Big Bang

Cosmic inflation [ edit | change source ]

The universe has grown a lot in a very short period of time. This phenomenon caused by an inflation is called "cosmic inflation".
The Big Bang theory brought new issues to cosmology. For example, he proposed that the universe was homogeneous and isotropic [8] , but did not explain why it should be so. However, in the simple version of the theory, there was no mention of a mechanism or functioning in the realization of the Big Bang, which caused homogeneity in the universe, there was no such thing. Thus, it was assumed that the cause or justification for inflation (the initial sudden, rapid expansion) initiated a process that caused the universe to become homogeneous and isotropic.
The inventor of the concept of "cosmic inflation" is Alan Guth, who was the first to propose a descriptive scenario of such a process. [25] François Englert and Alexei Starobinsky are also known as other names who worked on some problematic parts of this issue in the same period (1980). Guth later (in 1982) found in some studies that, according to the results he put forward in these studies, cosmic inflation, which contains the seeds of large astrophysical structures, not only provided the opportunity to explain the homogeneity of the universe, but also provided the opportunity to explain why the universe should contain some phenomena contrary to homogeneity.
Inflation must have occurred in an extremely hot (at temperatures between 10 14 and 10 19 GeV, that is, between 10 27 and 10 32 degrees) and early in the history of the universe, adjacent to the Grand Unified Epoch and the Planck Epoch. The fact that almost all of the issues raised by the Big Bang theory can be explained by the inflation process, and that other scenarios, although more complex, are seen as inadequate in explaining such issues, has given the inflation scenario a more prominent place in cosmology. From the detailed observation of the anisotropies of the cosmic background [26] , it became clear that inflationary models did not need to be reinforced with evidence, as they were well established. The fact that the inflation scenario is in harmony with the observations has placed it in the leading role in all relevant issues.
The inflation phase is the extremely rapid expansion of the universe over a certain period of time. This universe, whose density decreased due to expansion, was filled with a very homogeneous type of energy. This energy was then converted into particles that began to interact and heat up very quickly. These two phases that end inflation are called the "pre-heating phase" in terms of the explosive creation of the particles and the "warming phase" in terms of the thermalization of the particles. Although the general functioning of inflation is well understood, the functioning of the pre-warming and warm-up phases is not fully understood and is still the subject of various studies.

Age of Planck — Quantum Cosmology [ edit | change source ]

Beyond (before) the inflation phase, more generally, at temperatures such as the Planck temperature , we enter a field where current physical theories are no longer valid. This is an area where the concepts of quantum mechanics are valid, where a correction of the general theory of relativity will be in question . Although it has not been put forward yet, a theory of quantum gravity that will perhaps arise from string theory , which is still under development , will enable various speculations about the universe in the so-called Planck Age. Many authors, such as Stephen Hawking, have suggested various research methods that would allow attempts to describe the universe in these periods. This field of research is today called quantum cosmology .

Standard model of cosmology [ edit | change source ]

Proportional table of the elements that make up the universe according to the ΛCDM model, which is considered the best Big Bang model. As this table prepared by NASA shows, 95% of the universe consists of dark matter and dark energy types.
The "standard model of cosmology" is a logical consequence of the Big Bang view proposed in the first half of the 20th century. The "cosmology standard model", whose name is derived by analogy from the name of the standard model of particle physics , offers a definition of the universe that is compatible with the integrity of universe observations.
In particular, it stipulates the following two points:

• The observable universe arose from a dense and hot phase (the Big Bang). A mechanism during this phase ensured that the region we could access (observe) was homogeneous, but also showed some exceptions. This is probably an inflation type mechanism, although there are other mechanisms suggested.
• The current universe is filled with many types of matter:
  • Photons , which are particles representative of all types of electromagnetic radiation .
  • Neutrinos.
  • Baryonic matter that makes up atoms .
  • Dark matter , called dark matter , is one or more types of matter more massive than the stars that make up them, which is responsible for the structure of galaxies and is predicted by particle physics , although it has not been produced in a laboratory environment.
  • So-called dark energy is a type of energy with unusual properties that is responsible for the "acceleration of the expansion of the universe" observed today (and is probably not directly related to cosmic inflation).

A large part of astronomical observations now make use of these indispensable cornerstones when describing the universe as we know it. Cosmological research has mainly aimed to identify these types of matter, their properties, and the accelerated expansion scenario of the primordial universe. The three cornerstones of the "standard model of cosmology" require reference to physical phenomena that have not been observed in the laboratory: Cosmic inflation, dark matter and dark energy. There is no satisfactory cosmological model that assumes the absence of these cornerstones or any of them.

Another issue encountered when examining the evolution of the universe is the possible "radius of curvature" (the distance from the center of a sphere or ellipzoid object to the surface; if the object in question consists of a curved surface, the radius can be obtained by completing the curved surface into a spherical object). General relativity reveals that if the distribution of matter in the universe is homogeneous, then the geometry of the universe depends on only one parameter , the so-called “spatial curvature” [29] . Intuitively, one could say that this quantity relates to a distance scale beyond "euclidean geometry", which would no longer be valid in the circumstances in question. For example, the sum of the interior angles of a giant triangle whose vertices span several billion light-years may not equal 180 degrees. Although not confirmed, it is quite normal to encounter such phenomena in situations where distances greater than the distances of the observable universe are involved. [30]
However, another issue arises when the length scale called the "radius of curvature" tends to become smaller and smaller compared to the size of the observable universe . In other words, if the "radius of curvature" was larger than the size of the "observable universe" five billion years ago, it had to be smaller than the size of the "observable universe" today and the mentioned effects or consequences had to become visible. Continuing this reasoning, it can be said that the radius of curvature was infinitely larger than the size of the observable universe at the time of nucleosynthesis, since the effects or consequences due to curvature are not yet visible. The phenomenon that the radius of curvature remains larger than the radius of the observable universe is today called the flatness problem [31] .

The issue of monopoles [ edit | change source ]

Particle physics predicts that new particles gradually emerge during the cooling of the universe resulting from its expansion.
Some of these must have emerged during an event called phase change , which is thought to have occurred in the primordial universe . These particles, some of which are called monopolar or magnetic monopolar [32] , were stable, had to be numerous and extremely heavy ( 10 to 15 times the mass of a proton is one of their typical properties). If such particles had arisen, their contribution to the density of the universe would have been considerably higher than that of ordinary matter.
However, although the universe owes some of its density to types of matter that we do not know much about, there is absolutely no room in the universe for particles with an exceptional ratio such as that of monopoles. The issue of such heavy particles, which have not been determined whether they actually exist because they have not been discovered, although predicted by particle physics, is called the issue of monopoles.

The issue of the formation of structures [ edit | change source ]

Although observations show that the universe is homogeneous on large scales, it also shows that it contains deviations from homogeneity on small scales (planets, stars, galaxies, etc.), that is, it is not homogeneous.
Today, it is known and explained how, when certain conditions are met, a small inhomogeneity in the distribution of matter grows and develops until it creates an important astrophysical object that is denser than its surroundings. This is called the Jeans Instability mechanism. However, for such a process to occur, the existence of a small inhomogeneity must first be assumed, and furthermore, the diversity of observed astrophysical structures shows that the distribution of these inhomogeneities in terms of width and size follows a precise law known as the "Harrison-Zel'dovich spectrum". is subject to The first Big Bang models were inadequate to explain such turmoil or instability. Therefore, when the first Big Bang models were put forward, the issue of the formation of structures arose.

Suggested solutions [ edit | change source ]

About the horizon issue [ edit | change source ]

Panoramic view showing the distribution of galaxies beyond the Milky Way.
The issue of horizon and the issue of planarity can be considered within the scope of the same issue. As time progresses, expansion continues and larger regions containing more and more matter are passed. It is surprising that galaxies, whose number increases visibly as time progresses, have the same properties.
One solution to this problem lies in the idea that, early in the history of the universe, certain information about the state of the universe spread extremely rapidly throughout the universe. In such a case, regions of the universe that are extremely distant from each other may have exchanged information that would enable them to form similar formations. The obstacle to this solution is the special theory of relativity ; The special theory of relativity stipulates that nothing can move faster than light.
However, although the expansion of the universe has been very rapid, the limits of special relativity may have been somehow exceeded. In fact, in such a case, the distance between two regions of the universe could increase exponentially while the size of the observable universe remains constant. In other words, a region that is initially very small and homogeneous has the opportunity to reach an extremely large size compared to the region of the observable universe . When this phase with a constant expansion rate is completed, the homogeneous region of the universe in which we are located may be extremely larger than what we observe.
Friedmann equations show that such scenarios are possible, provided that the existence of an atypical type of matter in the universe is accepted.

About the flatness issue [ edit | change source ]

The perception of the curvature of a spheroid depends on the relative size of the region over which the measurement is made. As this dimension increases, the curve becomes increasingly visible. In the diagram, the global surface represents the expanding universe, and the colored (pink) part represents the observable part whose relative size increases over time. (Attention! The universe is not a sphere; in fact, it is represented by a surface here too.)
The problem of flatness can be solved in the same way. The gist of the matter is this: the "radius of curvature" is growing less rapidly than the size of the observable universe. However, this can no longer be true if the law governing expansion is different from the law governing the expansion of a universe filled with ordinary matter. Assuming the existence of a type of matter with atypical properties (e.g. negative pressure), the "radius of curvature" will grow faster than the size of the observable universe. If such an expansion phase occurred in the past and lasted long enough, it is not surprising that the radius of curvature is not measurable.

About the issue of unipoles [ edit | change source ]

The issue of magnetic monopoles can be solved by an accelerated expansion phase. This tends to reduce the density of all ordinary matter in the universe. However, in this case, a new issue arises: The accelerated expansion phase leaves behind a homogeneous but matter-free universe in the form of a spatial plane without bumps or pits.
The "cosmic inflation" scenario proposed by Alan Guth in the early 1980s was a solution that resolved all of these problems. In this solution, it is the type of "atypical matter" that has all the necessary properties that causes the accelerated expansion phase. [33] In the solution, the "scalar field" responsible for this expansion phase, which becomes unstable (variable) as a result of the accelerated expansion, [34] is gradually transformed into the "standard model" during the complex processes called "pre-warming" and "warming". ” [35] It breaks down into particles.
Although the first models presented regarding cosmic inflation had various technical problems, the later proposed models were developed to a reasonable state by eliminating these technical problems. An alternative solution to the cosmic inflation solution of the monopoles, planarity and horizon issues is presented by the Weyl curvature hypothesis [36] . [37]

About the formation of large structures [ edit | change source ]

In cosmic inflation, there are quantum fluctuations or fluctuations of all types of matter (as a result of Heisenberg's uncertainty principle ). One of the unexpected consequences of inflation is that these turbulences, initially quantum in nature, evolve to become ordinary classical densities during the “accelerated expansion phase.” Spectral calculations of these turbulences carried out within the scope of the "cosmological disturbances theory" revealed that these turbulences follow the pressures of the "Harrison-Zeldovitch spectrum" [38] .
Thus, cosmic inflation allows us to explain the emergence of small escapes or deviations from homogeneity in the universe. The unexpected success of the first cosmic inflation model led to the preparation of a more developed version: According to this model, the details of the small inhomogeneities created during the cosmic inflation phase could be the initial causes of the inhomogeneities in our current universe. The agreement between these predictions and the observations made by examining the data on "cosmic background fluctuations" observed by the COBE and WMAP satellites is interesting. This harmony, which is also seen in the results of the study called "galaxies catalogue" prepared by the SDSS (Sloan Digital Sky Survey) [39] team, dates back to the 20th century. It reveals one of the great achievements of cosmology.

Dark matter [ edit | change source ]

Main article: Dark matter
Strong gravitational lensing observed within Abell 1689 with the Hubble Space Telescope indicates the presence of dark matter - Enlarge image to see lensing curves
Various observations made in the 1970s and 1980s proved that there is not enough visible matter to explain the apparent influence of gravitational forces within and between galaxies. This determination naturally led to the conclusion that a maximum of 90% of the matter in the universe consists of a type of matter (dark matter) that does not emit light or interact with normal baryonic matter. Dark matter, in short, is a type of matter that does not emit light or does not reflect electromagnetic rays sufficiently to be directly detected. Although the existence of dark matter was initially a controversial issue, its existence has subsequently been well established by various observations, in particular the following: anisotropies in the cosmic microwave background [26] , velocity losses in galaxy clusters , large scale distributions of structures, and X-ray measurements in galaxy clusters. [40] Although no dark matter particle has been produced in a laboratory environment, evidence of the existence of dark matter is found especially in its gravitational effect on other matter. So far, many particles that could be dark matter particles have been presented to the scientific community as candidates, and many projects have been initiated to reveal or discover dark matter particles. [41]

Dark energy [ edit | change source ]

Main article: Dark energy
Measurements of the “ redshift ”-“apparent magnitude ” relationship in type Ia supernovae have shown that the expansion of the universe has accelerated since the universe reached half its current age. In explaining this acceleration, "general relativity" required that some of the energy in the universe must be composed of an element with large negative pressure, and this element or energy is today called "dark energy". The existence of dark energy is also understood in other ways.
Negative pressure is a type of vacuum energy. But it can be said that the true nature of dark energy is a remnant of one of the great mysteries of the Big Bang. According to some, it is a cosmological substance or a constant. The WMAP (Wilkinson Microwave Anisotropy Probe) satellite team in 2008, combined with data from the “ cosmic microwave background radiation ” and data from other sources, concluded that 72% of the present-day universe consists of dark energy, 23% dark matter, and 4.6% regular energy. showed that it consists of (ordinary) matter and less than 1% of it consists of neutrinos [42]. Although the energy density in matter decreases with the expansion of the universe, the density of dark energy remains constant. As a result, although matter has constituted a significant part of the entire energy of the universe in the past and still constitutes a considerable part, its contribution to the universe will decrease in the distant future and dark energy will become even more dominant.
In the ΛCDM model [43] , which is the current best Big Bang model, dark energy is explained by the existence of a cosmological constant in the general theory of relativity . However, the size of the constant that nicely explains dark energy is surprisingly small when it comes to predictions based on ideas about quantum gravity . The distinction between the cosmological constant and other dark energy explanations is already an area of ​​research, an active area of ​​study that is the subject of ongoing research.

Different cosmological models that accept cosmic inflation [ edit | change source ]

According to some models based on string theory, universes built on branes float in a multidimensional "superuniverse."
The belief that the Big Bang is based on the first or beginning moment of the history of the universe is a false belief. The Big Bang only shows that the universe went through a dense and hot period. There are various cosmological models that describe this dense and hot phase in very different ways.
In one of the first models presented, Georges Lemaître assumed an initial state in which the density of matter was the density of nuclear matter (10 15 g/cm³). Lemaître rightly thought that it was difficult to claim to know the behavior of matter at such densities with certainty and assumed that it was the disintegration of this unstable giant atomic nucleus that initiated the expansion. Lemaître had previously pointed out in 1931 that in describing the first moments of the history of the universe it would always be necessary to resort to quantum mechanics [44] and that the concepts of space and time would probably have lost their usual qualities. [45]
Today, different models have been created that complement the inadequacies of the classical Big Bang models and address cosmic inflation and the Big Bang from a different perspective. Some cosmic inflation models assume an infinite (eternal) universe, some models such as the pre-Big Bang assume that the initial state was not very dense, but then went through a rebound phase, and some models based on string theory assume that the "observable universe" is beyond four-dimensionality. They assume that it is immersed in space. [46] According to the latter models, the Big Bang and the expansion movement resulted from the collision between two branes [47] . [48] Some models also liken the movement of the universe to a repeating pulse (expansion and contraction).
In conclusion, it must be reiterated that the universe we observe was born from the Big Bang. According to the Big Bang theory, the elementary particles we know today were formed in this dense and hot period, and all the structures we observe in the universe were formed in the following periods.

Formation [ edit | change source ]

The conditions prevailing in the region of the observable universe during the initial period of the Big Bang were the same everywhere. On the other hand, it is seen that material elements are rapidly moving away from each other due to the expansion of the universe. The term Big Bang has been proposed as a term to express the violence of this expansion movement.
The Big Bang has no center or specific direction. What the universe was like in the past can only be understood by observing distant regions of the universe. The more distant a region in the universe can be observed, the more distant past in the history of the universe can be detected. However, what can be observed today is not directly the first period of the Big Bang itself, but the "cosmic background radiation", which is the luminous reflection of this hot phase in the history of the universe. This radiation is essentially uniform and can be observed in all directions. This shows that the Big Bang occurred in a very homogeneous manner in the regions where it was possible to observe it. The reason why the initial state of the Big Bang cannot be detected is that the primordial universe had a dim radiation due to its high density.
Contrary to popular belief, the Big Bang was not an explosion that occurred anywhere. The Big Bang or Big Bang is not, as some people think when they first hear the name, an explosion that took place at any point and threw out the matter that forms today's galaxies.

Philosophical consequences [ edit | change source ]

According to some philosophers, the solution proposed by the Big Bang, or at least in its simple model, was deemed compatible with the creationist idea. According to these philosophers, the basic idea was developed on the basis that it was compatible with the "Universe with a Beginning" proposed by Creationism. While the scientific community viewed the theory with suspicion, the majority of the public soon accepted this as confirmation of Creationism. In addition to previous interpretations of the beginning of the universe in theology and philosophy, this scientific development led to the confirmation or questioning of previous movements by different commentators in the fields of philosophy and theology. This point was made by Pope Pius XII. It was specifically stated by Pius. [ citation needed ] According to some, the chronology suggested by the Big Bang appeared to be contrary to the views of the founders of gravity theories such as Newton and Einstein , who believed that Creation was infinite. Lemaître had a different point of view than the one expressed by the Pope. On the other hand, there were those who claimed that Lemaître's religious beliefs helped him prepare the Big Bang model, even though it was not based on scientifically acceptable evidence. [49]
Some scientists have stated that astrology and cosmology data do not overlap with any philosophy or theology. [ citation needed ] However, some astrophysicists have argued that the subject can be associated with the existence of God. For example, American astrophysicist Hugh Ross made the following statement on the subject:
" Since time is the dimension in which events occur, if matter emerged with the Big Bang, then the cause that created the universe must be completely independent of time and space in the universe. This shows us that the Creator is above all dimensions in the universe ." [50]

Criticisms from scientists [ edit | change source ]

One of those who rejects the Big Bang theory and thinks that the theory has many aspects to criticize is Fred Hoyle, one of the architects of the "steady state theory". The following names can be given as examples of those who oppose the theory from the scientific world:

• Hannes Alfvén (1908-1995): Winner of the Nobel Prize in Physics in 1970 for his work in plasma physics. He completely rejected the Big Bang. He defends his own theory, the "plasma universe" theory.
• Edward Arthur Milne (1896–1950): Drawing on Newtonian cosmology, he argued that expansion was nothing but the movement of galaxies in a static universe.
• Arno Allan Penzias and Robert Woodrow Wilson: They were awarded the Nobel Prize in Physics in 1978 for their discovery of cosmological thermal radiation in 1968. Their discovery was later called the «cosmic microwave background radiation».

Despite its undeniable successes, some parts of the scientific world still oppose the Big Bang today. There are also some astronomers on this opposition side. Examples of these opponents include Geoffrey Burbidge, Fred Hoyle and Jayant Narlikar, who have developed a new version of the "steady state" [51] based on the creation of matter. [52] A recent criticism of the Big Bang concerns the discrepancy between the age of some distant cosmic objects, such as the Abell 1835 IR1916 and HUDF-JD2 galaxies, and the age of the younger universe. But most of the time, such problems arise from bad age estimates.

Current status [ edit | change source ]

The Big Bang theory is essentially based on two basic ideas: the universality of physical laws and the cosmological principle. The cosmological principle, as mentioned before, assumes that the universe is homogeneous and isotropic at macroscales. These ideas were previously hypotheses, but today they are supported by observations.
Observational developments in the field of observational cosmology provide definitive support for the Big Bang, at least this view is common among researchers working in this field. [53] The "steady state theory", which is the basic theory against the Big Bang, has become completely marginal today due to its inability to explain observations of the cosmic background radiation, the emission of light elements and the evolution of galaxies.
The Big Bang is actually a result of general relativity, which observations still cannot disprove. [54] Therefore, according to some, rejecting the Big Bang means rejecting general relativity.
On the other hand, it is a fact that many periods or phenomena are still not well known. For example, the baryogenesis period, when there is a slight excess of matter compared to antimatter , and details of the end of the cosmic inflation phase, especially the pre-heating and warming phases... Although Big Bang models, which have areas to be improved, are still under development, we can no longer grasp the general concept of the Big Bang. It has become difficult enough to argue.

According to the Big Bang theory, the future [ edit | change source ]

Illustration of a region of a universe undergoing the Great Crunch
Before the existence of dark energy was understood, cosmologists had developed two scenarios about the future of the universe. If the "mass density" of the universe was greater than the "critical density" [55], the universe would enter the collapse process after reaching its maximum size. It would become denser and hotter, and it would complete this process with a state similar to its initial state, called the "Big Crunch" [56] . [57] Alternatively to this scenario, if the density of the universe was equal to or below the "critical density", the expansion would slow but never stop. Star formation in interstellar gases would stop in all galaxies, and stars would turn into white dwarfs , neutron stars and black holes . The collisions between them would gradually lead to the formation of mass accumulation, that is, the formation of more massive objects and gradually becoming larger black holes. The average temperature of the universe would inexorably approach "absolute zero" (thermal death of the universe). Moreover, if the proton remained unstable, baryonic matter would disappear, leaving behind only radiation and black holes. Eventually, black holes would evaporate (disappear) by emitting " Hawking radiation ". Thus, the entropy of the universe would escalate to a point called the “thermal death of the universe” [58] from which no form of organized energy could save itself.
Modern "rapid expansion" observations show that today's "visible universe" will gradually slide beyond our "event horizon" and out of our contact possibilities. The subsequent situation or final outcome is unknown. The most advanced Big Bang model , the ΛCDM model, considers dark energy as a form of " cosmological constant ". This theory or model assumes that only limited gravitational systems such as galaxies can stay together, as they cannot escape thermal death. Other explanations for dark energy, called “phantom energy theories,” suggest that eventually galaxy clusters, stars, planets, atoms, etc. will separate through eternal expansion. [59] This is called the Big Rip [60] .