute zero temperature of -273.15 ° C, at that moment there will be nothing in the universe except for a few elementary particles. It will be dark and cold, there will be no chemical or physical reaction, the universe will be dead.