Big Bang Theory Explanation – By Stephen Hawking

Big Bang Explanation - By Stephen Hawking

Big Bang Theory Explanation – By Stephen Hawking

Hi, I am Stephen Hawking. All Friedman solutions that we learned in the previous article(Friedman Discovery) have the characteristic that at some point in the past, ten to twenty thousand million years ago, the distance between neighboring galaxies must have been zero.


At that time, what we call the Big Bang, the density of the universe, and the curvature of space-time would have been infinite. This means that the general theory of relativity – on which Friedman’s solutions are based – predicts that there is a singular point in the universe.


All our theories of science are built on the assumption that space-time is smooth and nearly flat, so they will all break down at the Big Bang singularity, where the curvature of space-time is infinite.


This means that even if the events occurred before the Big Bang, one could not use them to determine what would happen after, because the prediction on the Big Bang would be broken. Similarly, if we only know what has happened since the Big Bang, we cannot determine what happened before.



As far as we are concerned, events prior to the Big Bang cannot have any consequences, so they should not form part of the scientific model of the universe. So we should take them out of the model and say that time started with a big bang.


Many people don’t like the idea that time has a beginning, probably because it smells of divine intervention. (The Catholic Church, on the other hand, held onto the Big Bang Model and in 1951 it was officially declared biblical.)


Several attempts were made to avoid the conclusion that there had been a major explosion. The proposal that gained widespread support was called the Steady-State Theory. It was suggested in 1948 by two refugees from Nazi-occupied Austria, Hermann Bondi, and Thomas Gould, along with Britain’s Fred Hoyle, who worked with him on the development of radar during the war.


The idea was that as galaxies drifted away from each other, new galaxies were constantly being formed in the intervening space, from new matter that was constantly being created. Therefore the universe will appear the same almost at all times as well as at all points in space.



The steady-state theory required a modification of general relativity to allow for the continuous creation of matter, but the rate of induction was so low—about one particle per cubic kilometer per year—that it was not opposed to experimenting.


The theory was a good scientific theory, in the sense that it was simple and made definite predictions that could be verified by observation. One of these predictions was that the number of galaxies or similar objects in any volume of space should be the same wherever and whenever they are seen in the universe.


In the late 1950s and early 1960s, a survey of the sources of radio waves from outer space was conducted in Cambridge by a group of astronomers led by Martin Ryle. The Cambridge group showed that most of these radio sources must be outside our galaxy and that there were many more weak sources than strong sources.


He interpreted weak sources as being more distant and strong sources as being closer. Then fewer sources per unit volume of space appeared for nearby sources than for distant sources.


This could mean that we were at the center of a great region in the universe with sources being smaller than elsewhere. Alternatively, it could mean that in the past the sources were much larger than they are now when the radio waves set out on their journey. Either explanation contradicted the predictions of steady-state theory.


Furthermore, the discovery of microwave radiation by Penzias and Wilson in 1965 also indicated that the universe may have been much denser in the past. Therefore, the steady-state theory had to be regretfully abandoned. 



Another attempt to avoid the conclusion that there would have been a major explosion and, therefore, the beginning of time, was made in 1963 by two Russian scientists, Evgeny Lifshitz and Isaac Khalatnikov.


He suggested that the Big Bang might be a peculiarity of Friedman’s models alone, which were only conjectures for the real universe. Perhaps, of all the models that were almost like the real universe, only Friedman would have a big bang singularity.


In Friedman’s model, the galaxies are all moving directly away from each other. So it’s no surprise that they were all in the same place at some point in the past.


In the real universe, however, galaxies are not only moving directly away from each other – but they also have small edge velocities. So really they should never be in exactly the same place, only very close together.


Perhaps, then, the current expanding universe originated not from a big bang singularity, but from an earlier contraction phase; As the universe collapsed, all the particles in it may not have collided, but they may have blown past and then drifted away from each other, causing the current expansion of the universe. How then can we tell whether the real universe should have started with a big bang?


Lifshitz and Khalatnikov studied models of the universe that were roughly similar to Friedman’s model but took into account the irregularities and random velocities of galaxies in the real universe.



They showed that such models could start with a big bang, even though galaxies were no longer always moving directly away from each other. But he claimed that this was still only possible in some extraordinary models in which the galaxies were all moving in the right way.

He argued that since there seemed to be infinitely more Friedman-like models without the Big Bang singularity than with one, we should conclude that it was very unlikely that a major bang had occurred.


However, he later realized that there was a much more general class of models like Friedman’s that contained singularities, and in which the galaxies did not need to move in any particular way. So he withdrew his claim in 1970.


The work of Lifshitz and Khlatnikov was valuable because it showed that the universe could have a singularity – a big bang – if the general theory of relativity were correct. However, this did not solve the important question: does general relativity predict that there must be a big bang in our universe, the beginning of time?


The answer stemmed from a completely different approach introduced in 1965 by British physicist, Roger Penrose. He used the way light cones behave in general relativity, and the fact that gravity is always attractive, to show that a star that collapses under its own gravity becomes trapped in a region whose boundary is eventually shrinking to zero sizes.


This means that all matter in the star will be compressed into a region of zero volume, so the density of matter and the curvature of space-time become infinite. In other words, there is a singularity within a region of space-time known as a black hole.

Black Hole Singularity


At first glance, Penrose’s result had nothing to say about the question of whether there had been a big bang in the past. However, at the time Penrose formulated his theorem, I was a research student looking for a problem with which I could earn my Ph.D. Thesis


I realized that if one reverses the direction of time in Penrose’s theorem so that the collapse becomes an expansion, then the terms of his theorem would still hold, provided the universe is on a large scale like the Friedman model at the present time.


Penrose’s theorem showed that any collapsing star must end in a singularity; The reversal of time logic showed that any Friedman-like expanding universe would have started with a singularity.


For technical reasons, Penrose’s theorem required that the universe be infinite in space. So I could use it to prove that a singularity must occur only if the universe is expanding fast enough to avoid collapsing again because only the Friedman model was infinite in space.


During the next few years, I developed new mathematical techniques to take this and other technical conditions away from theorems that prove that singularities must exist.


The end result was a joint paper by Penrose and me in 1970, which proved that there must have been a big bang singularity, provided that general relativity is correct and there is as much matter in the universe as we see.


There was much opposition to our work, partly from the Russians, who followed the party line set by Lifshitz and Khlatnikov, and partly from those who felt that the whole idea of ​​the singularity was counterproductive and spoiled the beauty of Einstein‘s theory. done.


However, one cannot really argue with a mathematical theorem. So it is now generally accepted that the universe must have a beginning.

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So these are enough for today. Comment what is your opinion about this information “Big Bang Theory Explanation – By Stephen Hawking“. Thanks for reading.


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Information Source – Stephen Hawking Book “Theory of Everything

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