There is a critical density in theory. If the average density of matter in the universe is less than the critical density, the universe will continue to expand, which is called opening the universe; If the average density of matter is greater than the critical density, the expansion process will stop sooner or later, and then it will shrink. This is the so-called closed universe.
The problem seems simple, but it is not. The theoretical critical density is 5× 10-30g/cm3. But it is not so easy to determine the average density of matter in the universe. There is a vast intergalactic space between galaxies. If the mass of all luminescent substances observed at present is evenly distributed in the whole universe, the average density is only 2× 10-3 1g/cm3, which is far lower than the above critical density.
However, all kinds of evidence show that there are still so-called dark matter that has not been observed in the universe, and its quantity may far exceed that of visible matter, which brings great uncertainty to the determination of average density. Therefore, whether the average density of the universe is really less than the critical density is still a controversial issue. However, at present, it is more likely to open the universe.
When a star evolves to a later stage, it will throw some substances (gases) into interstellar space, which can be used to form the next generation of stars. This process will consume less and less gas, so that new stars cannot be formed in the end. 10 14 years later, all the stars will lose their luster and the universe will become dark. At the same time, stars will escape from galaxies because of interaction, and galaxies will shrink because of energy loss. In this way, the central part will produce a black hole, which will devour the stars passing by it and grow up.
After1017 ~1kloc-0/8, only black holes and some scattered death stars are left for a galaxy. At this point, the protons that make up the star are no longer stable. When the age of the universe reaches 1024, protons begin to decay into photons and various leptons. 1032, this decay process is completed, leaving only photons, leptons and some huge black holes in the universe.
10 100 years later, high-energy particles will escape from the huge black hole through evaporation and eventually disappear completely, and the universe will return to darkness. This may be the scene at the end of the universe, but it is still expanding constantly and slowly.
What will happen if the universe is closed? In a closed universe, the end time of the expansion process depends on the average density of the universe. If the average density is assumed to be twice the critical density, then according to a simple theoretical model, after 40-50 billion years, when the radius of the universe expands to about twice its current size, gravity will begin to prevail, the expansion will stop, and then the universe will begin to shrink.
In the future, the situation is almost like a cosmic movie that is shown backwards. All the major changes in the universe after the Big Bang will be reversed. After shrinking for tens of billions of years, the average density of the universe has roughly returned to its present state. However, the retrogression of galaxies far away from the earth will be replaced by the movement closer to the earth. In a few billion years, the background radiation of the universe will rise to 400 kHz and continue to rise, so the universe will become very hot, dense and shrinking faster and faster.
In the process of collapse, galaxies will merge with each other and stars will collide frequently. Once the temperature of the universe rises to 4000 kHz, electrons will be dissociated from atoms; When the temperature reaches several million degrees, all neutrons and protons are separated from the nucleus. Soon, the universe entered the "big collapse" stage, and all matter and radiation were swallowed into an infinitely high-density and infinitely small space very quickly, returning to the state when the Big Bang occurred.
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