Was the big bang a black hole?
Knowledge : After the big bang is before the big bang
Experience has shown that there is something that has preceded everything. For example, the sun and earth have not existed forever. They were formed 4.5 billion years ago from condensing gas and dust. Their hot origin was preceded by the collapse of an interstellar gas cloud. Could it be that something similar happened before the hot beginning of the universe? What was before the big bang?
Many researchers are now raising this question. Physicists like Martin Bojowald from Pennsylvania State University in the USA no longer see the Big Bang as an absolute beginning, but as a transition. His colleague Stephen Hawking from Cambridge has already tried in various ways to expand the Big Bang cosmology. According to Hawking, the Big Bang could have been preceded by the collapse of a precursor universe. And the doyen of the guild, Roger Penrose of Oxford University, now also believes that the world has always existed.
In his recently published book “Cycles of Time”, Penrose bridles the cosmic horse from behind: If our universe were to keep expanding and at some point only radiation remained, it would end in complete timelessness. All time standards were lost. But even then, a cyclical universe would be conceivable. A new world age could begin without a previous collapse. Time would simply return with a new big bang.
Penrose's theory of a universe oscillating between timelessness and time is unusual and, like his book, not easily accessible due to its mathematical complexity. But it is not the first time that he has shaken up the professional world with ambitious theses. Penrose was born the son of a geneticist in Colchester, UK, in 1931 and studied mathematics. In the 1960s he drew attention to himself with calculations on the collapse of stars.
Stars produce energy by converting hydrogen into helium and other elements. But what happens when a star that is much larger than our sun has burned its fuel? Can gravity be balanced by any internal forces even with such a massive star? Or does he inexorably collapse? Does he keep collapsing?
Researchers had long suspected that a state of infinite density could be avoided if the collapsing star is not symmetrical. Penrose came to a different conclusion. He showed that all star matter contracts to one point. This singularity is inevitable in the context of general relativity. It soon got the name "black hole".
Stephen Hawking, then a student, was impressed. He applied Penrose's mathematical techniques to the universe as a whole and traced the expanding space back to its origins. Once again, Einstein's theory led to a state of infinite density: the Big Bang, which apparently represented the beginning of time.
In the context of general relativity, the question of what was before the big bang is pointless. A timescale that extends beyond the Big Bang into the past loses all meaning. The beginning of the universe would also be beyond our knowledge. Because in the Big Bang the theory collapses.
Neither Hawking nor Penrose accepted this result, although it was derived from their own calculations and astronomical observations pointed in the same direction. In the 1960s, researchers discovered the cosmic background radiation, a kind of echo of the Big Bang (see box). The doubts about the existence of black holes have also vanished. Astronomers have long been on the trail of gigantic accumulations of matter in the heart of galaxies such as our Milky Way, as well as smaller black holes in other places in space.
Black holes curve space and take in matter like cosmic vacuum cleaners. However, cosmologists largely agree that Einstein could not have spoken the last word. To understand how matter inside black holes condenses in a tiny space or how the universe emerged from a hot core, the general theory of relativity must be combined with quantum physics. Because only quantum physics opens up the behavior of the world on a small scale.
But what happens to space-time on tiny length scales, smaller than an atomic nucleus? Is our concept of space and time completely dissolving? Is space-time becoming a quantum foam? Can the space-time continuum be replaced by discrete structures such as loops, with the help of which Bojowald risks a glimpse behind the Big Bang? Or do you need the ten- or eleven-dimensional geometry of string theory, with which Hawking is toying, to merge relativity and quantum physics?
The concepts are diverse. Even the aging string theory has hardly got beyond approaches. Penrose therefore puts on new strings. A big bang is not enough for him either. And again the mathematician from Oxford proves to be an original thinker.
The starting point for his considerations is the “dark energy”, which was only discovered in 1998. Nobody knows what is behind it. But the astrophysicists Saul Perlmutter, Brian Schmidt and Adam Riess were able to show through measurements of distant star explosions that the expansion of the universe is accelerating. For this finding, the researchers were awarded the Nobel Prize in Physics this week. So the universe is expanding faster and faster. The cause is precisely that "dark energy", a mysterious repulsive force.
This repulsion has dominated the energy balance of the universe for several billions of years and possibly for all eternity. Penrose therefore rejects the idea that space could collapse again at some point. Instead, there is much to suggest that the galaxy clusters are moving further and further apart.
The cosmos is thinning and getting colder. The last star could be extinguished in about 100 quadrillion years. Much later, the protons, the building blocks of atomic nuclei, might decay. After all, according to Hawking's theory, even black holes should evaporate. Above all, radiation would remain.
"What a depressing boredom!" Penrose said to himself. But boredom for whom? If there were hardly anything left besides massless light particles or gravitons, there would be no boredom. “The point is that there is no time for massless particles.” They all move at the speed of light. According to Einstein's theory, it would therefore take an infinitely long time before the internal clock of a light particle even made the first “tick”. In a radiation desert there would therefore no longer be any time scale. The universe would have “forgotten” time.
Penrose makes similar considerations for the Big Bang. In the hot beginning of space, all particles had such a high energy that they moved almost at the speed of light. This phase was also characterized by timelessness, at least as long as the particles had no rest mass. They probably only got their mass below a certain temperature threshold, comparable to many other phase transitions that occur when a system cools below a critical temperature value: liquid water suddenly freezes into ice, a non-conductor turns into a superconductor, a metal becomes a magnet .
The layman can hardly see any similarities between the hot initial and the cold final state of the cosmos. Penrose, however, identifies the two stages of development with one another. With a mathematical trick he purrs the final wasteland of space to one point. Since in a space filled with radiation neither time can be measured nor length scales can be defined, he no longer sees any difference between enormous expansion and extreme smallness. Not even between energies that are distributed over a larger or smaller volume, i.e. between a cold end and a hot beginning.
After the last particles have disappeared in space, a new big bang could occur in this way. The matter would then have evaporated to such an extent that it would be negligible compared to the energy of the vacuum. There would be a cosmic rebirth that has some parallels to “creation out of nothing”. The nothing that would be left would be an energy field, as it were, from which a successor universe would emerge, just as our universe would have emerged from a predecessor universe.
So far, many physicists have been skeptical about the theory. Can matter completely annihilate? How should charged particles such as electrons be made to disappear? The model cannot easily be brought into harmony with the current laws of physics. Is there any way to test the theory? Penrose is confident. He thinks it is possible to find traces of past world ages in the cosmic background radiation. Its time cycles are definitely worth considering. Because the theory solves some previous cosmological problems in an interesting way.
Most researchers now assume that our space inflated unimaginably quickly in the first fractions of a second. This “inflation” would make it understandable why space looks roughly the same in all directions.
So far, however, it is completely unclear what could have sparked this accelerated expansion immediately after the Big Bang. Penrose manages without inflation. In his theory, the phase of accelerated expansion lies before the Big Bang: at the end of the previous world age.
Cycles of time.
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