The interpretation of the origins of the Universe is a borderline matter, somewhere between science and philosophy: the possibility of empirical validation of the various hypotheses is often difficult, if not impossible, given that the conditions of the Big Bang and the instants following it cannot be recreated again.
Many theories have followed one another, including the Big Bang, which is the most accredited one, yes, but it is still only a theory because on its own it cannot give all the necessary answers. If for millennia cosmogony, the birth of the Universe, has been more material for myths, religions and philosophy, three scientific models are formulated in the twentieth century:
Closed Universe: also called Elliptical. The Universe would be roughly a sphere that expands to such an extent that the gravity of the matter and energy in it reverses its trend, causing it to contract until it collapses on itself and shrinks into a new singularity with the Big Crunch. The Density Parameter, called Omega and indicated with Ω0, is the ratio between the density and the critical density, which if it was to occur it would cause the infinite and eternal expansion of the cosmos. In this type of Universe, the value is greater than one, and its destiny is to collapse on itself. Moreover, in a Closed Universe, which is almost spherical, our geometric theorems, applicable on flat geometric planes, would not be valid on a cosmological level. Finally, in the Closed Universe, if one travels long enough following a straight line they would eventually return to the starting point
Open Universe: also called Hyperbolic. This model predicts a Universe that expands endlessly until it freezes with the Big Freeze, or disintegrates with the Big Rip. In this Universe, Omega is less than 1, and it will continue to expand until its self-destruction. Also, in this case our Euclidean geometry could not be applied on a cosmic level, while if we travel following a straight line we would never return to the starting point
Stationary Universe: also called Flat. The Universe would be a Euclidean space, in which Euclidean geometry would therefore apply everywhere and the hypotenuse would be equal to the sum of the squares of the catheti at every level. In the absence of dark energy, this Universe would become slower and slower, without ever stopping, while in the presence of it, it would initially decelerate its expansion, due to gravity, and then accelerate again. The possible fates of a Stationary Universe would be the same as the Open one, since Omega is equal to 1, and one indicates a situation of critical density and continuous growth.
In general, as far as the structure and destiny of the Universe are concerned, it is believed that the amount of dark energy that exists and how much it is connected to the expansion of the Universe itself, is fundamental. The most recent measurements seem to indicate that the factual nature of the Universe is flat, or very close to being flat, also because galaxies are moving away from each other. According to the Big Bang Theory, the fate of the Universe would be the Big Freeze.
The three models of the Universe: Closed, Open and Stationary
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End of universe – NASA