Is the big rip really devastating

Tear, freeze, collapse - everything is there

What does the future hold? This question has interested people at all times. And the longing to gain certainty about what is to come has remained to this day. Astrologers, cartelists and a host of other esoteric "professional groups" benefit from it. Even a scientific discipline, futurology, has made the systematic exploration of possible future developments on its flags. Most of the forecasts - and not just those from the crystal ball - have so far failed. The further the researchers' gaze moves away from the present, the more uncertain their statements become. Understandably, because the complex interlocking of upcoming events and their effects cannot be precisely recorded. "Futorology is the science of coffee grounds," Hans Magnus Enzensberger once remarked ironically. But none of this prevents the experts from devoting themselves to this topic again and again using a wealth of different methods and techniques. The cosmologists, whose research subject is the physical world as a whole, are no exception. The US-American researcher Katie Mack has just published an entire book on "The End of Everything" ("The End of Everything". Scribner, New York 2020, nee .; Penguin Random House, London 2021, brosch.), In which she is concerned with the possible future and the end of the universe.

Cosmologists also extrapolate the present into the future

The basis of all considerations about the fate of the universe is of course our current knowledge, especially the discovery of Edwin Hubble and others, that the cosmos is developing, i.e. has a life story that, according to the currently prevailing theory, began with the so-called Big Bang. Once something has started, the researchers say, it has to end again at some point - birth and death as the two cornerstones of everything that exists. But how could this question be approached at all? Basically, the cosmologists proceed in a similar way to the futurologists. They extrapolate the present into the future and then define "possible futures" with the aim of finally figuring out the most likely one of them.

The dominant developmental characteristic of the presence of the universe is its expansion. The space is getting bigger and bigger, the galaxies are all moving away from each other and the greater the speed, the further away they are. Originally, cosmologists had concluded from this that the expansion of the universe would have to slow down gradually due to the gravitation of the masses it contains, in order to finally come to a standstill and then turn into a contraction. The contraction got faster and faster and finally ended in a "big crunch", a great collapse, so that that state, which cannot be described by physics, emerges again from which the universe is supposed to have emerged, the cosmological singularity.

Calculations showed that the standstill would only occur after 80 billion years and the "crunch" would not occur until after 160 billion years. But then the "dark energy" was discovered, which causes an accelerated expansion. There can be no question of a gradual slowdown. The result is a completely different apocalypse of the universe. The entire universe with all its objects could tear apart because nothing can withstand the enormous expansion. The universe would then "already" end in a "big rip" after a little more than 150 billion billion years. But only if the current accelerated expansion continues.

We know from measurements that, for a reason that is not yet understood, it only started a few billion years ago, so it was by no means always there. So it could just as suddenly end again. If this happened, a constant unaccelerated expansion would result in a gradual extinction of the universe.

Our current knowledge of the development of stars, but also of the evolution of the cosmos, as well as the standard model of particle physics, allow us to sketch a rough outline of the fate that is likely to lie ahead of the universe under these conditions in model calculations.

The result is the opposite of a "big crunch". First of all, the galaxy clusters would become more and more spaced apart. In the individual star systems, the hundreds of billions of stars that make them up are constantly fusing hydrogen to helium. The number of galaxies in the clusters should decrease over time, as collisions over time cause them to grow into gigantic structures. If we spin this thought further, one very distant day all the star systems in our closer galactic neighborhood, the objects of the so-called local group, will have merged into a single giant galaxy. This process continues and eventually also includes the Virgo galaxy cluster, on the edge of which our local nebula group is located today.

In those long periods of time we are talking about, however, the raw material from which stars will one day become increasingly scarce. As there is less and less hydrogen, the process of star formation comes to a standstill, while at the same time the already existing stars gradually exhale their life. The "old" die, but no new generations grow back. The universe is losing one of its most remarkable properties at present: it is getting dark. After a total of ten trillion (i.e. after 10,000 billion) years it would only be filled with long-wave heat and radio radiation from objects in which nuclear fusion no longer takes place, i.e. which only cool down.

In the end, even the protons could decay

After 100 trillion years, there are only black dwarfs, that is, once cooled down white dwarfs, neutron stars and black holes. The total temperature of the room, inflated to an unimaginable size, is now only one Kelvin, i.e. one degree above absolute zero. Now, however, the time could have come in the ancient universe in which the protons also increasingly decay and thus all remaining objects (white dwarfs, neutron stars and planets) end their existence. Only the black holes have an even longer life expectancy. According to Stephen Hawking, it is almost 10,100 years for very massive objects, such as those observed in the center of galaxies. After this time, however, there are actually no more structures in the universe. On the other hand, neutrinos, electrons and positrons still wander through the thin sea of ​​radiation. According to this theory, the expansion would have frozen the universe - the temperature is almost at absolute zero. Actually there is nothing like at the beginning of the life story of the universe, which is now approaching a state of timeless eternity.

But while all these largely speculative scenarios take place in the distant future, we could also face a very quick end - and not just us, but the entire universe. The two physicists Sidney Coleman and Frank de Lucia shocked the scientific world in the 1980s with the thesis that the entire universe could suddenly disappear. They argued that the brief inflationary phase of the expansion of the universe came about through a spontaneous decay of the quantum vacuum. The existing vacuum was in an unstable state and changed into one with lower energy. The two physicists believe, however, that one cannot know whether this vacuum has already reached the state of lowest energy. If this is not the case, there could be another spontaneous breakdown of the vacuum - with devastating consequences for the entire universe. The two physicists are not worried that the first decay took place 10-35 seconds after the Big Bang and that the universe has now existed for 13.82 billion years. After all, even with the spontaneous radioactive decay of atoms, there are half-lives of a few minutes and others of several billion years.

If we live in a very long-lived, but unstable vacuum, this could break up at any point at any time for no reason, i.e. spontaneously, and this phase transition would spread across the entire universe with enormous speed. Not only are the large structures of the universe, the galaxy clusters, galaxies and stars destroyed, but also the elementary particles. It would be the end of the entire universe in its current state. For us humans this would most likely mean a painless death because it would come too quickly - without warning. Even if such an ultimate wave of annihilation had already started in a distant corner of space, we would not notice it until we were finally caught up in it ourselves.

At the end of her book, "Astrokatie" (according to the author's Twitter account) philosophizes on the basis of conversations with celebrities such as Freeman Dyson, Roger Penrose and Martin Rees about the unpleasantness of a limited lifespan of the universe. But maybe it will exist forever after all. In any case, this thesis is vehemently advocated by the young Hungarian scientist Anna Ijjas from the Max Planck Institute for Gravitational Physics in Hanover. She thinks she has good arguments for a cyclical universe that exists forever in an interplay of expansion and contraction. It even manages without a "big bang", because even before a universe that is contracting again after expansion changes into the cosmological singularity, there is a "original impact" and the next expanding universe is born. However, no cosmic futurologist currently knows which scenario is the most likely of all.

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