Big Bang
Big Bang
- The formation and expansion of the universe. According to the Big Bang model, the present-day universe emerged from an extremely dense and hot state a little more than 13.5 billion years ago and continues to expand today. The space containing galaxies is expanding metrically.physical cosmology
Big Bang · Universe - age of the universe
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The big bang (English: Big Bang ) is a theory and widely accepted [1] cosmological model of the evolution of the universe that postulates that the universe expanded from a point called the singularity point at its earliest 13.8 billion years ago . This theory , which was first put forward by Russian cosmologist and mathematician Alexander Friedmann and Belgian physicist priest Georges Lemaître [2] in the 1920s, has been widely accepted among scientists , especially physicists [3] , as it is supported by various evidence .
The basic idea of the theory is that the universe, which is still expanding, expanded from a hot and dense point, that is, the singularity point, at a certain time in the past. This hypothesis, which Georges Lemaître once called the "first atomic hypothesis", has now been established as the "big bang theory". The skeleton of the model is based on Einstein 's theory of general relativity , and the first Big Bang model was prepared by Alexander Friedmann. The model was later defended by George Gamow and his colleagues and was developed and presented by adding the first nucleosynthesis event [4] . [one]
After Edwin Hubble 's discovery of the relative redshift (in the light of galaxies) in distant galaxies in 1929 , this observation was taken as evidence that very distant galaxies and galaxy clusters had an "apparent speed" relative to our position. Those that move with the highest "apparent speed" are the furthest ones. [5] Since the distance between galaxy clusters is increasing, they must all have been together in the past. According to the Big Bang model, the universe was extremely dense and hot in its initial state before expansion. Experimental results with " particle accelerators " produced under conditions similar to this initial state confirm the theory. However, these accelerators have so far only been tested in high energy systems in laboratory environments. Leaving aside the phenomenon of the expansion of the universe, it is not possible for the Big Bang theory to provide any definitive explanation for this initial state without a finding regarding the first moment of expansion. The abundance of light elements in the cosmos that we observe today, in line with the first nucleosynthesis results accepted by the Big Bang theory , coincides with the predictions that light elements were formed in the nuclear processes in the first minutes of rapid expansion and cooling of the universe (The ratio of hydrogen and helium in the universe is the remainder of the Big Bang according to theoretical calculations). It matches the required ratio of hydrogen and helium. If the universe did not have a beginning, the hydrogen in the universe should have been completely burned and turned into helium.). In these first minutes, the cooling universe should have allowed the formation of some nuclei (Certain amounts of hydrogen , helium and lithium were formed).
The term Big Bang was first used by British physicist Fred Hoyle in 1949, during his speech on a radio (BBC) program called "The Nature of Things". [6] Hoyle is a scientist who made contributions to how light elements can form some heavy elements.
Most scientists believed that a Big Bang event occurred at the beginning of the universe , but it was not until 1964/1965 that the " cosmic microwave background radiation ", or "Big Bang" in the terms used by Georges Lemaître, was detected as evidence of the hot and dense period of the universe. They were convinced after the discovery of the 'pale light echo'.
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The Big Bang model is basically based on two assumptions: Albert Einstein's theory of general relativity and the cosmological principle. [7] The general theory of relativity explains the gravitational interaction of all objects without error. The discovery of general relativity by Albert Einstein in 1915 is considered the beginning of modern cosmology, which makes it possible to describe the universe in its entirety as a physical system, since the gradual evolution of the universe is described by general relativity.
Einstein was also the first person to use general relativity in describing space in its entirety, proposing a solution arising from general relativity (“Einstein universe”). At that time, this model gave rise to a new concept with Einstein's bold initiative: the cosmological principle. According to the cosmological principle, human beings do not have a privileged position in the universe, the universe is homogeneous and isotropic . In other words, no matter where and where one looks, the universe is homogeneous in terms of space; More clearly, the general view of the universe does not depend on the observer's location and the direction he is looking. This was considered a very daring hypothesis for the time; because at that time, when the debate about whether there were objects outside the Milky Way , later called the "Great Debate", was ongoing, no credible observation could provide the opportunity to confirm the existence of objects outside the Milky Way. Although the "cosmological principle" explains the macro properties of the universe, it also implies that the universe has no boundaries, so the Big Bang did not occur at a specific point in space, but simultaneously throughout all space. [1] On a macro scale, the universe is homogeneous and isotropic. [8] These two assumptions made it possible to calculate the history of the universe after Planck's time . Scientists are still trying to identify crucial events that occurred before "Planck time." [one]
Einstein concluded from the calculations he made with the theory of general relativity , which he put forward in 1915, that the universe could not be static. But at that time, the general acceptance was that the universe was static; That's why Einstein added the " cosmological constant " factor to his equations to correct his conclusion . Thus, Einstein implicitly added to the cosmological principle another hypothesis whose degree of verification today seems to have clearly diminished; this was the hypothesis that the universe was static, meaning that it did not evolve over time. This led him to change his initial solution by adding the term " cosmological constant " to his equations. But future developments would prove that he was wrong. For example, in the 1920s, Edwin Hubble discovered that some "nebulae", which we call galaxies today , were outside our galaxy, and that they were moving away from our galaxy, and that their speed of departure was proportional to their distance from our galaxy (Hubble's Law or Hubble Constant ). Since this discovery, no data has been found to confirm Einstein's "static universe hypothesis".
Already before Hubble's discovery, many physicists such as Willem de Sitter, Georges Lemaître and Alexandre Friedmann had found other "general relativity" solutions that described an "universe expansion". The models they put forward were accepted as soon as the expansion of the universe was discovered. Thus, a universe that had been expanding for billions of years was defined.
The Big Bang and its counterpart steady-state theory [ edit | change source ]
The discovery that the universe was expanding not only revealed that the universe was not static, but also allowed many different views to be put forward, both taking into account the "law of conservation of matter" and those that did not. Of these views, the view that initially assumed the creation of matter was the most popular at first. One of the reasons for this success was that the universe was considered infinite in this model called "steady state theory" . According to the "steady state theory" put forward by Fred Hoyle, there could be no contradiction between the age of the universe and the age of a celestial body. [9]
On the other hand, in the Big Bang hypothesis, the universe had a certain age that could be calculated from the expansion rate. In the 1940s, estimates of the expansion rate of the universe were greatly exaggerated, which caused estimates of the age of the universe to be well below reality. So much so that, according to the values reported by different dating methods that determine the age of the Earth, the Earth was older than the universe. This was only one of the difficulties that Big Bang-type models previously encountered in the face of various observations. But such difficulties became history with the precise determination of the expansion rate of the universe.
Observational evidence [ edit | change source ]
An artist's depiction of the WMAP satellite , which collects data to help scientists understand the Big Bang .
Later, two definitive observational evidence fully confirmed the Big Bang models: the discovery of the " cosmic microwave background radiation ", which is energy radiation (microwave field) that can be called a remnant of the hot phase of the history of the universe, and the measurement of the release of light elements, namely hydrogen , helium , which were formed during the first hot phase. measuring the release of different isotopes of lithium .
These two observations occurred in the early part of the 20th century and firmly established the Big Bang in cosmology as the defining model of the observable universe . In addition to the almost perfect agreement of this model with cosmological observations, other evidence has begun to appear confirming the model: the observation of galactic clusters and the measurement of "cosmic background cooling" (the temperature difference between several billion years ago and today).
Cosmic background [ edit | change source ]
Main article: Cosmic microwave background radiation
Cosmic microwave background radiation
Expansion naturally tells us that the universe was denser in the past. The possibility that the universe was warmer in the past was first mentioned by Georges Lemaître in 1934; But real research on this started only in the 1940s. The first ideas that the universe should be filled with radiation that loses energy through expansion, in a manner similar to the redshift in the radiation of distant astrophysical objects, came from George Gamow.
In fact, Gamow understood that the strong densities in the primordial universe must have enabled the establishment of a thermal balance between atoms and the subsequent existence of a radiation released by these atoms. Gamow improved Lemaitre's calculations in the 1940s and put forward a thesis based on the Big Bang. There had to be a certain amount of radiation left over from the Big Bang. In addition, this radiation should be equal throughout the universe. This radiation had to have a density commensurate with the density of the universe, and therefore, this radiation had to still be present, even though its density had now decreased greatly. Gamow, together with Ralph Alpher and Robert C. Herman, was the first person to understand that the present-day temperature of this radiation could be calculated from the age of the universe, the density of matter, and the emission of helium.
Although there are some today who call this radiation "fossil radiation", it is generally called " cosmic microwave background (or cosmological microwave background) radiation ". This radiation is equivalent to a low-temperature "dark body" radiation (2.7 °K), in accordance with Gamow's predictions. We owe this somewhat accidental discovery to Arno Allan Penzias and Robert Woodrow Wilson: In the 1960s, Arno Penzias and Robert Woodrow Wilson from Bell Laboratory in New Jersey were trying to measure obscure radio waves coming from the outer parts of the Milky Way . But instead they detected radiation coming from all over the sky. This radiation had the same brightness in all directions and appeared to come from an environment with a temperature of approximately 3 °K. [10] Interestingly, Penzias and Wilson, who received the Nobel Prize in Physics for their discoveries in 1978 , would later join the ranks of scientists who opposed the Big Bang theory, such as Fred Hoyle.
The "cosmic background" discovered in 1965 is one of the clearest evidence of the Big Bang. After this discovery, cosmic background fluctuations have been studied by the COBE (1992) and WMAP (2003) space satellites.
The existence of a "blackbody" radiation can be easily explained within the framework of the Big Bang model: In the past, the universe was hot and exposed to intense radiation. In this very high density universe of the past, there were many different interactions between matter and radiation. As a result, the radiation is thermalized, meaning that its electromagnetic spectrum is that of a "blackbody". On the other hand, in the "steady state theory" the existence of such radiation is almost unverifiable (although some of its proponents state otherwise…).
Although equivalent to low-temperature and low-energy radiation, the cosmic background does not appear to be the universe's largest form of electromagnetic energy at all: About 96% of the energy is present in the form of photons in that radiation , with the remaining 4% It originates from the radiation of stars in the "visible spectrum" [11] and the cold gas in galaxies ( in the infrared ). These other two sources are undoubtedly more energetic, but they emit fewer photons. In the "steady state theory" the existence of the "cosmic background" is assumed to be a result of the thermalization of stellar radiation, which is assumed to occur by the release of microscopic iron particles. However, this model is contradicted by observational data. (Also, in this case, the "cosmic background" cannot be explained as a dark body.)
In conclusion, it can be said that the discovery of the cosmic background has historically been conclusive evidence of the Big Bang.
First nucleosynthesis [ edit | change source ]
Since the discovery of the strong nuclear force and the understanding that it is the energy source of stars, the issue of explaining the release of various chemical elements in the universe has arisen. Around the 1950s, this oscillation was tried to be explained by two different processes - suggested by two different competing views:
Proponents of the "steady-state theory" believed that hydrogen was produced continuously throughout time, gradually transforming into helium and then into the heaviest elements at the heart of stars. The fission of both helium and heavy elements continued throughout time; for while the proportion of helium increased by the phenomenon of nucleosynthesis, it seemed to decrease in proportion by the phenomenon of the production of hydrogen. In contrast, Big Bang supporters believed that all elements, from helium to uranium , were produced during the hot phase of the early universe.
The current thesis is based on both hypotheses. Accordingly, helium and lithium were indeed produced during the initial nucleosynthesis. The main evidence for this comes from the study of the emission of so-called light elements (hydrogen, helium, lithium) in distant quasars . According to the Big Bang model, their relative oscillations are tightly dependent on a single parameter that has remained constant since the first nucleosynthesis; This is related to the density of photons and the density of baryons . Based on this single parameter, which can also be measured by other methods, the release of isotopes of helium (He) and lithium (Li) can be explained. At the same time, an increase in the fission of helium is observed within nearby galaxies, which can be considered a sign of the gradual development of the “interstellar medium” through elements synthesized by stars.
Evolution of galaxies [ edit | change source ]
Hubble Ultra Deep Field image of space acquired by the Hubble Space Telescope. It shows galaxies as they were in an ancient era when the universe was younger, denser and hotter. This image was created by combining data collected by the Hubble Space Telescope from a small region of the Fornax Constellation between September 24, 2003 and January 16, 2004.
The Big Bang model assumes that the homogeneous universe was more homogeneous in the past than it is today. Proof is provided through observation of the radiating cosmic background. The cosmic background radiation shows extraordinary isotropy [8] .
In this case, astrophysical structures (galaxies, galaxy clusters) did not exist in the first period of the Big Bang, they must have formed gradually afterwards. The process behind their formation has been known since James Jeans's work in 1902; This process is known as Jeans Instability .
Therefore, according to the Big Bang model, the galaxies we observe today were formed later, and these first galaxies in the past were not very similar to the neighboring galaxies we observe in our immediate surroundings. Although the speed of light is a tremendous speed, since it is a specific speed, we only need to look at distant celestial objects to understand what the universe was like in the past. (For example, observing a celestial object that is one billion light years away from our planet also means that we see the state of that object one billion years ago, considering that the light coming from that object to the Earth departed from its source one billion years ago.)
Observations of distant galaxies showing redshift according to Hubble's Law show that the first galaxies were indeed sufficiently different from the later ones. At that time, intergalactic interactions were greater; A small number of giant galaxies emerged after mergers between galaxies. Likewise, the class formations of spiral, elliptical and " irregular galaxies " have emerged with changes over time.
All these observations of distant galaxies were made with relatively meticulous studies; Because distant galaxies are poorly illuminated (due to their distance), their good observation requires sensitive and perfect observation instruments. With the inauguration of the Hubble Space Telescope in 1990 and subsequent large observatories such as the VLT, [12] Keck [13] and Subaru [14] , the observation of large "redshift" galaxies showed us that the galaxy clusters predicted by "models of the formation and evolution of galaxies" It provides the opportunity to verify evolutionary phenomena.
The study of stars and galaxies in the first generation became one of the main subjects of astronomical research at the beginning of the 21st century.
Temperature measurement of the cosmic background at the large "redshift" [ edit | change source ]
In December 2000, Raghunathan Srianand, Patrick Petitjean, and Cédric Ledoux succeeded in measuring the temperature of the "cosmic background" in an "interstellar cloud" where they observed absorption of radiation emitted by the background quasar PKS 1232+0815 at redshift 2.57 degrees.
Examining the spectral lines would allow understanding the chemical composition of the cloud, and detecting the lines corresponding to the transitions between different energy levels of various atoms or ions present in the cloud would also allow understanding its temperature. The chemical properties of this cloud, detected by a highly discriminatory spectrometer (Very Large Telescope's UVES spectrometer), enabled the temperature of the "cosmic background radiation" to be distinguished for the first time. Srianand, Petitjean, and Ledoux determined that the temperature of the cosmic background radiation is between 6 and 14 °K (Kelvin); That is, given that the cloud was at a redshift of 2.33.771 degrees, it was in agreement with the Big Bang prediction of 9.1 °K.
Their discoveries were published in Nature, one of Britain's scientific journals. [15]
Chronology of the Big Bang [ edit | change source ]
The chronological stages of the Big Bang are explained in reverse, that is, from present to past:
A diagram of the expansion of the universe from the early Big Bang to the present day. It was prepared in 2006 with WMAP satellite data.
The universe today (+ 13.7 billion years) [ edit | change source ]
Our universe is now extremely less dense (there are only a few atoms per cubic meter in the universe) and colder (2.73 kelvin, that is -270°C) than it was in the past. Although there are some very hot astrophysical objects (stars), it can be said that the radiation that the universe is currently exposed to is very weak. The low frequency of stars in the universe plays a big role in this phenomenon, that is, the distance between a star at any point in the universe and the closest star to it is extremely large. Astronomical observation teaches us that stars and galaxies existed very early in the history of the universe, less than a billion years after the early Big Bang.
Merger [ edit | change source ]
300,000 years after the Big Bang, when the universe was a thousand times hotter and a billion times denser than it is today, stars and galaxies did not yet exist. The first visible image of the universe 300,000 years after this big bang was photographed. This photo taken by NASA's COBE satellite in 1992 appeared to be in full accordance with astrophysicists' calculations. This is the period when the density of the universe drops to a level sufficient for light to spread. Previously, the main obstacle to the propagation of light was the presence of "free electrons". During the cooling of the universe, these "free electrons" came together in atomic nuclei to form atoms . That's why this period is called the "unification period". Since it is also the period when light begins to spread, this period is also referred to as the "period of separation of matter and radiation". The radiation we call cosmic background radiation is the radiation or lights that have survived from this period to the present day. According to data from NASA's WMAP satellite in 2006, a clearer map of the universe was created 380,000 years after the Big Bang. According to these results, it was determined that 12% of the universe consists of atoms, 15% of photons, 10% of neutrons and 63% of dark matter. In the light of these results, since 12% of the universe consisted of atoms 380,000 years after the Big Bang, the beginning of the time when the first atoms began to form and therefore light could be emitted through the arrangement of free electrons around the atomic nucleus should be 300,000 years after the Big Bang. 380,000 years can only be considered as the time when the "unification period" was completed. Moreover, since a map of the situation 300,000 years after the Big Bang can be drawn using the 1992 data of the COBE satellite, the beginning of the time when light can spread freely in the universe should be accepted as 300,000 years. This is an indication that free-floating electrons first began to line up around the atomic nucleus at this time, in other words, the first atoms began to form. To accept otherwise would require acknowledging that the COBE satellite's data is invalid. Such a situation is not mentioned in NASA sources. As a result, the period of 380,000 years does not replace 300,000 years, it reflects the situation at the time when the WMAP satellite observed it to create a clearer map of the universe. [16]
Initial nucleosynthesis (+ 3 minutes) [ edit | change source ]
300,000 years after the first period of the Big Bang, the universe consisted of a " plasma of electrons and atomic nuclei " (Accepting this period as 380,000 years contrasts with the 2006 data of the WMAP satellite. Because, as stated in the paragraph above, according to the results announced by NASA, the universe It was determined that 380,000 years after the Big Bang, 12% turned into atoms.). [17] When the temperature is high enough, atomic nuclei cannot exist; In this case, we can talk about a mixture of protons , neutrons and electrons . Under the conditions prevailing in the primordial universe, nucleons could combine into atomic nuclei only when the temperature dropped below 0.1 MeV ( Electron Volt , approximately one billion degrees). However, under these conditions, it is not possible to form atomic nuclei heavier than lithium. Therefore, the only atomic nuclei formed in this phase, which starts approximately one second after the start of the Big Bang and lasts approximately three minutes, are hydrogen, helium and lithium nuclei. Therefore, this phase or period is called the first nucleosynthesis. Today, modern cosmology researchers regard the subject of early nucleosynthesis as a completed issue in terms of observation and understanding of the results.
Annihilation of electron-positron pairs [ edit | change source ]
Annihilation of electron-positron pairs
The temperature of the universe, which exceeded 0.5 MeV (five billion degrees) just before the first nucleosynthesis, which started when the temperature was 0.1 MeV ( Electron Volt ) , was equivalent to the mass energy of electrons [18] . Beyond this temperature, interactions between electrons and photons can spontaneously create electron-positron pairs. Although these pairs can spontaneously disappear, they are constantly recreated as the temperature exceeds the 0.5 MeV threshold. As the temperature drops below this threshold, almost all of these pairs disappear as photons, giving way to excess electrons arising from baryogenesis [19] .
Separation of neutrinos [ edit | change source ]
Just before this period, the temperature was above 1 MeV (ten billion degrees), sufficient for various interactions of electrons, photons and neutrinos . From this temperature onwards, these three species are in a state of “thermal equilibrium” [20] . When the universe cools, neutrinos cease to interact, although electrons and photons continue their interactions. This period is also the period of separation of neutrinos. Therefore, there is a "cosmic background of neutrinos" that has properties similar to the "cosmic background radiation" we know. The existence of a “cosmic background” of neutrinos playing an indirect role has been indirectly confirmed by the results of the first nucleosynthesis. [21] Although direct detection of the cosmic background of neutrinos is extremely difficult with current technological possibilities [22] , there has been no debate about their existence.
Baryogenesis [ edit | change source ]
It deals with subatomic particles and their interactions, in which various particles and fundamental interactions ( fundamental forces ) are treated as merely different aspects of “elementary entities” (neutron, proton, electron) (for example, electromagnetism and the weak nuclear force can be described as two aspects of a single interaction). Particle physics is based on the general idea supported by experiments. More generally, it is assumed that the laws of physics and the universe become more "symmetrical" at higher temperatures. For example, it is accepted that matter and antimatter existed as quantitative partners in the universe in the past. Current observations show that antimatter is virtually non-existent in our observable universe . [23] In this case, the existence of matter is due to a slight excess of matter over antimatter at a certain time (matter predominates over antimatter). [24] During the subsequent evolution of the universe, matter and antimatter disappeared in equal quantities, leaving behind the slightest excess of matter. Since this ordinary matter consists of particles called baryons , the phase in which this excess of matter occurs is called baryogenesis. Little is known about this phase or process. For example, the temperature gradient during this event varies between Big Bang models (this is one of the differences between different Big Bang models). The conditions necessary for baryogenesis to occur are called "Sakharov conditions" due to the work of Russian physicist Andréi Sakharov in 1967.
The era of the "grand united" [ edit | change source ]
A growing number of indications suggest that weak and strong electromagnetic forces are simply different manifestations of a single interaction (force). This situation is now generally included within the scope of the "Grand Unified Theory" (Eng. Grand unification theory or Grand Unified theory ), known as GUT in English . It is thought that this interaction or force manifests itself at temperatures above 10 16 GeV (10 29 degrees). Therefore, the universe must have probably gone through a phase where the GUT theory found application. Although its nature is still unknown, this phase would have been involved in the origin of baryogenesis and possibly dark matter .
