Nothing is eternal. And our universe, of course, will also die. Rumor has it that it will be eternal expansion and, in the end, death from entropy. The universe is expanding and entropy is growing and will continue to grow until everything we hold dear dies. But this is sentiment, and we are human scientists, so we wonder what the end of the universe will look like? What will it be accompanied by? No, well, curious.
There will be no stars left in the night sky
In 150 billion years, the night sky on Earth will look very different. As the universe tends towards its thermal death, space expands faster than the speed of light. We know that the speed of light is the rigid speed limiter for all objects in the universe. But this only applies to objects that are in space, not the fabric of spacetime itself. It's hard to figure out on the fly, but the fabric of spacetime is already expanding faster than the speed of light. And in the future this will entail strange consequences.
Since space itself is expanding faster than light, there is a cosmological horizon. Any object that goes beyond this horizon will require us to be able to observe and record data about it using particles traveling faster than light. But there are no such particles. As soon as objects leave the cosmological horizon, they become inaccessible to us. Any attempt to contact or interact with distant galaxies beyond this horizon will require technology from us that can move faster than the expansion of space itself. So far, only a few objects are outside our cosmological horizon. But as dark energy accelerates expansion, everything will eventually be out of reach of our eyes.
What does this mean for the Earth? Imagine staring into the night sky 150 billion years from now. The only thing that will be seen is a few stars that remain within the cosmological horizon. In the end, they will also leave. The night sky will be completely clear, like the tabula rasa. Astronomers of the future will not be able to prove that there is any other object in the universe. All the stars and galaxies that we see now will disappear. For us, only the Solar System will remain in the entire Universe. True, the Earth is unlikely to live up to this, but more on that below.
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Life after the death of the Sun will not disappear
Everyone knows that stars do not last forever. Their lifespan begins with their formation, continues throughout the main sequence phase (which accounts for most of the star's life), and ends with the death of the star. In most cases, stars swell up to several hundred times their normal size, ending the main sequence phase, and with this swallow up any planets that come close to them.
However, for planets that orbit the star at great distances (outside the "freezing line" of the system), these new conditions may actually become warm enough to support life. According to a recent study by the Carl Sagan Institute at Cornell University, this situation in some stellar systems could last for billions of years and lead to the emergence of completely new forms of extraterrestrial life.
In about 5.4 billion years, our Sun will exit the main sequence phase. Having exhausted the hydrogen fuel in the core, the ashes of inert helium that will collect there will become unstable and collapse under the influence of their own weight. This will lead to the fact that the core heats up and becomes denser, which, in turn, will lead to an increase in the size of the Sun - the star will enter the phase of the "branch of red giants".
This period will begin when our Sun becomes a subgiant and will slowly double in size over about one and a half billion years. It will expand at a faster rate for the next half a billion years, until it is 200 times its current size and several thousand times brighter. Then it will officially become a red giant and its diameter will be approximately 2 AU. e. - The sun will go beyond the current orbit of Mars.
Obviously, the Earth will not survive the appearance of a red giant in the solar system, like Mercury, Venus or Mars. But beyond the freezing line, where it is cold enough for volatile compounds - water, ammonia, methane, carbon dioxide and carbon monoxide - to remain frozen, gas giants, ice giants and dwarf planets will remain. And a total thaw will begin.
In short, when a star expands, its "habitable zone" will do the same, spanning the orbits of Jupiter and Saturn. When this happens, a previously uninhabited place - like the moons of Jupiter and Saturn - may suddenly become residential. The same is true for many other stars in the Universe, which are destined to become red giants as they grow up and die.
When our Sun reaches the red phase of the giant branch, it will have only 120 million years of active life. This time is not enough for new life forms to appear and develop, capable of becoming truly complex (like humans and other mammalian species). But according to a study recently published in The Astrophysical Journal, some planets near other red giants in our universe can remain inhabited for much longer - up to nine billion years or more in some cases.
For you to understand, nine billion years is twice the current age of the Earth. Assuming that the worlds of interest to us will have the right composition of elements, they will have enough time to give rise to new complex forms of life. The study's lead author, Professor Lisa Kaltenneger, is also the director of the Carl Sagan Institute. She knows firsthand how to search for life in the Universe:
“As a star grows older and brighter, the habitable zone moves outward and you are essentially seeing a second life for the planetary system. Currently, objects in the outer regions are frozen in our solar system, like Europa and Enceladus, the moons of Jupiter and Saturn. After our yellow Sun expands enough to become a red giant and turn the Earth into a scorched desert, there will still be regions in our solar system - and in other systems as well - where life could flourish."
As a star expands, it loses mass and pushes it outward in the form of the solar wind. Planets that orbit close to a star or have low surface gravity can lose their atmosphere. On the other hand, planets with sufficient mass (or located at a safe distance) can preserve this atmosphere. In the context of our solar system, this means that in a few billion years, worlds like Europa and Enceladus (which may already have life hiding under ice shells) could become a paradise for life.
Our Sun will become a black dwarf
At the moment, our universe has many different types of stars. Red dwarfs - cool stars that emit red light - are among the most common. There are also many white dwarfs in the universe. These are the stellar remains of dead stars, made up of degenerate matter held together by quantum effects. Currently, astronomers believe that white dwarfs have an almost infinite life span. But after a certain time, even they will die and become exotic stars: black dwarfs.
Such a fate awaits our Sun too. In the distant future, our Sun will eject its outer layers and turn into a white dwarf star that will remain for billions of years. But one day, even white dwarfs will start to cool down. After 10 (to the power of 100) years, they will cool down to a temperature equal to the temperature of the microwave background radiation, a few degrees above absolute zero.
When this happens, our star will become a black dwarf. Because this type of star is so cold, it will be invisible to the human eye. For anyone who tries to find the Sun that gave us life, it will be impossible to do it using optical systems. He will have to look for it by gravitational effects. Most of the stars we see in the night sky will become black dwarfs (another reason why the night sky will become clear). But for our warm Sun it is especially offensive.
Strange stars
By the time our Sun becomes a black dwarf, stellar evolution has already been completed. New stars will not be born. Instead, the universe will be flooded with cold star remains. And this will allow the Universe to start creating strange stars that are significantly different from what we know.
One of these is a frosty or cold star. When stars in the universe burn out their nuclear fuel, they increase their metallicity. In astronomy, it is a measure of the elements in a star that are heavier than helium - virtually all elements, starting with lithium. As the metallicity of a star increases, they become colder as the heavier elements release less energy during fusion. Finally, the stars will become so cold that they have a temperature of 0 degrees, the freezing point of water.
Looking further into the future, there will be an even stranger star. In about 10 (to the power of 1500) years in the future, entropy will take its toll, and the universe will be essentially dead. In these cold times, quantum effects will govern the universe.
Quantum tunneling will allow light elements to be synthesized into an unstable form of iron. It, in turn, will decay into a more stable isotope, emitting a weak amount of energy. These iron stars will be the only star shape possible at this time. But they are only found in models in which astronomers do not believe in proton decay, so this idea is not the most popular.
All nucleons will decay
Let's rewind from a point of 10 (to the power of 15) years after the Big Bang to a point of 10 (to the power of 34) years. If the human race is not dead by that time, we certainly will not survive this era. As mentioned above, astronomers constantly argue about whether the proton will decay by the end of time. Let's say yes.
Nucleons are particles in the nucleus of an atom, protons and neutrons. Free neutrons are known to decay with a half-life of 10 minutes. But protons are incredibly stable. No one has seen firsthand the decay of a proton. But towards the end of the universe, everything will change.
Physicists assume that the half-life of a proton is 10 (to the power of 37) years. We have not seen this decay because the universe is not old enough yet. In the decay epoch (10 (to the power of 34) - 10 (to the power of 40) years), the protons will finally begin to decay into positrons and pions. By the end of the decay epoch, all protons and neutrons in the Universe will run out.
Obviously, life in the Universe will start having problems. If we assume that the human race survived the change of the Sun and migrated to more friendly parts of the Universe, at some point the laws of physics will begin to dictate the death of the human race. Our bodies and all interstellar objects are made of nucleons. When they disintegrate, any life will end, since the atoms themselves will cease to exist. Life will not be able to continue to exist in such conditions (and in such a form) and the Universe will plunge into the era of black holes.
Black holes will flood the universe
When the nucleons disappear, black holes will enter into law and will rule the Universe from 10 (to the power of 40) years after the Big Bang to 10 (to the power of 100) years. From this moment we begin to talk about times so long that it is completely impossible to understand them with our minds. After a time much longer than the present age of the universe, black holes will remain the only structures.
When the nucleons leave, the main subatomic particles will be leptons - electrons and positrons. They will fuel black holes. By absorbing the remnants of matter in the Universe, black holes themselves will emit particles that will fill the Universe with photons and hypothetical gravitons. But black holes are destined to die, as Stephen Hawking decided.
According to Hawking, black holes evaporate due to their radiation. When they radiate, they lose mass in the form of energy. This process takes a long time, so we know practically nothing about it. It takes 10 (to the power of 60) years for a black hole to completely evaporate, so this process has not yet proceeded to the end for a century of our Universe. But, as we said, eventually black holes will also die. Only massless particles and a few scattered leptons will remain of them, which will lazily interact and lose their energy.
An atom of a new type will appear
With only a few subatomic particles left of our universe, it may seem like there’s nothing more to talk about. But life can appear even in this worst of worlds.
For many years, particle researchers have talked about positronium, the atom-like bond between a positron and an electron. These two particles have opposite charges. (The positron is the antiparticle of the electron). Therefore, they will be electromagnetically attracted. When a pair of such particles begins to interact, they can have rudimentary orbits and atomic behavior.
Since positronium will be rare, this model of positronium "chemistry" cannot be called complete. But very curious things can come out of these strange "atoms". First, they can exist in giant orbits that cover interstellar space. As long as two particles interact, they will be able to maintain a pair regardless of distance.
During the era of black holes, some of these "atoms" will have diameters spanning distances greater than our present observable universe. Positronium atoms composed of leptons will survive the decay of a proton and pass through the era of black holes. In addition, black holes will create positronium atoms in the process of radiation. After a certain time, the positron-electron pairs will also decay. But before that, the Universe can give birth to a completely indescribable life.
Everything will slow down, even the very thought
When the era of black holes comes to an end and even these stellar giants disappear into darkness, only a few things will remain in our universe, mainly diffuse subatomic particles and the remaining atoms of positronium. After that, everything in the Universe will happen extremely slowly, any event can last for eons. According to some theoretical physicists, such as Freeman Dyson, life may reappear in the universe at this time.
After a long, long time, organic evolution can begin to develop from positronium. The creatures that will appear will be very different from anything we know. For example, they can be huge, spanning interstellar distances. Since there is nothing else left in the Universe, they will have where to turn around. But since these life forms will be huge, they will think much slower than us. In fact, it can take trillions of years for such a creature to create even one thought.
It may seem strange to us, but since these creatures will exist on huge time intervals, such a thought will be instant for them. They will exist for an incredibly long time, watching the Universe fly past them. But they will sink into oblivion.
The end of "macrophysics"
By this time, the Universe will reach almost the maximum state of entropy, that is, it will become a homogeneous field of energy and several subatomic particles. This will be after the era of black holes, much later after 10 (to the power of 100) years. Space will expand so much, and dark energy will become so powerful that even black holes will cease to exist and the universe will lose massive objects.
It is difficult to imagine such a universe. Just think about it: stars will stop forming because the subatomic particles that make up matter will be separated by such distances that they cannot meet in any way, traveling at the speed of light. Even positronium atoms cannot appear.
Physics will come to an end. The only physical model that will continue to work will be quantum mechanics. Quantum effects will occur even at huge interstellar distances, in a gigantic time frame. Eventually, the temperature of the universe will drop to absolute zero: there will be no energy left to be converted into work. In some models, the expansion of space will grow, tearing spacetime apart. The universe will cease to exist.
Is it possible to escape from all this?
Until now, our journey to the end of the universe has been accompanied by only dark and depressing events. But physicists do not lose their optimism and sketch out possible ways for humanity to survive the end times and even restart our universe.
The most promising way to escape our universe with maximum entropy is to use black holes until the decay of photons makes life impossible. Black holes remain very mysterious objects, but theorists propose to use them to enter new universes.
Modern theory suggests that bubble universes are constantly being born in our own universe, forming new universes with matter and the possibility of life. Hawking believes black holes may be the gateways to these new universes. But there is one problem. Once you cross the border of the black hole, there is no turning back. Therefore, if humanity decides to go to a black hole, it will be a one-way trip.
First, you have to find a spinning black hole massive enough to survive the trip across the event horizon. Contrary to popular belief, massive black holes are safer to travel through. Space travelers of the future may hope that the trip will not end badly, but they will not be able to contact their friends on this side of the black hole and inform them of the result. Each ride will be a leap of faith.
But there is a way to make sure that a new universe awaits us on the other side. According to Alan Guth, the newborn Universe needs only 10 (to the 89 power) protons, 10 (to the 89 power) electrons, 10 (89 power) positrons, 10 (89 power) neutrinos, 10 (89 power) antineutrinos, 10 (to the power of 79) protons and 10 (to the power of 79) neutrons for the start. It may seem like a lot, but in total it is no more than a brick.
The humans of the future could generate a false vacuum - a region of space with the potential for expansion - using a super-strong gravitational field. In the distant future, humans could have acquired the technology to create a false vacuum and start their own universe. Since the initial inflation of the universe lasts a fraction of a second, the new universe will expand instantly and become a new home for humans. A quick jump through the wormhole and we are saved.
Random quantum tunneling could restart the universe
What will happen to the universe that we left behind? After a while, it will finally reach its maximum entropy and become completely uninhabitable. But even in this dead universe, life will have a chance. Researchers in quantum mechanics are aware of the effect of quantum tunneling. This is when a subatomic particle can enter an energy state that is impossible classically.
In classical mechanics, for example, the ball cannot spontaneously pick up and roll up a hill. This is a forbidden energy state. Elementary particles also have forbidden energy states from the point of view of classical mechanics, but quantum mechanics turns everything upside down. Some particles can "tunnel" into these energy states.
This process is already taking place in the stars. But when applied to the end of the universe, a strange possibility arises. Particles in classical statistical mechanics cannot move from a higher state of entropy to a lower one. But with quantum tunneling, they can and will. Physicists Sean Carroll and Jennifer Chen proposed the idea that after a certain time, quantum tunneling could spontaneously reduce entropy in a dead universe, lead to a new Big Bang and restart the universe. But don't hold your breath. For a spontaneous decrease in entropy to happen, you have to wait 10 (to the power of 10) ^ (to the power of 10) ^ (to the power of 56) years.
There is another theory that gives us hope for a new universe - this time from mathematicians. In 1890, Henri Poincaré published his recurrence theorem, according to which, after an incredibly long time, all systems return to a state very close to their original state. This also applies to thermodynamics, in which random thermal fluctuations in a universe with high entropy can cause it to return to its original state, after which everything will start again. Time will pass, and the universe can form again, and the creatures who will live in it will not have the slightest idea that they live in our universe.
ILYA KHEL