The big crunch theory, the Great Crack or the Great Implosion, proposes that the universe will eventually contract to re-form a great singularity in space-time. It is, therefore, a kind of reversal of the big Bang.
If the universe had its beginnings in the violent expansion of a singularity in which all space-time was concentrated, then its end is precisely the reverse process. The idea is not new, since scientists have long wondered if gravity, the great architect of matter, will also one day be able to cause its total collapse.
The Big Crunch talks about what the end of the universe would be like if the force of gravity prevails. It is not a theory about its origin, although there is another, that of the oscillating universe, which combines the Big Crunch with the big Bang to create an image of infinite universes in constant cycles of expansion and contraction.
Although the universe is expanding at the moment, and there is much evidence to support this, the Big Crunch suggests that gravity will at some point be powerful enough to stop this expansion..
And not only stop it, but reverse it to the point of causing a continuous contraction, bringing stars and galaxies closer and closer. But it wouldn't be all, by contracting, the universe would progressively heat up to an unimaginable scale, causing the planets to become inhospitable to life..
The compression will continue until space-time and everything in it is reduced to a singularity from which a new universe could eventually be born. Or maybe not, since at the moment there is no way to know.
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Although for now the evidence that the universe is expanding is unquestionable, the force of gravity never ceases to be present, being able to become the dominant force at any moment and cause planets, stars and galaxies to come closer together..
Scientists propose that the current expansion is due to dark energy, a field that fills all of space but whose true nature is unknown, although it is thought to be generated by space itself, and increases as space grows..
And the more the universe expands, the more space is created and with it more dark energy with negative pressure, creating a scenario of indefinite and increasingly rapid expansion..
However, if you start from a closed universe, the expansion cannot go on forever and the dark energy must necessarily weaken, although it is not known when that would begin to happen. Some believe that it has already begun, even though the universe appears to be increasing its rate of expansion..
This weakening will make gravity take a preponderant role, causing the density of the universe to increase again. A density of at least 3 atoms / cubic meter is estimated to be necessary for this to happen.
In this way, the galaxies will get closer and closer, arriving at a time when they all form a colossal galaxy that will then concentrate to give rise to a unique black hole, a singularity of incredibly small dimensions..
It is a kind of big Bang in reverse, although the characteristics of this new extremely hot universe would be quite different, since the density would no longer be uniform.
According to a cosmological criterion, if the density of the universe is homogeneous, its curvature is determined by the average density, said curvature being constant. The indicator is the curvature parameter Ωo:
Ωo = average density of the universe / critical energy density
Where the critical energy density is that of a flat universe, devoid of curvature. There are three possibilities for this parameter: Ωo = 1, greater than 1 or less than 1. When Ωo> 1 we have a spherical or closed universe, in which the Big Crunch is a very real possibility.
Current measurements point to a plane geometry universe, hence the Big Crunch hypothesis does not currently have the support of the majority in the scientific community, except for some cosmologists, as we will see shortly..
The Big Crunch hypothesis comes from many scientists for whom the idea of a constantly expanding universe is not admissible. Admitting that the Big Crunch is a real possibility, the universe would have a beginning and an end, which can be reassuring for many..
On the other hand, for other scientists the Big Crunch is accepted when it is part of an endless cycle of expansions and contractions proposed in the theory of the oscillating universe, because this would avoid thinking precisely about the beginning of the universe as such, and how disturbing it is to speculate about what was there before there was.
For these reasons, many researchers continue to work on creating new models of the universe. Some have proposed modifications in the value of the cosmological constant, a constant proposed by Albert Einstein so that the solutions of his field equations would lead to a stable universe.
According to the most recent astronomical data, the cosmological constant, denoted by the uppercase Greek letter lambda, has a value of: Λ = 10-46 km-two.
Some cosmologists claim that an even lower value of this constant, which is already tiny, surely leads to a finite universe, in which the contraction of space is feasible. In this way the Big Crunch would be a viable end of the universe..
Also known as the pulsating universe theory or Big Bounce, it has many points in common with the Big Crunch.
It was proposed by the mathematician Richard Tolman (1881-1948), who postulated that the universe expands by the impulse coming from the big Bang, but then expansion stops when gravity becomes the dominant force.
The above has been happening periodically, therefore the universe does not have, nor has it ever had a beginning or end.
Apart from the Big Crunch and the oscillating universe theory, a good part of the cosmologists affirm that the universe will end up with the Big Rip rather: it is probably the expansion that ends up annihilating the matter, dividing it more and more.
And another part of scientists considers that the expansion entails a continuous cooling. As is known, the movements of the constituent particles of matter cease when they reach absolute zero, an inconceivably cold temperature that has not yet been reached..
If the universe is open, the expansion can continue indefinitely, as its temperature gets closer and closer to absolute zero. This cooling, known as the Big Freeze, will cause the eventual heat death of the universe in the distant future..
Two important facts mean that many scientists do not believe in the Big Crunch as an alternative in the evolution of the universe.
The first is that the universe is currently expanding, a fact confirmed experimentally by the observation of distant supernova stars and measurements of the cosmic background radiation, remnant of the big Bang.
Of course, there is a possibility that in the future it will stop doing it, since there is enough time for it and most importantly: there are many things that we still do not know about the universe.
The second is that measurements of the curvature of the universe suggest that the geometry of the universe is flat. And in a geometry like this, the Big Crunch is not possible. This was revealed by the results of the Planck mission, which indicate that the density of the universe is 5% greater than that required for it to be closed..
The Planck mission is a project of the European Space Agency, which consists of an artificial satellite equipped to collect data on the nature of space. It was launched in 2009 from French Guiana and is equipped with probes, detectors and telescopes.
Among those who defend the possibility of an imminent collapse of the Big Crunch-type universe are Nemanja Kaloper and Antonio Padilla. These researchers work with a model in which they have modified the value of the cosmological constant, obtaining a stable and closed universe..
Their results have been published in Physical Review Letters, however, at the moment there are no observations to support this new model.
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