The oscillating universe theory or cyclical universe proposes that the universe expands and contracts indefinitely. Richard Tolman (1881-1948), a mathematician at the California Institute of Technology, proposed a mathematically based theory of the pulsating universe around 1930.
But the idea was not new to Tolman's time, since ancient Vedic scriptures had already proposed something similar around 1500 BC, stating that the entire universe was contained in a cosmic egg called Brahmanda.
Thanks to Edwin Hubble (1889-1953), it is proven that the universe is currently expanding, which, according to most astronomers, is currently accelerating.
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What Tolman proposes is that the expansion of the universe occurs thanks to the initial impulse of the Big Bang and will stop once said impulse ceases due to the action of gravity..
Indeed, the Russian cosmologist Alexander Friedmann (1888-1925) had already introduced mathematically in 1922 the idea of a critical density of the universe, below which it expands without gravity being able to prevent it, while above it, the same gravitation prevents expansion and causes its contraction until it collapses.
Well, in his theory Tolman predicts that the density of the universe will reach a point at which the expansion stops thanks to the gravitational brake, and the contraction phase will begin, called Big crunch.
During this phase, the galaxies will grow closer and closer to form a huge, incredibly dense mass, causing the predicted collapse..
The theory also postulates that the universe does not have a specific beginning and end, as it is built and destroyed alternately in cycles of millions of years..
Most cosmologists accept the Big Bang theory as the origin of the universe, which was formed through the great primordial explosion, from a specific form of matter and energy of unimaginable density and enormous temperature..
From this great initial atom emerged the elementary particles that we know: protons, electrons and neutrons, in the form called ylem, a Greek word that the sage Aristotle had used to refer to primordial substance, the source of all matter.
The ylem it gradually cooled as it expanded, becoming less dense each time. This process left a radiation footprint on the universe, which has now been detected: the microwave radiation background..
The elementary particles began to combine with each other and form the matter we know in a matter of minutes. So the ylem it was transformed successively into one and another substance. The idea of ylem It is precisely the one that gave rise to the pulsating universe.
According to the theory of the pulsating universe, before reaching this expansive phase in which we are now, it is possible that another universe similar to the current one existed, which contracted until forming the ylem.
Or maybe ours is the first of the cyclic universes that will take place in the future..
According to Tolman, every oscillation sequence in the universe begins with a Big Bang, in which the ylem gives rise to all the matter we know and ends with the Big Crunch, the collapse in which the universe collapses.
In the period of time that elapses between one and the other, the universe expands until gravity puts a stop to it.
However, as Tolman himself realized, the problem lies in the second law of thermodynamics, which states that the entropy - degree of disorder - of a system never decreases.
Therefore each cycle would have to be longer than the previous one, if the universe were able to keep a memory of its previous entropy. By increasing the duration of each cycle, there would come a point where the universe would tend to expand indefinitely.
Another consequence is that according to this model, the universe is finite and at some distant point in the past it must have had an origin.
To remedy the problem, Tolman stated that by including relativistic thermodynamics, such restrictions would disappear, allowing an indefinite series of contractions and expansions of the universe..
The Russian cosmologist Alexander Friedmann, who was also a great mathematician, discovered three solutions to Einstein's equations. These are 10 equations that are part of the theory of relativity and that describe how space-time curves due to the presence of matter and gravity.
Friedmann's three solutions lead to three models of the universe: one closed, one open, and a third flat. The possibilities offered by these three solutions are:
-An expanding universe can stop expanding and contract again.
-The expanding universe may reach a state of equilibrium.
-Expansion can continue to infinity.
The expansion rate of the universe and the amount of matter present in it are the keys to recognize the correct solution among the three that were mentioned..
Friedmann estimated that the critical density referred to at the beginning is about 6 hydrogen atoms per cubic meter. Remember that hydrogen and helium are the main products of the ylem after the Big Bang and the most abundant elements in the universe.
Until now, scientists agree that the density of the current universe is very low, in such a way that it is not feasible to generate a force of gravity that slows down the expansion..
Then our universe would be an open universe, which could end in the Great Tear or Big Rip, where matter is separated into subatomic particles that never come back together. This would be the end of the universe we know.
But you have to take into account the existence of dark matter. Dark matter cannot be seen or detected directly, at least for now. But its gravitational effects do, since its presence would explain the gravitational alterations in many stars and systems..
Since dark matter is believed to occupy up to 90% of the universe, it is possible that our universe is closed. In that case, gravity would be able to compensate for the expansion, taking it to the Big Crunch, as described before..
In any case, it is a fascinating idea, which still has a lot of room for speculation. In the future it is possible that the true nature of dark matter, if it exists, will be revealed.
There are already experiments for this in the laboratories of the International Space Station. Meanwhile on the ground, experiments are also being carried out to obtain dark matter from normal matter. The resulting findings will be key to understanding the true nature of the universe..
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