The Cosmic Jojo Big Bang
The big bang that started the universe was followed by a long rumble, which according to current knowledge became more and more complicated. Finally there was a peaceful silence in which timid little voices ask: "Yes, but…?"
Where does the universe come from? And why does it look the way it looks? These are just two of the really big questions that modern physics is currently grappling with and have been making scientists' heads explode for decades.
Everything seemed to have been solved perfectly. Edwin Hubble had experimentally established that distant galaxies are rapidly receding from us - the further away, the faster. Aha, he concluded, then space is apparently expanding. And if it expands and we let time run backwards in our minds, then in ancient times everything must have been concentrated on an infinitesimally small point. Sort of like a yeast dough in a tin that is unfortunately simmering in the back seat of a parked car in the blazing sun. Yeast dough like infinitely compressed space-time will, at the given point in time, comply with its urge to expand and, in the case of the dough, cause a mess, in the case of space-time it will open up a universe. The former is annoying, the latter gratifying, because thanks to this so-called Big Bang, we humans, including physicists, have received a home.
The scenario is well known and also famous among astrophobic fellow citizens because of its apparent clarity. Unfortunately, as a result, more and more experts stumbled over unexplained corners of the theory and contradictory observations. It was found that, in stark contrast to a yeast dough, the universe did not always expand more slowly with increasing age, but faster. And right after the Big Bang, it must have been particularly fixed for a short time.
In addition, it had received a certain amount of energy at the start, which should have been preserved as vacuum energy until today. For physicists, unlike laypeople, a vacuum is much more than just nothing. It has an energy density that, according to Einstein's E=mc2, constantly spawns and explodes pairs of particles. In short: there is a lot going on in the vacuum – thanks to vacuum energy. Theoretically, their density should be around 1093 grams per cubic centimeter, because theoretically it is constant and therefore theoretically as large as in the Big Bang. It's just lower in practice. Much lower. Very, very much lower. Let's say: by a factor of 10120 lower. A mere 7x10-30 grams per cubic centimeter are observed. Apparently there is a certain discrepancy between the model and the stubborn reality.
Which wouldn't be too bad. If only the density of the vacuum energy were not equivalent to the cosmological constants that Einstein introduced in his general theory of relativity as a repulsive component – the antagonist to gravitation, so to speak. An energy density obeying the theory would result in correspondingly greater repulsion, which in turn would have given the Big Bang extra boost, and with an infinitesimally short boom! the universe would have gotten so big and thin that nobody would write or read this text.
Which brings us to the humble but immensely difficult question of why we can write and read texts. Or to put it another way: Why are the actual circumstances in the universe such that life, and especially us, exist at all?
The scientific answer is: I don't know! All she has to show is speculation. For example, there is the idea that there is not one universe, but innumerable many universes, and in most things things do not fit to life. Or it's all in one universe, but that has different bubble-shaped experimentation areas with different settings. We therefore sit in an accidentally privileged corner that allows life or creates it compellingly. So this anthropic principle boils down to: We are there because we are there and therefore can ask such stupid questions.
A reasoning that doesn't fully satisfy every researcher. Among others, Paul Steinhardt from Princeton University in the USA and Neil Turok from the University of Cambridge in Great Britain yearn for an alternative explanation. They believe that the problem with the much too low energy density of the vacuum is basically just a question of age. Assuming the universe is a system that is diligently reborn, it could not go back a mere 13.8 billion years, as science currently suggests. Rather, its cumulative age could be trillions, quadrillions, trillions…fantatillion years. Under such circumstances, an extremely slowly decreasing energy density is sufficient to get from the theoretical start value to the practical current level.
As usual in science, this idea had to be analyzed and calculated mathematically. It turned out that such a cyclical universe, which spreads out in countless big bangs, at some point contracts again and expands again, would be quite possible. Fortunately, the cosmological constant assumes values that are quite life-friendly for the longest time in every speck of the universe. The anthropic "It just happens to be like that" becomes "It just happens everywhere".
Strangely enough, the professional world does not cheer with enthusiasm at the sight of the cyclical process. On the one hand, the idea of the oscillating cosmos already existed in a similar form and never really caught on. On the other hand, it doesn't explain all the oddities of space either. For example, the relationship between matter density and energy density, which also has a sensitive influence on the friendliness of life. As a result, we still don't really know much about life, the universe and everything else. But maybe one day the many interesting ideas will give us the correct answer.