How did our universe come about? How did it turn into seemingly endless space? And what will it become after many millions and billions of years? These questions tormented (and continue to torment) the minds of philosophers and scientists, it seems, since the beginning of time, while giving rise to many interesting and sometimes even crazy theories. Today, most astronomers and cosmologists have come to a general agreement that the Universe as we know it appeared as a result of a giant explosion that generated not only the bulk of matter, but was the source of the basic physical laws according to which the cosmos that surrounds us exists. All this is called the Big Bang theory.
The basics of the Big Bang theory are relatively simple. In short, according to her, all matter that existed and exists now in the Universe appeared at the same time - about 13.8 billion years ago. At that moment in time, all matter existed in the form of a very compact abstract ball (or point) with infinite density and temperature. This state was called the singularity. Suddenly, the singularity began to expand and spawned the universe as we know it.
It is worth noting that the Big Bang theory is only one of many proposed hypotheses of the origin of the Universe (for example, there is also a theory of a stationary Universe), but it has received the widest recognition and popularity. It not only explains the source of all known matter, the laws of physics and the great structure of the universe, it also describes the reasons for the expansion of the universe and many other aspects and phenomena.
Chronology of events in the Big Bang theory
Based on knowledge of the current state of the Universe, scientists suggest that everything should have started from a single point with infinite density and finite time, which began to expand. After initial expansion, the theory says, the universe went through a cooling phase that allowed subatomic particles and later simple atoms to appear. Giant clouds of these ancient elements later, thanks to gravity, began to form stars and galaxies.
All this, according to scientists, began about 13.8 billion years ago, and therefore this starting point is considered the age of the universe. Through the study of various theoretical principles, experiments involving particle accelerators and high-energy states, as well as through astronomical studies of the distant corners of the Universe, scientists derived and proposed a chronology of events that began with the Big Bang and led the Universe ultimately to the state of cosmic evolution, which takes place now.
Promotional video:
Scientists believe that the earliest periods of the birth of the universe - lasting from 10-43 to 10-11 seconds after the Big Bang - are still the subject of controversy and discussion. Considering that the laws of physics that we now know could not exist at this time, it is very difficult to understand how the processes in this early Universe were regulated. In addition, experiments using those possible types of energies that could be present at that time have not yet been carried out. Be that as it may, many theories about the origin of the universe ultimately agree that at some point in time there was a starting point from which it all began.
The era of the singularity
Also known as the Planck era (or Planck era), it is taken to be the earliest known period in the evolution of the universe. At this time, all matter was contained in a single point of infinite density and temperature. During this period, scientists believe that the quantum effects of gravitational interaction dominated the physical, and none of the physical forces were equal in strength to gravity.
The Planck era supposedly lasted from 0 to 10-43 seconds and is named so because its duration can only be measured by Planck time. Due to the extreme temperatures and infinite density of matter, the state of the universe during this time period was extremely unstable. This was followed by periods of expansion and cooling that led to the emergence of fundamental forces of physics.
Approximately in the period from 10-43 to 10-36 seconds, the process of collision of transition temperature states took place in the Universe. It is believed that it was at this moment that the fundamental forces that govern the present universe began to separate from each other. The first step in this department was the emergence of gravitational forces, strong and weak nuclear interactions and electromagnetism.
In the period from about 10-36 to 10-32 seconds after the Big Bang, the temperature of the Universe became sufficiently low (1028 K), which led to the separation of electromagnetic forces (strong interaction) and weak nuclear interaction (weak interaction).
The era of inflation
With the appearance of the first fundamental forces in the Universe, the era of inflation began, which lasted from 10-32 seconds according to Planck time to an unknown point in time. Most cosmological models assume that the universe was evenly filled with high-density energy during this period, and that incredibly high temperatures and pressures led to its rapid expansion and cooling.
It started at 10-37 seconds, when the phase of transition, which caused the separation of forces, was followed by an exponential expansion of the Universe. In the same period of time, the Universe was in a state of baryogenesis, when the temperature was so high that the disordered movement of particles in space occurred at a near-light speed.
At this time, pairs of particles - antiparticles are formed and immediately colliding colliding, which is believed to have led to the dominance of matter over antimatter in the modern Universe. After the end of inflation, the Universe consisted of quark-gluon plasma and other elementary particles. From that moment on, the Universe began to cool down, matter began to form and combine.
The era of cooling
With a decrease in density and temperature inside the Universe, a decrease in energy began to occur in each particle. This transitional state lasted until the fundamental forces and elementary particles came to their current form. Since the energy of the particles has dropped to values that can be achieved today within the framework of experiments, the actual possible presence of this time period causes much less controversy among scientists.
For example, scientists believe that 10-11 seconds after the Big Bang, the energy of the particles has decreased significantly. At about 10-6 seconds, quarks and gluons began to form baryons - protons and neutrons. Quarks began to predominate over antiquarks, which in turn led to the predominance of baryons over antibaryons.
Since the temperature was no longer high enough to create new proton-antiproton pairs (or neutron-antineutron pairs), mass destruction of these particles followed, which led to the remainder of only 1/1010 of the number of original protons and neutrons and the complete disappearance of their antiparticles. A similar process took place about 1 second after the Big Bang. Only the "victims" this time were electrons and positrons. After the mass destruction, the remaining protons, neutrons and electrons stopped their random motion, and the energy density of the universe was filled with photons and, to a lesser extent, neutrinos.
During the first minutes of the expansion of the Universe, the period of nucleosynthesis (synthesis of chemical elements) began. Due to the drop in temperature to 1 billion kelvin and the decrease in energy density to about values equivalent to the density of air, neutrons and protons began to mix and form the first stable isotope of hydrogen (deuterium), as well as helium atoms. Nevertheless, most of the protons in the universe remained as incoherent nuclei of hydrogen atoms.
About 379,000 years later, electrons combined with these hydrogen nuclei to form atoms (again, mostly hydrogen), while radiation separated from matter and continued to expand almost unhindered through space. This radiation is usually called relic radiation, and it is the oldest source of light in the Universe.
With the expansion, the CMB gradually lost its density and energy, and at the moment its temperature is 2.7260 ± 0.0013 K (-270.424 ° C), and its energy density is 0.25 eV (or 4.005 × 10-14 J / m³; 400–500 photons / cm³). The relic radiation extends in all directions and over a distance of about 13.8 billion light years, but estimates of its actual propagation say about 46 billion light years from the center of the universe.
Age of Structure (Hierarchical Age)
Over the next several billion years, denser regions of matter, almost evenly distributed in the Universe, began to attract each other. As a result, they became even denser, began to form clouds of gas, stars, galaxies and other astronomical structures that we can observe at the present time. This period is called the hierarchical era. At this time, the Universe that we see now began to take its shape. Matter began to combine into structures of various sizes - stars, planets, galaxies, galactic clusters, as well as galactic superclusters, separated by intergalactic barriers containing only a few galaxies.
The details of this process can be described according to the idea of the amount and type of matter distributed in the Universe, which is represented in the form of cold, warm, hot dark matter and baryonic matter. However, the modern standard cosmological model of the Big Bang is the Lambda-CDM model, according to which dark matter particles move slower than the speed of light. It was chosen because it solves all the contradictions that appeared in other cosmological models.
According to this model, cold dark matter accounts for about 23 percent of all matter / energy in the universe. The proportion of baryonic matter is about 4.6 percent. Lambda CDM refers to the so-called cosmological constant: a theory proposed by Albert Einstein that characterizes the properties of a vacuum and shows the balance between mass and energy as a constant static quantity. In this case, it is associated with dark energy, which serves as an accelerator for the expansion of the universe and keeps the giant cosmological structures largely homogeneous.
Long-term predictions about the future of the universe
Hypotheses that the evolution of the universe has a starting point naturally lead scientists to questions about the possible end point of this process. If the Universe began its history from a small point with infinite density, which suddenly began to expand, does this mean that it will also expand infinitely? Or, one day it will run out of expansive force and a reverse compression process will begin, the end result of which will be the same infinitely dense point?
The answers to these questions have been the main goal of cosmologists from the very beginning of the debate about which cosmological model of the Universe is correct. With the adoption of the Big Bang theory, but largely thanks to the observation of dark energy in the 1990s, scientists came to an agreement on two most likely scenarios for the evolution of the universe.
According to the first, called the "big compression", the Universe will reach its maximum size and begin to collapse. This scenario will be possible if only the mass density of the Universe becomes greater than the critical density itself. In other words, if the density of matter reaches a certain value or becomes higher than this value (1-3 × 10-26 kg of matter per m³), the Universe will begin to contract.
An alternative is another scenario, which states that if the density in the Universe is equal to or below the critical density, then its expansion will slow down, but never completely stop. This hypothesis, dubbed the "thermal death of the universe," would continue to expand until star formation ceases to consume interstellar gas within each of the surrounding galaxies. That is, the transfer of energy and matter from one object to another will completely stop. All existing stars in this case will burn out and turn into white dwarfs, neutron stars and black holes.
Gradually, black holes will collide with other black holes, which will lead to the formation of larger and larger ones. The average temperature of the Universe will approach absolute zero. The black holes will eventually "evaporate", releasing their last Hawking radiation. Eventually, the thermodynamic entropy in the Universe will become maximum. Heat death will come.
Modern observations that take into account the presence of dark energy and its effect on the expansion of space have prompted scientists to conclude that over time, more and more space in the universe will pass beyond our event horizon and become invisible to us. The final and logical result of this is not yet known to scientists, but "heat death" may well be the end point of such events.
There are other hypotheses regarding the distribution of dark energy, or rather, its possible types (for example, phantom energy). According to them, galactic clusters, stars, planets, atoms, nuclei of atoms and matter itself will be torn apart as a result of its endless expansion. This evolutionary scenario is called the "big gap". According to this scenario, the expansion itself is the cause of the death of the Universe.
History of the Big Bang theory
The earliest mention of the Big Bang dates back to the early 20th century and is associated with observations of space. In 1912, the American astronomer Vesto Slipher conducted a series of observations of spiral galaxies (which originally appeared to be nebulae) and measured their Doppler redshift. In almost all cases, observations have shown that spiral galaxies are moving away from our Milky Way.
In 1922, the outstanding Russian mathematician and cosmologist Alexander Fridman derived the so-called Friedman equations from Einstein's equations for the general theory of relativity. Despite Einstein's advancement of the theory in favor of a cosmological constant, Friedmann's work showed that the universe was rather expanding.
In 1924, Edwin Hubble's measurements of the distance to the nearest spiral nebula showed that these systems are in fact other galaxies. At the same time, Hubble began developing a series of distance subtraction metrics using the 2.5-meter Hooker telescope at Mount Wilson Observatory. By 1929, Hubble had discovered a relationship between distance and the receding rate of galaxies, which later became Hubble's Law.
In 1927, the Belgian mathematician, physicist and Catholic priest Georges Lemaitre independently arrived at the same results as shown by Friedmann's equations, and was the first to formulate the relationship between the distance and the speed of galaxies, offering the first estimate of the coefficient of this relationship. Lemaitre believed that at some time in the past, the entire mass of the universe was concentrated in one point (atom).
These discoveries and assumptions sparked a lot of controversy between physicists in the 20s and 30s, most of whom believed that the universe was in a stationary state. According to the model established at that time, new matter is created along with the infinite expansion of the Universe, being evenly and equally distributed in density throughout its entire length. Among the scholars supporting it, the idea of the Big Bang seemed more theological than scientific. Lemaitre has been criticized for bias based on religious bias.
It should be noted that other theories existed at the same time. For example, Milne's model of the Universe and the cyclical model. Both were based on the postulates of Einstein's general theory of relativity and subsequently received support from the scientist himself. According to these models, the universe exists in an endless stream of repeated cycles of expansion and collapse.
After World War II, a heated debate erupted between proponents of a stationary model of the universe (which was actually described by astronomer and physicist Fred Hoyle) and proponents of the Big Bang theory, which was rapidly gaining popularity among the scientific community. Ironically, it was Hoyle who coined the phrase "big bang", which later became the name of the new theory. It happened in March 1949 on the British radio BBC.
Eventually, further scientific research and observations spoke more and more in favor of the Big Bang theory and increasingly questioned the model of a stationary universe. The discovery and confirmation of the CMB in 1965 finally solidified the Big Bang as the best theory of the origin and evolution of the universe. From the late 1960s to the 1990s, astronomers and cosmologists conducted even more research into the Big Bang and found solutions to many of the theoretical problems that stand in the way of this theory.
These solutions include, for example, the work of Stephen Hawking and other physicists who have proven that the singularity was the undeniable initial state of general relativity and the cosmological model of the Big Bang. In 1981, physicist Alan Guth developed a theory describing the period of rapid cosmic expansion (inflationary epoch), which solved many previously unresolved theoretical questions and problems.
In the 1990s, there was an increased interest in dark energy, which was seen as the key to solving many unresolved issues in cosmology. In addition to the desire to find an answer to the question of why the universe is losing its mass along with the dark mother (the hypothesis was proposed back in 1932 by Jan Oort), it was also necessary to find an explanation for why the universe is still accelerating.
Further research progress is due to the creation of more advanced telescopes, satellites and computer models that have allowed astronomers and cosmologists to look further into the universe and better understand its true age. The development of space telescopes and the emergence of such as, for example, the Cosmic Background Explorer (or COBE), the Hubble Space Telescope, the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck Space Observatory, have also made an invaluable contribution to the study of the issue.
Today, cosmologists can measure various parameters and characteristics of the Big Bang theory model with fairly high accuracy, not to mention more accurate calculations of the age of the space around us. But it all started with the usual observation of massive space objects located many light years from us and slowly continuing to move away from us. And even though we have no idea how this will all end, it won't take too long by cosmological standards to figure it out.