The universe is not eternal. What are the possible endings? All the theories point to the same end.

The universe is not eternal, which is a very difficult statement to accept, because the end and extinction are the most terrifying things.

This is just like life will eventually die out, people are powerless to stop it but have to admit this fact, so they fantasize about reincarnation to comfort themselves. Although there is no evidence for this, if anyone dares to question it, they will inevitably be opposed, because questioning reincarnation is equivalent to erasing people's hope for "eternal life". Similarly, if the universe is not eternal, then reincarnation has no meaning, so this is bound to be an unacceptable statement. In Newton's time, people believed that the universe was static and eternal, but with the introduction of general relativity, the problem became a little different. People began to realize that the universe is not static, let alone eternal.

The universe is not eternal. This is not only difficult for ordinary people to accept, but even Einstein, the originator of the theory of general relativity, was unwilling to believe it.

For a long time, Einstein tried to correct his formula and tried to add a so-called cosmological constant to his equation. The only purpose of doing so was to hope that the final solution of the general theory of relativity formula would conform to the static eternal universe of the Newton era, but Einstein failed. In the end, he accepted the conclusions drawn by many scientists based on general relativity: the universe is expanding, the universe is not static, and it is not eternal. The universe has a beginning, and this beginning is the "Big Bang" hypothesis. Although it is a hypothesis, it is the most well-documented and most recognized theory of the origin of the universe so far. All current research on the universe is basically based on this foundation.

The Big Bang created the universe, and then the universe began to expand. 13.8 billion years have passed, and the expansion of the universe has never stopped.

The expansion of the universe is the expansion of space itself. It occurs at every point in the universe. We can imagine the universe as a balloon with many small dots of varying distances on it. Then we start to inflate the balloon, and the distance between these points begins to gradually increase. This is the expansion of space itself.

Will the universe continue to expand? What will be its final outcome? The final outcome of the universe is closely related to the shape of the universe. It is generally believed that the universe may have three different shapes.

The first view is that the universe is closed, which is also the view that most people are more likely to accept.

A closed universe is like a sphere, finite but unbounded.

Since the universe is closed, the expansion of the universe will gradually weaken due to the gravitational force of the internal matter, and eventually the expansion will stop. The universe will begin to change from expansion to contraction under the action of gravity. This contraction process will gradually accelerate, and the entire universe will collapse again into a point with infinite density and infinitely small volume, that is, a singularity. After a long, long time, the balance inside this singularity will be broken due to some unknown factors, so the Big Bang will occur, and a new universe will be born.

The second view is that the universe is not closed, but flat. A flat universe is like a plane that is constantly expanding, making the plane bigger and bigger. However, the expansion will not continue endlessly, and its speed will gradually weaken.

As the expansion rate slows down, the flat universe will eventually come to a stop, but before that, everything in the universe will return to equilibrium.

We know that there are various forms of matter in the universe, but all matter is essentially made up of elementary particles. As time goes by, all the stars in the universe will eventually run out of fuel, they will collapse, and then cool. Of course, massive stars will collapse into black holes, but black holes will also gradually evaporate due to Hawking radiation. Although this process is extremely slow, the universe has plenty of time. Slowly, the universe became colder and colder, and the temperature finally reached absolute zero. At the same time, due to continuous expansion, the matter in the universe gradually moved away, and finally all the matter was torn into the form of elementary particles, and evenly distributed under the action of space expansion.

The universe eventually reached a state of absolute equilibrium, and the entropy in the universe increased to its maximum. This is heat death.

The third view is that the universe is completely open. An open universe is the opposite of a closed universe. It presents a state of negative curvature. In this state, the expansion of the universe will not stop, but will continue forever, so its ending is the same as that of a flat universe, which is heat death. Universes of different shapes have different endings, but whether it is heat death or collapse, although they seem to be completely opposite, they all point to the same end, that is, extinction. This may seem unacceptable, but it is the closest to the truth. Nothing we have seen so far can last forever, so clinging to eternity will only add to our troubles. When you begin to accept the fact that "death represents the end of everything", you will find that your life will become different, and every minute and every second will be wonderful. This feeling is a bit like a person who becomes fearless in life after learning that he has a terminal illness.

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