Why was space-time in a singularity
What was before the big bang?
There are different theories on this question. According to the classic Big Bang theory, our universe began with the Big Bang about 13.82 billion years ago. At that moment, space and time also emerged. According to this theory, therefore, the question of the “before” makes no sense. Newer theories that include quantum theory in their considerations assume that the Big Bang did not exist in the conventional understanding and that one or more other universes existed before our universe.
Our universe today is expanding, i.e. the galaxies are moving away from each other. If one now mentally rewinds the history of the universe, one arrives at a point in which all mass and space-time were united in one point without expansion. Physicists call this point singularity. The density and curvature of space are infinitely great at this point. Time also ends at this big bang singularity. There can therefore be no “before”. This scenario is a consequence of the theory of relativity.
At this point, Einstein's theory is no longer sufficient to describe the physical processes, because it only takes into account gravity, but not the quantum effects that played a role in the vicinity of the singularity. The researchers faced a similar problem when trying to explain why atoms are stable. According to classical mechanics, the electrons should have constantly lost energy and eventually crashed into the nucleus. Only quantum mechanics, according to which energy is not given off in arbitrary but only in discrete amounts, could explain the stability.
In order to describe the physical processes at the beginning of the universe, physicists all over the world are now trying to unite quantum theory with relativity theory to form a theory of quantum gravity. There are different models for this, of which the so-called string theory and loop quantum gravity are currently favored. What all these theories have in common is that by taking quantum effects into account, they rule out the creation of a singularity. Mass and space-time were compressed to a tiny volume, but not in an unexpanded point. This clears the way for the hypothesis that something existed before our universe.
In string theory there are subatomic threads or strings, the different vibrations of which represent different particles of matter. A scenario from the beginning of the world based on string theory assumes that our four-dimensional universe is one of many branes (derived from membrane) and thus part of a higher-dimensional universe. If two such branes collide, a big bang occurs. The kinetic energy of the branes is transformed into matter and radiation. The collision would be the beginning of our four-dimensional universe, but not the higher-dimensional universe of which it is part. How the higher-dimensional universe came into being or whether it has always existed, this theory cannot make any statement about it either.
According to the loop quantum gravity, space and time are not continuous but consist of discrete pieces. Before our universe, so the supporters of this theory suspect, there was already a universe with similar physical properties that contracted further and further down to a tiny size. Then it expanded again. So our universe began with a big bounce - the big impact. This process can be illustrated using a balloon, from which the air escapes and thus the volume decreases until the parts of the envelope collide. The walls of the balloon can penetrate each other. The process of contraction then continues, so to speak, with the opposite sign. Contraction becomes expansion, except that what was inside is now outside and vice versa. As a result, the previous universe differs from our universe in some properties. Only recently, American researchers succeeded in calculating the existence and properties of this predecessor universe.
All these scenarios are still based on theoretical considerations, but the physicists hope to soon find measurable indicators for one or the other model with the help of gravitational wave detectors, among other things.
The question was answered by Prof. Thomas Thiemann from the Max Planck Institute for Gravitational Physics in Potsdam.
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