A possible definition of the Universe is that it is everything that influenced us in the past, influences us in the present and that may influence us in the future. This means that anything that can be discovered belongs to our Universe, as this thing has somehow influenced us. If there is anther Universe, it cannot be discovered.
The known Universe is made up of galaxies, stars, nebulae, planets, satellites, comets, asteroids and radiation. It is possible that there is also matter in a form not yet detected. The currently known Universe has a radius of about 20 billion light years, containing about 100 billion galaxies, including Our Galaxy, also called the Milky Way. An age of about 20 billion years is assumed for the Universe. The study of the origin and evolution of the Universe is called Cosmology.
There are several theories about the origin and evolution of the Universe:
Static Model
Stationary (or stable) model
Expansive Model
Cyclic Model
Static Model
Cosmological Principle: The Universe looks the same to any observer, except for local characteristics. In the static model, it is assumed that the Universe had, has and will always have the same aspect, that is, that it does not undergo any type of evolution. This model has the drawback of being contradictory to observations, which show that important changes occur in the elements that make up the observable Universe.
Stationary Model
Observations show that the Universe is expanding. This contradicts the Static model, as it implies a decrease in the density of the Universe. To get around this problem, this model defines the Perfect Cosmological Principle: the Universe looks the same to any observer at any time, except for local characteristics. To guarantee the maintenance of the density of the Universe, despite the expansion, this model assumes the spontaneous generation of matter. There is no evidence that this hypothesis is valid, but there is also nothing to refute it.
Expansive Model
By observing the differences between the colors of light that galaxies emit and those that we receive from them, it can be seen that galaxies are moving away from each other. The apparent change in color is called the redshift, or “redshift”, and the movement away from these galaxies is known as the Recession of the Galaxy. Knowing the expansion of the Universe, a new definition of the Cosmological Principle can be stated: The Universe is homogeneous and isotropic for any observer that participates in its expansion. Astronomer Hubble discovered that the further a galaxy is from us, the faster it is moving away from us. This principle is called Hubble's Law.
There is no doubt that the Universe is expanding. But will this expansion continue at the current pace, slow down or increase the pace?
Cyclic Model
If the mass of the Universe is greater than a certain critical value, then the gravity of the Universe is large enough to gradually stop the expansion and impose a contraction process on the Universe. Current estimates of the mass of the Universe give a value slightly lower than the minimum necessary for the Universe to undergo this contraction, but...! How is the mass of the Universe determined? By sampling. A certain region of space is imagined that can be considered as representative of the density of the Universe. The mass of this region is calculated, taking into account the stars, nebulae and dust existing there.
It turns out that there are indications of the existence of Black Holes, which, because they are difficult to detect, are not included in this estimate. Also, it seems that there are neutrinos that have mass. As neutrinos are by-products of nuclear fusion that occur inside stars, there must be a very large number of neutrinos scattered throughout the Universe. If this is true, it is very likely that the mass of the Universe is greater than what is currently estimated, in which case it is possible that the mass exceeds the critical mass, causing the Universe to stop expanding and start contracting.With this contraction, it is permissible to think that after a certain moment the expansion starts again and that the process is cyclical.
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