As the universe continues to expand, will there be a critical point? What will be the final destination?

This article is based on answering netizens' questions: Is the universe also continuously expanding like a living organism, and will it really explode when it reaches a critical point?

The question itself is not accurate and the metaphor is not appropriate.

The expansion of the universe is due to some driving force. All galaxies and cosmic objects are moving apart from each other and becoming increasingly sparse, while the entire universe is getting bigger and bigger like a balloon being blown up. There is no continuous expansion of living organisms. There is only metabolism, birth, aging, sickness and death. This is the law of evolution of life.

So these are two different things and cannot be compared.

Let’s first talk about the expansion of the universe.

The standard model of the universe now recognized by the scientific community is the Big Bang theory, which states that the universe was born from an infinitely small singularity, and has been expanding since the Big Bang. Now it has formed an observable universe with a radius of 46.5 billion light years. Outside the observable universe, there is a universe that can never be seen. This part is called the unobservable universe, and it is currently impossible to estimate how big this part of the universe is.

The foundation of the standard cosmological model is the theory of cosmic expansion, which was discovered by American astronomer Edwin Hubble, who established Hubble's law. Based on the fact that the universe has been expanding, we can infer that the further we go back in time, the smaller the universe becomes. In the earliest past, all galaxies would be squeezed into one point, which is the singularity.

According to the expansion rate, this singularity exploded 13.82 billion years ago, so the age of the universe is 13.82 billion years. However, the Hubble constants measured by scientists so far are inconsistent and not unified, so the calculated age of the universe is different, ranging from 11.4 billion to 15 billion years old. Here we use the more common 13.82 billion years.

The Hubble constant is the rate at which galaxies are moving away from us at a distance of one million parsecs (about 3.26 million light-years) measured by scientists through various scientific methods and equipment. The measured value is between 67.8 km/s and 82.4 km/s.

The evidence of the expansion of the universe is that distant galaxies are moving away from us at high speeds. They look the same from all directions, which is isotropy. Moreover, the speed at which they move away from us is linearly proportional to the distance, that is, the farther away, the faster they move away. Some people use a balloon as an analogy, and all the galaxies are the stars on the surface of the balloon, and these stars continue to separate as the balloon expands.

Therefore, all the galaxies in the universe are moving away from each other, and standing at the position of each galaxy, the result is the same, which also shows that the universe has no center.

But the expansion of the universe cannot be simply likened to a balloon. The dimensional theory involved is very complicated, so I won’t go into detail here. The balloon metaphor is just a figurative and general metaphor, and you can understand it. As for whether the balloon will be blown up, leading to the destruction of the universe, this is a bit like the Big Rip theory, and you can understand it yourself.

These evidences were discovered by Edwin Hubble in the early 20th century. Einstein personally went to the observatory where Hubble was working to verify them, thereby modifying the formulas of field theory.

The expansion of the universe is governed by dark energy

Modern research has found that in our universe, visible matter, including stars, galaxies, nebula dust, etc., only accounts for 4.9% of the total mass-energy, invisible dark matter accounts for 26.8%, and dark energy accounts for 68.3%. Therefore, the entire evolution of the universe is actually a game between dark matter and dark energy.

Dark energy mainly manifests itself as repulsion, which makes matter and celestial bodies have a tendency to keep moving apart. Therefore, dark energy is the ultimate force that dominates the expansion of the universe. Modern research has found that the rate of expansion of the universe is getting higher and higher. According to the Hubble constant obtained from various scientific observations, at the edge of the observable universe, the rate of expansion of the universe is more than three times the speed of light.

Although dark matter is invisible, it has mass. According to Einstein's general theory of relativity, any object with mass will cause the curvature of space-time, which manifests as a gravitational effect. Therefore, the gravitational force of dark matter is much greater than that of visible matter. They exist in the space between galaxies and galaxy clusters and are the main force that keeps the celestial bodies in the universe from dispersing. The reason why galaxies and galaxy clusters do not fall apart when the velocity dispersion is very high is the gravitational pull of dark matter.

Therefore, the entire process of cosmic evolution is actually a game between dark matter and dark energy, and it is unpredictable who will win in the end.

I want to emphasize here that whether it is the expansion of the universe or the standard cosmological model, as well as the theory of dark matter and dark energy, these are not my own random words, but the result of hundreds of years of observation, discovery and experimental research by the scientific community. There is a lot of evidence to support them, and they are already the basic consensus of the scientific community. As a popular science writer, Spacetime Communications only spreads scientific common sense to readers in a popular way.

Is there a critical point for the expansion of the universe that determines the fate of the universe?

At present, the scientific community has not yet reached a conclusion on whether there is a critical point for the expansion of the universe. This depends on the final outcome of the game between dark matter and dark energy. If dark energy always has the upper hand, the universe will expand forever; if dark matter eventually prevails, the universe will stop expanding. This is the critical point, and then the universe will begin to shrink.

Whether the universe has a critical point determines the fate of the universe. If it continues to expand forever, the universe will become more and more empty, and all the stars will no longer be able to see each other, and eventually the universe will become cold and dead. The so-called cold and dead means that after all the energy of the stars is exhausted, the black holes will eventually evaporate, the universe will have no energy, the temperature will return to absolute zero, and time and space will end.

If the expansion of the universe reaches a critical point and then begins to contract, the contraction will become faster and faster, and eventually form a collapse trend. All galaxies are squeezed together and collapsed into a singularity, and time and space disappear. This theory is called the Big Collapse Theory. The universe is born from a singularity and returns to a singularity, and the cycle repeats.

It seems that no matter whether the universe has a critical point or not, there is no explosion. In addition, there are two other theories about the fate of the universe, one is called heat death and the other is called the Big Rip.

The so-called heat death is based on the second law of thermodynamics, which holds that the reason why this world exists is because of the flow of energy. In an isolated system, high temperature will continuously exchange heat with low temperature, eventually reaching thermal equilibrium and terminating the flow. No energy flow is equal to death.

This theory holds that as an isolated system, the entropy of the universe will increase over time, developing from order to disorder. When the entropy reaches its maximum value, all matter reaches thermal equilibrium, and the entire universe no longer has the energy to maintain movement and life. Time and space stop, and the universe dies.

The Big Rip means that the universe is expanding faster and faster, and eventually all matter in the universe can no longer withstand the increasing expansion speed and is torn apart, including galaxies, stars, planets, and every atom and elementary particle. From then on, everything returns to nothingness and returns to the state before the birth of the universe with vacuum quantum fluctuations, and the universe disappears.

Therefore, whether the expansion of the universe will have a critical point, and how the universe will eventually die, the current theoretical framework cannot determine. Some studies have shown that the fastest way to die is the Big Rip, which will probably happen in about 20 billion years; the rest of the ways to die will take at least trillions of years.

If you have any opinions on this, please feel free to discuss. Thank you for reading.

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