There are no coincidences, they are physics!
, we titled an article a long time ago.
The topic led from networks (one does not know whether to look at networks to cry and laugh, or stay away from strong emotions) to comments on the determinism of science and the implications of quantum physics.
Both derivatives enter into our topic today, which deals with one of the three most serious problems that the most accepted cosmological theory suffers from, which is based on the expansion of the universe from the one known as the Big Bang.
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Are you sure the Big Bang was the beginning of everything?
Among these serious problems it is not found that in our current paradigm we need dark matter that we do not find in laboratories, or that the observations imply the existence of a cosmological constant or a dark energy that explains that the expansion of the universe is accelerating.
These are minutiae compared to the fact that the universe that we observe with fascination is "so perfect", and it must have been even more so in the past, that it seems a lie.
Let's get into trouble.
Using his theory of relativity, Albert Einstein initially described, 104 years ago, a universe in which matter is uniformly distributed on large enough scales, fulfilling the so-called Cosmological Principle. But he not only considered that there was homogeneity in space, but also in time: the universe was and will always be the same. What Einstein liked was a static universe, and his equations imply that such a universe must be finite. For everything that exists to be finite, somehow a ray of light must be kept "inside the universe" (not very fortunate the phrase, by definition there is nothing outside!, But language has many limitations when faced with the modern cosmology), so its trajectory must be curved. It is what is known as a universe with spherical geometry,which does not mean that the universe is a sphere, since the concept of sphere only makes sense if there is something outside of it.
It is not easy for our minds to digest that the universe has spherical geometry and that at some point a ray of light can return to the point from which it started, as we could travel across the earth's surface in a straight line and reach the same point.
We are talking about a universe in 3 spatial dimensions, completely curved.
Again, not easy to assimilate, we like Euclidean geometry, which is what we learn in school when we draw triangles and apply the Pythagorean Theorem.
It is not easy for our minds to digest that the universe has spherical geometry and that at some point a ray of light can return to the point from which it started.
So when Einstein was informed from various sources, for example Edwin Hubble, that observations of galaxies indicated that the universe is expanding, it is not static, and when on the other hand he was told that his own equations could reliably describe that expansion, and that as a result the geometry of the universe could be Euclidean or flat, surely everyone breathed easy. Humans are very squared-minded, Cartesian, everything for us is easy if the parallel lines are always parallel, and if a ray of light will never return to us after traveling the entire universe.The priest Georges Lemaître told Einstein that his equations on the evolution of the universe could also serve to explain an expanding universe with a past in which everything was concentrated in a very small volume, a singularity, the origin of the Big Bang. That is to say, a non-static universe, something that Einstein described as based "on correct mathematical calculations", but with an "abominable understanding of physics".
From those times when modern cosmology was born, and until almost the end of the 20th century, astrophysicists tried to understand if the expansion of the universe was going to stop and reverse at some point, which implied a spherical universe; it was going to stop and become more static, which implied a flat universe; or the universe was going to expand forever, which is known as a hyperbolic universe. Things got complicated in the last years of the 20th century, when data was taken that implied that the universe would continue to expand forever, but would remain flat, even though what is known as dark energy will cause the expansion to occur at an ever-increasing speed, looking more and more like another universe that a friend of his, Willem de Sitter, studied with Einstein's equations.
The fact is that the most recent experiments that measure the past, present and future of the universe, and its geometry, which is related to this temporal evolution, tell us that the universe is flat with an accuracy of 0.5% (describe the implications of "Be flat" we leave it for another day). Of all the geometries that the universe could have, it is just flat, what our minds understand best. And not only that, those Einstein equations that can be used to describe the evolution of the universe as a whole tell us that the density of the universe shortly after the Big Bang, one billionth of a second or nanosecond (how long it takes for light to arrive from his mobile in his eyes), could only have a very precise value so that today the cosmos is as it is. That value is known up to the 25th decimal place, more or less.If the decimal number 25 had been different a nanosecond after the Big Bang, if quantum fluctuations had not had such restrictive properties, the universe would not be as it is, it would not be flat, we would not exist. The mind of astrophysicists, very Cartesian in the sense of skeptical and methodical, cannot accept it: Too much coincidence and, for some, too determinism in the history of the cosmos!
What we have just described is what is known as the flatness problem. It is an intrinsic problem of the Big Bang theory that by itself (leaving out what is known as inflation, a quite later and much less proven complementary theory) it cannot solve, it is one of its great failures. A solution may be, as happens to us on Earth, that we live and know such a small area of the universe that it seems flat to us, even though the universe actually has another geometry. It would be the end of the
flat
universe
, among which now are practically all astrophysicists, relying on data and knowing the meaning and limitations of the claim that the universe is flat.
Pablo G. Pérez González
is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute of Aerospace Technology (CAB / CSIC-INTA)
Cosmic Void
is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name "cosmic vacuum" refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of
Pablo G. Pérez González
, researcher at the Center for Astrobiology;
Patricia Sánchez Blázquez
, Professor at the Complutense University of Madrid (UCM);
and
Eva Villaver
, researcher at the Center for Astrobiology
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