Who are we? Where do we come from? What is our purpose? These are questions that have inspired, and often tormented, philosophers, theologians and scholars since ancient times, in search of answers that we may never have. Science, then, has tried to explain life and the reality that surrounds us "traveling" in time to the origin of the universe, the primordial moment from which everything began. Origin that, according to astrophysicists around the world, was triggered by a giant explosion: the Big Bang.
According to the theory of the scientist Georges Lemaître, later supported and developed by George Gamow, during its birth the universe would have expanded from a point of infinite density until self-generated. Consequently, the cosmological model of Big Bang is based on the idea that the universe began to expand at very high speed in a finite time in the past from a condition of extreme curvature, temperature and density, generating spacetime. Not only that, it is believed that this process continues to this day.
What is the universe
Before we too launch into deep space and get to somehow explain the origin of the universe, it is appropriate to define what exactly is meant by the definition Universe, in scientific terms of course. According to the literature, the universe is commonly defined as the complex that encompasses all of space and what it contains, namely matter and energy, planets, stars, galaxies, and the contents of intergalactic space.
The observable part of it, at least currently and with the technologies available to us, is about 93 billion light years in diameter, suggesting to experts that it has been governed by the same physical laws and constants for most of its history and throughout the same observable extent. Not only that, its "structure" absolutely allows inferences in the initial parts, making it particularly difficult to reconstruct its history with exactitude.
In any case, as known, the theory of the Big Bang, in Italian "Grande Scoppio", is the most accredited cosmological model that describes the birth of the universe. To hear the calculations, based of course on our local time frame, this event would have occurred about 13.8 billion years ago. Theoretically, the maximum observable distance is contained in the observable universe. Studies of several supernovas have shown that the universe continues to expand steadily, and many models have been created to predict its ultimate fate. In 1929, Edwin Hubble announced the discovery of the expansion of the universe.
By observing various galaxies, he was able to discover that most of them were moving away from Earth, considering that their spectra were oriented towards the red. He also observed that the greater was their distance from Earth and the higher was their speed of removal. A situation that is explained by the first cosmological principle, according to which "the structure and properties of the universe on a large scale are everywhere and at all times the same". In short, it is the famous formula v = H r, where v is the speed of receding, r the distance and H a number called Hubble constant, is 53 Km/s per million parsec.
Origin of the universe: the theory of Big Bang
The observations of Hubble put in crisis the dominant conception that saw the universe as stationary. If the universe is expanding, its density must in fact necessarily be variable. Since then, the theory has been appropriately modified with the statement that there must be a continuous creation of matter equal to one hydrogen atom per cubic centimeter every million billion years. Only later it has been assumed that the universe was born from an explosion occurred over 13 billion years ago and that the motion of receding was the consequence of this explosion.
Explosion known as Big Bang. Physicists, on their part, are uncertain about what preceded the great cosmic explosion: some propose models of cyclic universe, others describe an initial state without boundaries, from which spacetime has gone to emerge and expand at the time of the Big Bang. Others even hypothesize that our universe is only one of many that may exist.
The Big Bang model, a term coined by Fred Hoyle in 1949 during a radio program on the BBC, and used at the time with derogatory meaning, is universally recognized as the closest model to explain the origin of the universe, predominant in the scientific community on the basis of evidence and astronomical observations. Specifically, the good correspondence of the cosmic abundance of light elements such as hydrogen and helium with the values predicted as a result of the process of primordial nucleosynthesis, combined with the existence of the cosmic background radiation, with a spectrum in line with that of black body, have convinced most scientists that an event similar to the Big Bang occurred more than 13 billion years ago.
When humanity was only the projection of a very distant future. Before the Big Bang, it is imagined that the universe was concentrated in a sphere, with infinite temperatures and densities. Inside it there had to be a plasma, that is a particular state of matter in which nuclei and electrons are free to move chaotically, and that is made of quarks and gluons. The decidedly unstable situation caused the explosion, followed by four successive phases, commonly identified as "eras": the quantum age era, the electroweak era, the radiation-dominated era, and the matter-dominated era.
The Big Bang theory is based on two fundamental assumptions, namely the universality of the laws of physics and the cosmological principle that states that on a large scale the universe is homogeneous and isotropic. Although the reconstruction is accurate and credible under certain conditions, the theory as often happens has limitations. It is enough to think that, ideally proceeding backwards in time, in an inverse process to expansion, density and temperature increase until an instant in which around these values tend to infinity and volume tends to zero. The observation implies that the current physical theories are no longer applicable, generating the process commonly known as singularity. Just for this reason the Big Bang seems not adequate to explain the initial condition, but it provides a good description of the evolution of the universe from a certain moment onwards of its long history.
Beyond the Big Bang
The origin of the universe according to the Big Bang model is therefore based on theories certainly reliable and confirmed by observations only for the description of the evolution of the universe from the primordial nucleosynthesis onwards. Consequently, subsequent statements about the global shape of the universe and its evolution in the distant future are particularly uncertain. Given the finiteness of the speed of light, the observation is limited by what we can define as an insurmountable horizon, which does not allow to make other extrapolations.
The model, then, is based on assumptions related to the topological properties of space-time and its regularity and is in fact only and exquisitely hypothetical. The current technological progress, and the recent observation of dark energy, however, have allowed us to refine our knowledge of the universe, including its evolution.
Between the new best known theories, we have that the universe would continue to expand up to a certain point, then going to stop its growth. For other experts, the speed at which the universe expands will progressively reduce and the acceleration reached 0 will become negative, leading to a Big Crunch and returning to the state of singularity. For others, the universe will never stop expanding, generating cyclically the conditions for the existence of life forms, even if primordial.
The last hypothesis is linked to the name of Alan Guth, that in 1984 has been accepted by most of the scholars, and considered extremely credible. Certainly, the Big Bang theory, formulated by Alexander Friedmann in 1929 and completed by George Gamow in 1940, maintains its authority, even if it recognizes its own limits. What was there before the Big Bang remains for now a mystery, since astrophysicists refuse to advance too many hypotheses, leaving us a story that perhaps will never have an explanation capable of satisfying everyone.
Origin of the universe: the alternative theories
Treating a subject so vast and typically divisive, the origin of the universe is a question that has necessarily inspired many minds. Leading to as many theories and the subsequent birth of a non-standard cosmology. With this term, as you may have already understood, we refer to any cosmological model of the universe that has been, or still is, proposed as an alternative to the Standard Model of cosmology, or its reference to the Big Bang. In the history of space observation, there have been many researchers who have explicitly criticized the "Big Bang" model, either by rejecting its fundamental assumptions altogether, or by adding new ones deemed crucial to establishing new theoretical models of the universe.
Between the 1940s and 1960s, in particular, the scientific community was sharply divided between fervent supporters of the Big Bang and those of competing theories based on the steady state model. Today, there are very few astrophysicists who discuss its actual validity. Among the exponents of non-standard cosmology deserve a mention several names of the scientific world active especially in the twentieth century, such as Fred Hoyle, Paul Dirac, Kurt Gödel, Geoffrey Burbidge, Margaret Burbidge, Halton Arp, Jayant V. Narlikar, Hannes Alfvén, Eric Lerner, Dennis Sciama, Ernst Mach, Thomas Gold, Hermann Bondi, Fritz Zwicky, Christof Wetterich, Mordehai Milgrom, and Johan Masreliez.