Principles of cosmology.

 

                                 Cosmology.

 The beginning of cosmology.

  Edwin Hubble showed at the beginning of the XX century that the universe was in constant expansion, since he, with his rustic telescope, observed that the Andromeda galaxy had a slight red shift in its light spectrum, which was only explicable by the expansion of the universe. Breaking completely with the belief of the time inspired by Newton, that the universe was eternal and motionless, ushered the in a new era of cosmology.

Friedman, during the years 1922-1924 with Robertson and Walker, proposed a model that was the exactly solution to Einstein's field ecuations , in which an in expansion, homogeneous and isotropic universe was described and three standard models of universe were created. Three years later Lemaitre would join to the group, proposing an universe in expansion without center, with beginning in time and he was the fist in derive what now is know as Hubble's law. As counterpart to Lemaitre's theory comes the theory of a stationary universe.

Early universe.

Decades later the physicist Weinberg gives a detailed explanation of the first three minutes of the universe, at the beginning of the entire universe it would be in a "particle soup" at one hundred billion degrees, being four billion times denser than water ground. and it was expanding at great speed. One tenth of a second after the temperature dropped to thirty billion degrees, the density was thirty thousand times that of water on earth, the rate of expansion also decreased, and the balance between neutrons and protons was broken. When our universe was only 1.1 seconds, it cooled to ten billion degrees and its density was three hundred and sixty times that of water and there were 66% protons..

    At fourteen seconds the photons no longer produce electron-positron pairs by annihilation, leaving only electrons, the temperature drops to three billion degrees and the first nuclei of deuterium, tritium and helium are formed, although due to extreme conditions, these nuclei are rapidly destroyed..

  Finally, after three minutes, the temperature would be one billion degrees, the electrons and positrons are eliminated, there are more photons than neutrinos, and stable nuclei are created for the first time, through the genesis nucleus predicted by the standard model, of Hydrogen (75%) and Helium (25%).

  The Cosmic microwave background shows us how photons that move randomly without any discernible source, is the residue of the "flash" of the big bang itself. This background radiation predicted by Gamow (1948) and accidentally observed by Penzias and Wilson (1965) gives conclusions that classical theory cannot explain, the first problem is known as “The horizon problem” since the CMB shows us a extremely uniform radiation distribution, this means that remote regions of the universe that were never in contact and therefore in thermal equilibrium, have the same temperature.

Inflationary model.

  The inflationary model was the first proposal made by the american physicist and cosmologist Alan Guth in 1981 and independently Andrei LInde and Andreas Albrecht together with Paul Steinhard they gave it its modern form.

This theory tries to solve the so-called horizon problem which consist in understanding why at present the distribution of matter and radiation in the universe is practically homogeneous in all its regions from any point of view. Nevertheless there are certain inhomogeneous zones (galaxies) that previously to the inflationary model we could not explain in addition to the fact that the spacetime curvature would have to have trillions times lower to be able to explain the currently observed flatness.

    Despite the fact that the inflationary model is not proven, it makes considareble experimental predictions. This model consist of an exponential expansion of the space-time fabric in which at the beginning of time the inflaton field exists which at certain moment was excited at an specific point and gerenated a vertiginous expansion of the universe. In a fraction of a second the universe expanded at least a quintillion times; until the inflaton field was relaxed, transferring all that energy to the other fields, to be later converted into matter (big bang or reheating).

    By expanding so rapidly any space-time curvature was completely smoothed out, by way of analogy it is the equivalent of inflating a balloon. However, due to the quantum fluctuations of the inflaton field, irregularities were generated in space-time, which despite being very small, with the expansion that occurred, their size would become considerable, serving as gravitational wells for the formation of future galaxies (problem of horizon solution).

  The inflationary theory predicts that the universe must be significantly flat, which can be verified experimentally, because the density of matter in a flat universe is directly related to its expansion speed..

  In addition, this powerful inflation must have generated gravitational waves that propagate through space-time, which, although incredibly small, can be detected by the alteration they produce in the light wave of the CMB..       

    Gravitational interactions, in this case, circumvent (but do not violate) neither the first nor the second law of thermodynamics. However, while there is a consensus that this solves the initial conditions problem, it has been disputed by some, as the Universe is much more unlikely to come from a quantum fluctuation. Donald Page has been a staunch critic of inflation for this anomaly. He stressed that the thermodynamic arrow of time needed low entropy initial conditions, which could be highly probable. According to them, rather than solving this problem, the inflation theory aggravates it more - overheating at the end of the inflation era increases entropy, making it necessary for the initial state of the Universe to be even more orderly than in others Big Bang theories without inflation phase.

Higgs field.

  In the extreme conditions presented by the inflationary model, the existence of the Higgs field was theorized, which explains the fundamental origin of the mass of elementary particles, this field has its own particle, which has no spin, electric charge or color and it is also very unstable. The higgs field permeates all space and all particles when interacting with it (by friction) acquire mass, while those that do not, such as the photon, do not have any mass. And although there is nothing true about it, there are theories that state that the Higgs scalar field is exactly the same as the inflaton field.

   The higgs field is fundamental to understanding the grand unification theory, which tells us that the 4 fundamental interactions of the universe could have been part of a single extremely symmetric fundamental law. This symmetry would be visible only in a state of the universe with a temperature such as that which would have occurred in the first 10 ^ -25 seconds after the big bang and this symmetry would slowly lose itself and the universe would change phase as certain elements condensed. higgs fields. The first, not yet experimentally demonstrated, would be the highly unified Higgs, which would need a temperature greater than 10 ^ 28 Kelvins in order not to condense.

  On the other hand, there is, already experimentally demonstrated, the electroweak theory proposed by Steven Weinberg in the 60s, he tells us that prior to the condensation of the electroweak Higgs field, at a temperature of about 10 ^ 14 Kelvins and 10 ^ -12 seconds after the big bang, the electromagnetic field and the weak nuclear field merge into one and become completely indistinguishable from each other. These advances in the theory of grand unification are reflected in what is now M-theory, which comes from string theory and this theory gives us new possible advances in what is the inflationary model. An example of this is the cyclical model that Turok and Steinhardt proposed, which basically is that our universe would be a 3-brane joined by small elastic bands to another 3-brane that could be considered as another universe, these 3-branes collide and bounce generating a cycle that implies that each one of them drives the cosmological evolution of the other. The tremendous energy of the collision deposits a significant change of high radiation, energy, and matter on each bouncing brane, and the detailed properties of this matter and radiation have an almost identical profile to that produced in the inflationary model.

    However, these inflationary models fail when we consider what occurs at instant zero (in the case of the cyclical in the exact moment of collision of the 3-branes). So cosmology continues to need a rigorous solution to his singular beginning.

    Recommended books: The fabric of the cosmos -Brian Greene and The emperor's new mind -Roger Penrose.

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-Jordi Bustos