CMB radiation
Extragalactic microwave background radiation occurs in the frequency range from 500 MHz to 500 GHz, corresponding to wavelengths from 60 cm to 0.6 mm. This background radiation carries information about the processes that took place in the Universe before the formation of galaxies, quasars and other objects. This radiation, called the cosmic microwave background radiation, was discovered in 1965, although it was predicted back in the 40s by George Gamow and has been studied by astronomers for decades.
In the expanding Universe, the average density of matter depends on time - in the past it was higher. However, during expansion, not only the density changes, but also thermal energy matter, which means that at the early stage of expansion the Universe was not only dense, but also hot. As a consequence, in our time residual radiation should be observed, the spectrum of which is the same as the spectrum of an absolutely solid body, and this radiation should be in highest degree isotropic. In 1964, A.A. Penzias and R. Wilson, testing a sensitive radio antenna, discovered very weak background microwave radiation, which they could not get rid of in any way. Its temperature turned out to be 2.73 K, which is close to the predicted value. From isotropy experiments it was shown that the source of the microwave background radiation cannot be located inside the Galaxy, since then a concentration of radiation towards the center of the Galaxy should be observed. The source of radiation could not be located inside the Solar system, because There would be a daily variation in radiation intensity. Because of this, a conclusion was made about the extragalactic nature of this background radiation. Thus, the hypothesis of a hot Universe received an observational basis.
To understand the nature of the cosmic microwave background radiation, it is necessary to turn to the processes that took place in the early stages of the expansion of the Universe. Let us consider how the physical conditions in the Universe changed during the expansion process.
Now every cubic centimeter of space contains about 500 relict photons, and there is much less matter per volume. Since the ratio of the number of photons to the number of baryons during expansion is maintained, but the energy of photons during the expansion of the Universe decreases over time due to the red shift, we can conclude that at some time in the past the energy density of radiation was greater than the energy density of matter particles. This time is called the radiation stage in the evolution of the Universe. The radiation stage was characterized by equality of temperature of the substance and radiation. At that time, radiation completely determined the nature of the expansion of the Universe. About a million years after the expansion of the Universe began, the temperature dropped to several thousand degrees and a recombination of electrons, which were previously free particles, took place with protons and helium nuclei, i.e. formation of atoms. The Universe has become transparent to radiation, and it is this radiation that we now detect and call relict radiation. True, since that time, due to the expansion of the Universe, photons have decreased their energy by about 100 times. Figuratively speaking, cosmic microwave background quanta “imprinted” the era of recombination and carry direct information about the distant past.
After recombination, matter began to evolve independently for the first time, regardless of radiation, and densities began to appear in it - the embryos of future galaxies and their clusters. This is why experiments to study the properties of cosmic microwave background radiation - its spectrum and spatial fluctuations - are so important for scientists. Their efforts were not in vain: in the early 90s. The Russian space experiment Relikt-2 and the American Kobe discovered differences in the temperature of the cosmic microwave background radiation of neighboring areas of the sky, and the deviation from the average temperature is only about a thousandth of a percent. These temperature variations carry information about the deviation of the density of matter from the average value during the recombination epoch. After recombination, matter in the Universe was distributed almost evenly, and where the density was at least slightly above average, the attraction was stronger. It was density variations that subsequently led to the formation of large-scale structures, galaxy clusters and individual galaxies observed in the Universe. According to modern ideas, the first galaxies should have formed in an epoch that corresponds to redshifts from 4 to 8.
Is there a chance to look even further into the era before recombination? Until the moment of recombination, it was the pressure of electromagnetic radiation that mainly created the gravitational field that slowed down the expansion of the Universe. At this stage, the temperature varied in inverse proportion to the square root of the time elapsed since the expansion began. Let us consider successively the various stages of expansion early universe.
At a temperature of approximately 1013 Kelvin, pairs of various particles and antiparticles were born and annihilated in the Universe: protons, neutrons, mesons, electrons, neutrinos, etc. When the temperature dropped to 5*1012 K, almost all protons and neutrons were annihilated, turning into radiation quanta; Only those for which there were “not enough” antiparticles remained. It is from these “excess” protons and neutrons that the matter of the modern observable Universe mainly consists.
At T = 2*1010 K, all-penetrating neutrinos stopped interacting with matter - from that moment a “relict neutrino background” should have remained, which may be able to be detected during future neutrino experiments.
Everything that has just been said took place at super high temperatures in the first second after the expansion of the Universe began. A few seconds after the “birth” of the Universe, the era of primary nucleosynthesis began, when nuclei of deuterium, helium, lithium and beryllium were formed. It lasted approximately three minutes, and its main result was the formation of helium nuclei (25% of the mass of all matter in the Universe). The remaining elements, heavier than helium, made up a negligible part of the substance - about 0.01%.
After the era of nucleosynthesis and before the era of recombination (about 106 years), a quiet expansion and cooling of the Universe occurred, and then - hundreds of millions of years after the beginning - the first galaxies and stars appeared.
In recent decades, the development of cosmology and elementary particle physics has made it possible to theoretically consider the very initial, “superdense” period of the expansion of the Universe. It turns out that at the very beginning of the expansion, when the temperature was incredibly high (more than 1028 K), the Universe could be in a special state in which it expanded with acceleration, and the energy per unit volume remained constant. This stage of expansion was called inflationary. Such a state of matter is possible under one condition - negative pressure. The stage of ultra-rapid inflationary expansion covered a tiny period of time: it ended at about 10–36 s. It is believed that the real “birth” of elementary particles of matter in the form in which we know them now occurred just after the end of the inflationary stage and was caused by the decay of a hypothetical field. After this, the expansion of the Universe continued by inertia.
The inflationary universe hypothesis answers a number of questions important issues cosmologies, which until recently were considered inexplicable paradoxes, in particular on the question of the cause of the expansion of the Universe. If in its history the Universe really went through an era when there was a large negative pressure, then gravity inevitably should have caused not attraction, but mutual repulsion of material particles. And this means that the Universe began to expand rapidly, explosively. Of course, the model of the inflationary Universe is only a hypothesis: even an indirect verification of its provisions requires instruments that simply have not yet been created. However, the idea of the accelerated expansion of the Universe at the earliest stage of its evolution has firmly entered into modern cosmology.
Speaking about the early Universe, we are suddenly transported from the largest cosmic scales to the region of the microcosm, which is described by the laws quantum mechanics. The physics of elementary particles and ultra-high energies is closely intertwined in cosmology with the physics of giant astronomical systems. The largest and the smallest are connected here with each other. This is what Amazing beauty our world, full of unexpected connections and deep unity.
The manifestations of life on Earth are extremely diverse. Life on Earth is represented by nuclear and prenuclear, single- and multicellular creatures; multicellular, in turn, are represented by fungi, plants and animals. Any of these kingdoms unites various types, classes, orders, families, genera, species, populations and individuals.
In all the seemingly endless diversity of living things, several different levels of organization of living things can be distinguished: molecular, cellular, tissue, organ, ontogenetic, population, species, biogeocenotic, biosphere. The listed levels are highlighted for ease of study. If we try to identify the main levels, reflecting not so much the levels of study as the levels of organization of life on Earth, then the main criteria for such identification should be the presence of specific elementary, discrete structures and elementary phenomena. With this approach, it turns out to be necessary and sufficient to distinguish molecular genetic, ontogenetic, population-species and biogeocenotic levels (N.V. Timofeev-Resovsky and others).
Molecular genetic level. When studying this level, apparently, the greatest clarity was achieved in the definition of basic concepts, as well as in the identification of elementary structures and phenomena. The development of the chromosomal theory of heredity, the analysis of the mutation process, and the study of the structure of chromosomes, phages and viruses revealed the main features of the organization of elementary genetic structures and related phenomena. It is known that the main structures at this level (codes of hereditary information transmitted from generation to generation) are DNA differentiated by length into code elements - triplets of nitrogenous bases that form genes.
Genes at this level of life organization represent elementary units. The main elementary phenomena associated with genes can be considered their local structural changes (mutations) and the transfer of information stored in them to intracellular control systems.
Convariant reduplication occurs according to the template principle by breaking the hydrogen bonds of the DNA double helix with the participation of the enzyme DNA polymerase. Then each of the strands builds a corresponding strand, after which the new strands are complementarily connected to each other. The pyrimidine and purine bases of the complementary strands are held together by hydrogen bonds by DNA polymerase. This process is carried out very quickly. Thus, the self-assembly of Escherichia coli DNA, consisting of approximately 40 thousand nucleotide pairs, requires only 100 s. Genetic information is transferred from the nucleus by mRNA molecules to the cytoplasm to ribosomes and there participates in protein synthesis. A protein containing thousands of amino acids is synthesized in a living cell in 5–6 minutes, and faster in bacteria.
The main control systems, both during convariant reduplication and during intracellular information transfer, use the “matrix principle”, i.e. are matrices next to which the corresponding specific macromolecules are built. Currently, the code embedded in the structure of nucleic acids, which serves as a matrix for the synthesis of specific protein structures in cells, is being successfully deciphered. Reduplication, based on matrix copying, preserves not only the genetic norm, but also deviations from it, i.e. mutations (the basis of the evolutionary process). Sufficiently accurate knowledge of the molecular genetic level is a necessary prerequisite for a clear understanding of life phenomena occurring at all other levels of life organization.
The content of the article
CMB RADIATION, cosmic electromagnetic radiation, coming to Earth from all sides of the sky with approximately the same intensity and having a spectrum characteristic of black body radiation at a temperature of about 3 K (3 degrees on the absolute Kelvin scale, which corresponds to –270 ° C). At this temperature, the main share of radiation comes from radio waves in the centimeter and millimeter ranges. The energy density of the cosmic microwave background radiation is 0.25 eV/cm 3 .
Experimental radio astronomers prefer to call this radiation “cosmic microwave background” (CMB). Theoretical astrophysicists often call it “relict radiation” (the term was proposed by the Russian astrophysicist I.S. Shklovsky), since, within the framework of the generally accepted theory of the hot Universe today, this radiation arose at the early stage of the expansion of our world, when its matter was almost homogeneous and very hot. Sometimes in scientific and popular literature you can also find the term “three-degree cosmic radiation”. Below we will call this radiation “relict radiation”.
The discovery of the cosmic microwave background radiation in 1965 was of great importance for cosmology; it became one of the most important achievements of natural science of the 20th century. and, of course, the most important for cosmology after the discovery of the redshift in the spectra of galaxies. Weak relict radiation brings us information about the first moments of the existence of our Universe, about that distant era when the entire Universe was hot and no planets, no stars, no galaxies existed in it. Detailed measurements of this radiation carried out in recent years using ground-based, stratospheric and space observatories lift the curtain on the mystery of the very birth of the Universe.
Hot Universe theory.
In 1929, American astronomer Edwin Hubble (1889–1953) discovered that most galaxies are moving away from us, and the faster the further the galaxy is located (Hubble's law). This was interpreted as a general expansion of the Universe, which began approximately 15 billion years ago. The question arose about what the Universe looked like in the distant past, when galaxies just began to move away from each other, and even earlier. Although the mathematical apparatus based on general theory Einstein's relativity and describing the dynamics of the Universe, was created back in the 1920s by Willem de Sitter (1872–1934), Alexander Friedman (1888–1925) and Georges Lemaitre (1894–1966), about physical condition The Universe in the early era of its evolution was unknown. It was not even certain that there was a certain moment in the history of the Universe that could be considered the “beginning of expansion.”
Development nuclear physics in the 1940s allowed the development of theoretical models of the evolution of the Universe in the past, when its matter was supposed to be compressed to a high density at which nuclear reactions were possible, began to be developed. These models, first of all, were supposed to explain the composition of the matter of the Universe, which by that time had already been measured quite reliably from observations of the spectra of stars: on average, they consist of 2/3 of hydrogen and 1/3 of helium, and all other chemical elements taken together account for no more than 2%. Knowledge of the properties of intranuclear particles - protons and neutrons - made it possible to calculate options for the beginning of the expansion of the Universe, differing in the initial content of these particles and the temperature of the substance and the radiation that is in thermodynamic equilibrium with it. Each of the options gave its own composition of the original substance of the Universe.
If we omit the details, then there are two fundamentally different possibilities for the conditions in which the beginning of the expansion of the Universe took place: its matter could be either cold or hot. The consequences of nuclear reactions are fundamentally different from each other. Although the idea of the possibility of a hot past of the Universe was expressed by Lemaitre in his early works, historically it was the first to consider the possibility of a cold beginning in the 1930s.
In the first assumptions, it was believed that all matter in the Universe first existed in the form of cold neutrons. It later turned out that this assumption contradicts observations. The fact is that a neutron in a free state decays on average 15 minutes after its occurrence, turning into a proton, electron and antineutrino. In an expanding Universe, the resulting protons would begin to combine with the remaining neutrons, forming the nuclei of deuterium atoms. Further, a chain of nuclear reactions would lead to the formation of nuclei of helium atoms. More complex atomic nuclei, as calculations show, practically do not arise. As a result, all matter would turn into helium. This conclusion is in sharp contradiction with observations of stars and interstellar matter. The prevalence of chemical elements in nature rejects the hypothesis that the expansion of matter begins in the form of cold neutrons.
In 1946 in the USA, a “hot” version of the initial stages of the expansion of the Universe was proposed by Russian-born physicist Georgy Gamow (1904–1968). In 1948, the work of his collaborators, Ralph Alpher and Robert Herman, was published, which examined nuclear reactions in hot matter at the beginning of cosmological expansion in order to obtain the currently observed relationships between the amounts of various chemical elements and their isotopes. In those years, the desire to explain the origin of all chemical elements by their synthesis in the first moments of the evolution of matter was natural. The fact is that at that time they mistakenly estimated the time that had elapsed since the beginning of the expansion of the Universe as only 2–4 billion years. This was due to the overestimated value of the Hubble constant, which resulted from astronomical observations in those years.
Comparing the age of the Universe at 2–4 billion years with the estimate of the age of the Earth - about 4 billion years - we had to assume that the Earth, Sun and stars were formed from primordial matter with a ready-made chemical composition. It was believed that this composition did not change any significantly, since the synthesis of elements in stars is a slow process and there was no time for its implementation before the formation of the Earth and other bodies.
The subsequent revision of the extragalactic distance scale also led to a revision of the age of the Universe. The theory of stellar evolution successfully explains the origin of all heavy elements (heavier than helium) by their nucleosynthesis in stars. There is no longer any need to explain the origin of all elements, including heavy ones, at the early stage of the expansion of the Universe. However, the essence of the hot Universe hypothesis turned out to be correct.
On the other hand, the helium content of stars and interstellar gas is about 30% by mass. This is much more than can be explained by nuclear reactions in stars. This means that helium, unlike heavy elements, should be synthesized at the beginning of the expansion of the Universe, but at the same time in limited quantities.
The main idea of Gamow's theory is precisely that the high temperature of a substance prevents the transformation of all the substance into helium. At the moment of 0.1 seconds after the start of expansion, the temperature was about 30 billion K. Such hot matter contains many high-energy photons. The density and energy of photons are so high that light interacts with light, leading to the creation of electron-positron pairs. The annihilation of pairs can, in turn, lead to the production of photons, as well as to the emergence of neutrino and antineutrino pairs. In this “seething cauldron” there is an ordinary substance. At very high temperatures, complex atomic nuclei cannot exist. They would be instantly smashed by the surrounding energetic particles. Therefore, heavy particles of matter exist in the form of neutrons and protons. Interactions with energetic particles cause neutrons and protons to rapidly transform into each other. However, the reactions of combining neutrons with protons do not occur, since the resulting deuterium nucleus is immediately broken up by high-energy particles. Yes, because high temperature At the very beginning, the chain leading to the formation of helium breaks.
Only when the Universe, expanding, cools to a temperature below a billion kelvins, some amount of the resulting deuterium is already stored and leads to the synthesis of helium. Calculations show that the temperature and density of a substance can be adjusted so that by this moment the proportion of neutrons in the substance is about 15% by mass. These neutrons, combining with the same number of protons, form about 30% of helium. The remaining heavy particles remained in the form of protons - the nuclei of hydrogen atoms. Nuclear reactions end after the first five minutes after the expansion of the Universe begins. Subsequently, as the Universe expands, the temperature of its matter and radiation decreases. From the works of Gamow, Alfer and Herman in 1948 it followed: if the theory of the hot Universe predicts the emergence of 30% helium and 70% hydrogen as the basic chemical elements of nature, then the modern Universe must inevitably be filled with a remnant (“relic”) of the primordial hot radiation, and modern temperature This CMB should be around 5 K.
However, on Gamow's hypothesis the analysis different options The beginning of the cosmological expansion has not ended. In the early 1960s, an ingenious attempt to return to the cold version was made by Ya.B. Zeldovich, who suggested that the original cold matter consisted of protons, electrons and neutrinos. As Zeldovich showed, such a mixture, upon expansion, turns into pure hydrogen. Helium and other chemical elements, according to this hypothesis, were synthesized later when stars formed. Note that by this time astronomers already knew that the Universe is several times older than the Earth and most of the stars around us, and data on the abundance of helium in prestellar matter was still very uncertain in those years.
It would seem that the decisive test for choosing between the cold and hot models of the Universe could be the search for cosmic microwave background radiation. But for some reason, for many years after the prediction of Gamow and his colleagues, no one consciously tried to detect this radiation. It was discovered completely by accident in 1965 by radio physicists from the American Bell company R. Wilson and A. Penzias, who were awarded the Nobel Prize in 1978.
On the way to detecting cosmic microwave background radiation.
In the mid-1960s, astrophysicists continued to theoretically study the hot model of the Universe. The calculation of the expected characteristics of the cosmic microwave background radiation was carried out in 1964 by A.G. Doroshkevich and I.D. Novikov in the USSR and independently by F. Hoyle and R. J. Taylor in the UK. But these works, like more early works Gamova and her colleagues did not attract attention. But they have already convincingly shown that cosmic microwave background radiation can be observed. Despite the extreme weakness of this radiation in our era, it, fortunately, lies in that region of the electromagnetic spectrum where all other cosmic sources generally emit even weaker radiation. Therefore, a targeted search for the cosmic microwave background radiation should have led to its discovery, but radio astronomers did not know about it.
This is what A. Penzias said in his Nobel lecture: “The first published recognition of cosmic microwave background radiation as a detectable phenomenon in the radio range appeared in the spring of 1964 in a short article by A.G. Doroshkevich and I.D. Novikov, entitled Average density radiation in the Metagalaxy and some issues of relativistic cosmology. Although English translation appeared in the same year, but a little later, in the well-known journal “Soviet Physics - Reports”, the article apparently did not attract the attention of other specialists in this field. This remarkable paper not only deduces the spectrum of the CMB as a black-body wave phenomenon, but also clearly focuses on the twenty-foot horn reflector at Bell Laboratory at Crawford Hill as the most suitable instrument for detecting it!” (quoted from: Sharov A.S., Novikov I.D. The Man Who Discovered the Explosion of the Universe: The Life and Work of Edwin Hubble. M., 1989).
Unfortunately, this article went unnoticed by both theorists and observers; it did not stimulate the search for cosmic microwave background radiation. Historians of science are still wondering why for many years no one tried to consciously look for radiation from the hot Universe. It is curious that past this discovery - one of the largest in the 20th century. – the scientists walked by several times without noticing him.
For example, cosmic microwave background radiation could have been discovered back in 1941. Then the Canadian astronomer E. McKellar analyzed the absorption lines caused by interstellar cyanogen molecules in the spectrum of the star Zeta Ophiuchi. He came to the conclusion that these lines in the visible region of the spectrum can only arise when light is absorbed by rotating cyanogen molecules, and their rotation should be excited by radiation with a temperature of about 2.3 K. Of course, no one could have thought then that the excitation of rotational levels of these molecules caused by cosmic microwave background radiation. Only after its discovery in 1965 were the works of I.S. Shklovsky, J. Field and others published, in which it was shown that the excitation of the rotation of interstellar cyanogen molecules, the lines of which are clearly observed in the spectra of many stars, is caused precisely by relict radiation.
An even more dramatic story occurred in the mid-1950s. Then the young scientist T.A. Shmaonov, under the guidance of famous Soviet radio astronomers S.E. Khaikin and N.L. Kaidanovsky, measured radio emission from space at a wavelength of 32 cm. These measurements were made using horn antenna, similar to the one used many years later by Penzias and Wilson. Shmaonov carefully studied possible interference. Of course, at that time he did not yet have at his disposal such sensitive receivers as the Americans later acquired. The results of Shmaonov’s measurements were published in 1957 in his candidate’s thesis and in the journal “Instruments and Experimental Techniques”. The conclusion from these measurements was: “It turned out that absolute value The effective temperature of the background radio emission... is 4 ± 3 K." Shmaonov noted the independence of the radiation intensity from the direction in the sky and from time. Although the measurement errors were large and there is no need to talk about any reliability of the number 4, it is now clear to us that Shmaonov measured precisely the cosmic microwave background radiation. Unfortunately, neither he himself nor other radio astronomers knew anything about the possibility of the existence of cosmic microwave background radiation and did not attach due importance to these measurements.
Finally, around 1964, the famous experimental physicist from Princeton (USA), Robert Dicke, consciously approached this problem. Although his reasoning was based on the theory of an “oscillating” Universe, which repeatedly experiences expansion and contraction, Dicke clearly understood the need to search for cosmic microwave background radiation. On his initiative, at the beginning of 1965, the young theorist F. J. E. Peebles carried out the necessary calculations, and P. G. Roll and D. T. Wilkinson began to build a small low-noise antenna on the roof of the Palmersky physical laboratory in Princeton. It is not necessary to use large radio telescopes to search for background radiation, since the radiation comes from all directions. Nothing is gained from having a large antenna focus the beam onto a smaller area of the sky. But Dicke’s group did not have time to make the planned discovery: when their equipment was already ready, they only had to confirm the discovery that others had accidentally made the day before.
Discovery of cosmic microwave background radiation.
In 1960, an antenna was built in Crawford Hill, Holmdel (New Jersey, USA) to receive radio signals reflected from the Echo satellite balloon. By 1963, this antenna was no longer needed to work with the satellite, and radio physicists Robert Woodrow Wilson (b. 1936) and Arno Elan Penzias (b. 1933) from the Bell Telephone laboratory decided to use it for radio astronomical observations. The antenna was a 20-foot horn. Together with a state-of-the-art receiving device, this radio telescope was at the time the most sensitive instrument in the world for measuring radio waves coming from wide areas of the sky. First of all, it was planned to measure the radio emission of the interstellar medium of our Galaxy at a wavelength of 7.35 cm. Arno Penzias and Robert Wilson did not know about the theory of the hot Universe and did not intend to look for cosmic microwave background radiation.
To accurately measure the radio emission of the Galaxy, it was necessary to take into account all possible interference caused by radiation from the Earth's atmosphere and surface of the Earth, as well as interference arising in the antenna, electrical circuits and receivers. Preliminary tests of the receiving system showed slightly more noise than expected, but it seemed plausible that this was due to a slight excess of noise in the amplifier circuits. To get rid of these problems, Penzias and Wilson used a device known as a "cold load": the signal coming from the antenna is compared with the signal from artificial source, cooled with liquid helium at a temperature of about four degrees above absolute zero (4 K). In both cases, the electrical noise in the amplification circuits must be the same, and therefore the difference obtained by comparison gives the signal power coming from the antenna. This signal contains contributions only from the antenna device, the earth's atmosphere, and an astronomical source of radio waves within the antenna's field of view.
Penzias and Wilson expected that the antenna device would produce very little electrical noise. However, to test this assumption, they began their observations at a relatively short waves 7.35 cm long, at which radio noise from the Galaxy should be negligible. Naturally, some radio noise was expected at this wavelength and from the earth’s atmosphere, but this noise should have a characteristic dependence on direction: it should be proportional to the thickness of the atmosphere in the direction in which the antenna is looking: a little less in the direction of the zenith, a little more in direction of the horizon. It was expected that after subtracting the atmospheric term with its characteristic directional dependence, there would be no significant signal left from the antenna and this would confirm that the electrical noise produced by the antenna device was negligible. After this, it will be possible to begin studying the Galaxy itself at long wavelengths - about 21 cm, where the radiation Milky Way has quite noticeable significance. (Note that radio waves with wavelengths of centimeters or decimeters, up to 1 m, are usually called “microwave radiation.” This name is given because these wavelengths are shorter than the ultrashort waves used in radar at the beginning of World War II .)
To their surprise, Penzias and Wilson discovered in the spring of 1964 that they were receiving quite a noticeable amount of direction-independent microwave noise at the 7.35 cm wavelength. They found that this “static background” did not change depending on the time of day, and later discovered that it did not depend on the time of year. Consequently, this could not be radiation from the Galaxy, because in this case its intensity would vary depending on whether the antenna was looking along the plane of the Milky Way or across it. Moreover, if this were the radiation from our Galaxy, then the large spiral galaxy M 31 in Andromeda, similar in many respects to ours, should also emit strongly at a wavelength of 7.35 cm, but this was not observed. The absence of any variation in direction in the observed microwave noise strongly indicated that these radio waves, if they actually existed, did not come from the Milky Way, but from a much larger volume of the Universe.
It was clear to the researchers that they needed to test again to see if the antenna itself might be producing more electrical noise than expected. In particular, it was known that a pair of pigeons had nested in the antenna horn. They were captured, mailed to the Bell site at Whippany, released, rediscovered a few days later in their position in the antenna, recaptured, and finally subdued by more drastic means. However, during the rental of the premises, the pigeons coated the inside of the antenna with what Penzias called a “white dielectric substance”, which when room temperature could be a source of electrical noise. At the beginning of 1965, the antenna horn was dismantled and all the dirt was cleaned out, but this, like all other tricks, gave a very small reduction in the observed noise level.
When all sources of interference were carefully analyzed and taken into account, Penzias and Wilson were forced to conclude that the radiation was coming from space, and from all directions with the same intensity. It turned out that space radiates as if it were heated to a temperature of 3.5 kelvin (more precisely, the achieved accuracy allowed us to conclude that the “temperature of space” is from 2.5 to 4.5 kelvin). It should be noted that this is a very subtle experimental result: for example, if an ice cream bar was placed in front of the antenna horn, it would shine in the radio range, 22 million times brighter than the corresponding part of the sky. Considering the unexpected result of their observations, Penzias and Wilson were in no hurry to publish. But events developed against their will.
It so happened that Penzias called on a completely different matter his friend Bernard Burke from the Massachusetts Institute of Technology. Shortly before this, Burke had heard from his colleague Ken Terner at the Carnegie Institution about a talk he had in turn heard at Johns Hopkins University, given by the Princeton theorist Phil Peebles, working under the direction of Robert Dicke. In this talk, Peebles argued that there must be background radio noise left over from the early Universe that now has an equivalent temperature of about 10 K.
Penzias called Dicke and the two research groups met. It became clear to Robert Dicke and his colleagues F. Peebles, P. Roll and D. Wilkinson that A. Penzias and R. Wilson had discovered the cosmic microwave background radiation of the hot Universe. The scientists decided to simultaneously publish two letters in the prestigious Astrophysical Journal. In the summer of 1965, both works were published: by Penzias and Wilson on the discovery of the cosmic microwave background radiation, and by Dicke and his colleagues - with its explanation using the theory of the hot Universe. Apparently not entirely convinced of the cosmological interpretation of their discovery, Penzias and Wilson gave their note a modest title: Antenna excess temperature measurement at 4080 MHz. They simply announced that "measurements of the effective zenith temperature of the noise... gave a value 3.5 K higher than expected" and avoided any mention of cosmology except to say that " possible explanation observed excess noise temperature is given by Dicke, Peebles, Roll and Wilkinson in a companion letter in the same issue of the journal."
In subsequent years, numerous measurements were made at various wavelengths from tens of centimeters to a fraction of a millimeter. Observations have shown that the spectrum of the cosmic microwave background radiation corresponds to Planck's formula, as it should be for radiation with a certain temperature. This temperature was confirmed to be approximately 3 K. A remarkable discovery was made, proving that the Universe was hot at the beginning of its expansion.
This is the complex interweaving of events that culminated in the discovery of the hot Universe by Penzias and Wilson in 1965. The establishment of the fact of ultra-high temperatures at the beginning of the expansion of the Universe was the starting point important research, leading to the revelation of not only astrophysical mysteries, but also the secrets of the structure of matter.
The most accurate measurements of cosmic microwave background radiation were carried out from space: this is the Relikt experiment on the Soviet Prognoz-9 satellite (1983–1984) and the DMR (Differential Microwave Radiometer) experiment on the American COBE satellite (Cosmic Background Explorer, November 1989–1993). the latter made it possible to most accurately determine the temperature of the cosmic microwave background radiation: 2.725 ± 0.002 K.
Microwave background as “new ether”.
So, the spectrum of the cosmic microwave background radiation corresponds with very high accuracy to the radiation of an absolutely black body (i.e., it is described by Planck’s formula) with a temperature T = 2.73 K. However, small (about 0.1%) deviations from this average temperature are observed depending on in which direction in the sky the measurement is being taken. The fact is that the cosmic microwave background radiation is isotropic only in the coordinate system associated with the entire system of retreating galaxies, in the so-called “accompanying reference frame,” which expands along with the Universe. In any other coordinate system, the intensity of radiation depends on the direction. This is primarily caused by the movement of the measuring device relative to the CMB: the Doppler effect leads to the “blueing” of photons flying towards the device, and to the “reddening” of photons catching up with it.
In this case, the measured temperature compared to the average (T 0) depends on the direction of movement: T = T 0 (1 + (v/c) cos i), where v is the speed of the device in the coordinate system associated with the cosmic microwave background radiation; c – speed of light, i– the angle between the velocity vector and the direction of observation. Against the background of a uniform temperature distribution, two “poles” appear - warm in the direction of movement and cool in the opposite direction. Therefore, such a deviation from homogeneity is called “dipole”. The dipole component in the distribution of the cosmic microwave background radiation was discovered during ground-based observations: in the direction of the constellation Leo, the temperature of this radiation was 3.5 mK above the average, and in the opposite direction (the constellation Aquarius) the same amount below the average. Consequently, we are moving relative to the CMB at a speed of about 400 km/s. The accuracy of the measurements turned out to be so high that even annual variations in the dipole component were discovered, caused by the Earth's revolution around the Sun at a speed of 30 km/s.
Measurements with artificial satellites The lands have significantly refined these data. According to COBE, after taking into account the Earth's orbital motion, it turns out that solar system moves in such a way that the amplitude of the dipole component of the temperature of the cosmic microwave background radiation is D T = 3.35 mK; this corresponds to a speed of movement V = 366 km/s. The Sun moves relative to the radiation in the direction of the border of the constellations Leo and Chalice, to a point with equatorial coordinates a = 11 h 12 m and d = –7.1° (epoch J2000); which corresponds to galactic coordinates l = 264.26° and b = 48.22°. Taking into account the movement of the Sun itself in the Galaxy shows that, relative to all galaxies of the Local Group, the Sun moves at a speed of 316 ± 5 km/s in the direction l 0 = 93 ± 2° and b 0 = –4 ± 2°. Therefore, the movement of the Local Group itself relative to the cosmic microwave background radiation occurs at a speed of 635 km/s in a direction of about l= 269° and b= +29° . This is approximately at an angle of 45° relative to the direction towards the center of the Virgo galaxy cluster.
Studying the movements of galaxies on an even larger scale shows that the collection of nearby galaxy clusters (119 clusters from the Abel catalog within 200 Mpc of us) moves as a whole relative to the CMB at a speed of about 700 km/s. Thus, our neighborhood of the Universe floats in the sea of cosmic microwave background radiation at a noticeable speed. Astrophysicists have repeatedly drawn attention to the fact that the very fact of the existence of cosmic microwave background radiation and the dedicated reference system associated with it assigns this radiation the role of “new ether”. But there is nothing mystical about this: all physical measurements in this reference system are equivalent to measurements in any other inertial system countdown. (A discussion of the problem of the “new ether” in connection with Mach’s principle can be found in the book: Zeldovich Ya.B., Novikov I.D. Structure and evolution of the Universe. M., 1975).
Anisotropy of cosmic microwave background radiation.
The temperature of the cosmic microwave background radiation is only one of its parameters that describe the early Universe. The properties of this radiation also preserve other clear traces of a very early era in the evolution of our world. Astrophysicists find these traces by analyzing the spectrum and spatial inhomogeneity (anisotropy) of the cosmic microwave background radiation.
According to the theory of the hot Universe, after about 300 thousand years after the start of expansion, the temperature of the substance and the associated radiation decreased to 4000 K. At this temperature, photons could no longer ionize hydrogen and helium atoms. Therefore, at that epoch corresponding to the redshift z = 1400, a recombination of hot plasma occurred, as a result of which the plasma turned into a neutral gas. At that time, of course, there were no galaxies or stars. They arose much later.
Having become neutral, the gas filling the Universe turned out to be practically transparent to cosmic microwave background radiation (although in that era these were not radio waves, but light from the visible and near infrared ranges). Therefore, ancient radiation reaches us almost unhindered from the depths of space and time. But still, along the way it experiences some influences and, as an archaeological site, bears traces of historical events.
For example, during the era of recombination, atoms emitted many photons with an energy of the order of 10 eV, which is tens of times higher than the average energy of photons of equilibrium radiation of that era (at T = 4000 K there are very few such energetic photons, about one billionth of their total number). Therefore, recombination radiation should greatly distort the Planck spectrum of the CMB in the wavelength range of about 250 μm. True, calculations have shown that the strong interaction of radiation with matter will lead to the fact that the released energy will mainly “dissipate” over a wide region of the spectrum and will not distort it much, but future accurate measurements will be able to notice this distortion.
And much later, during the era of the formation of galaxies and the first generation of stars (at z ~ 10), when a huge mass of almost cooled matter again experienced significant heating, the spectrum of the cosmic microwave background radiation could again change, since, scattering on hot electrons, low-energy photons increase their energy (the so-called “reverse Compton effect”). Both effects described above distort the spectrum of the cosmic microwave background radiation in its short-wavelength region, which has so far been least explored.
Although in our era most of ordinary matter is densely packed in stars, and those in galaxies, yet even close to us, cosmic microwave background radiation can experience a noticeable distortion of the spectrum if its rays pass through a large cluster of galaxies on the way to Earth. Typically, such clusters are filled with rarefied but very hot intergalactic gas, having a temperature of about 100 million K. Scattering on fast electrons of this gas, low-energy photons increase their energy (the same inverse Compton effect) and move from the low-frequency, Rayleigh-Jeans region of the spectrum into the high-frequency, Vinov region. This effect was predicted by R.A.Sunyaev and Ya.B.Zeldovich and discovered by radio astronomers in the direction of many galaxy clusters in the form of a decrease in the radiation temperature in the Rayleigh-Jeans region of the spectrum by 1–3 mK. The Sunyaev-Zeldovich effect was the first to be discovered among the effects that create anisotropy of the cosmic microwave background radiation. Comparison of its value with the X-ray luminosity of galaxy clusters made it possible to independently determine the Hubble constant (H = 60 ± 12 km/s/Mpc).
Let's return to the era of recombination. At an age of less than 300,000 years, the Universe was an almost homogeneous plasma, shuddering from sound, or more precisely, infrasound waves. Calculations by cosmologists say that these waves of compression and expansion of matter also generated fluctuations in the radiation density in the opaque plasma, and therefore now they should be detected in the form of a barely noticeable “swell” in the almost uniform cosmic microwave background radiation. Therefore, today it must come to Earth from different sides with slightly different intensities. IN in this case we're talking about not about trivial dipole anisotropy caused by the movement of the observer, but about intensity variations actually inherent in the radiation itself. Their amplitude should be extremely small: approximately one hundred thousandth of the radiation temperature itself, i.e. on the order of 0.00003 K. They are very difficult to measure. The first attempts to determine the magnitude of these small fluctuations depending on the direction in the sky were made immediately after the discovery of the cosmic microwave background radiation itself in 1965. Later they did not stop, but the discovery took place only in 1992 using equipment taken outside the Earth. In our country, such measurements were carried out in the Relikt experiment, but these small fluctuations were more confidently recorded from the American COBE satellite (Fig. 1).
Recently, many experiments have been carried out and planned to measure the amplitude of fluctuations of the cosmic microwave background radiation on various angular scales - from degrees to arcseconds. Various physical phenomena, which occurred in the very first moments of the life of the Universe, should have left their characteristic imprint in the radiation coming to us. The theory predicts a certain relationship between the sizes of cold and hot spots in the CMB intensity and their relative brightness. The dependence is very peculiar: it contains information about the processes of the birth of the Universe, what happened immediately after the birth, as well as about the parameters of today’s Universe.
The angular resolution of the first observations - in the Relikt-2 and COBE experiments - was very poor, approximately 7°, so information about fluctuations of the cosmic microwave background radiation was incomplete. In subsequent years, the same observations were carried out using both ground-based radio telescopes (in our country, the RATAN-600 instrument with an empty aperture with a diameter of 600 m is used for this purpose), and radio telescopes that were ascended to balloons into the upper layers of the atmosphere.
A fundamental step in the study of the anisotropy of cosmic microwave background radiation was the “Boomerang” experiment (BOOMERANG), carried out by scientists from the USA, Canada, Italy, England and France using an unmanned NASA (USA) balloon with a volume of 1 million cubic meters, which was carried out from December 29, 1998 to January 9, 1999 circle at an altitude of 37 km around the South Pole and, having flown about 10 thousand km, dropped the gondola with instruments by parachute 50 km from the launch site. The observations were carried out with a submillimeter telescope with a main mirror with a diameter of 1.2 m, at the focus of which was placed a system of bolometers cooled to 0.28 K, which measured the background in four frequency channels (90, 150, 240 and 400 GHz) with an angular resolution of 0.2–0 ,3 degrees. During the flight, observations covered about 3% of the celestial sphere.
The temperature inhomogeneities of the cosmic microwave background radiation with a characteristic amplitude of 0.0001 K recorded in the Boomerang experiment confirmed the correctness of the “acoustic” model and showed that the four-dimensional space-time geometry of the Universe can be considered flat. The information obtained also made it possible to judge the composition of the Universe: it was confirmed that ordinary baryonic matter, which makes up stars, planets and interstellar gas, makes up only about 4% of the mass; and the remaining 96% are contained in as yet unknown forms of matter.
The Boomerang experiment was perfectly complemented by a similar experiment, MAXIMA (Millimeter Anisotropy eXperiment IMaging Array), mainly carried out by scientists in the USA and Italy. Their equipment, which flew into the stratosphere in August 1998 and June 1999, examined less than 1% of the celestial sphere, but with a high angular resolution: about 5". The balloon made night flights over the continental United States. The telescope's main mirror had a diameter of 1.3 m. The receiving part of the equipment consisted of 16 detectors covering 3 frequency ranges.Secondary mirrors were cooled to cryogenic temperatures, and bolometers - even to 0.1 K. low temperature it was possible to maintain up to 40 hours, which limited the duration of the flight.
The MAXIMA experiment revealed a small “ripple” in the angular distribution of the temperature of the cosmic microwave background radiation. Its data was supplemented by observations from a ground-based observatory using the DASI (Degree Angular Scale Interferometer) installed by radio astronomers University of Chicago(USA) on South Pole. This 13-element cryogenic interferometer observed in ten frequency channels in the range 26–36 GHz and revealed even smaller fluctuations of the cosmic microwave background radiation, and the dependence of their amplitude on the angular size well confirms the theory of acoustic oscillations inherited from the young Universe.
In addition to measuring the intensity of cosmic microwave background radiation from the Earth's surface, space experiments are also planned. In 2007, it is planned to launch the Planck radio telescope (European Space Agency) into space. Its angular resolution will be significantly higher, and its sensitivity will be approximately 30 times better than in the COBE experiment. Therefore, astrophysicists hope that many facts about the beginning of the existence of our Universe will be clarified (see Fig. 1).
Vladimir Surdin
Literature:
Zeldovich Ya.B., Novikov I.D. Structure and evolution of the Universe. M., 1975
Cosmology: theory and observations. M., 1978
Weinberg S. The first three minutes. Modern view of the origin of the Universe. M., 1981
Silk J. Big Bang. Birth and evolution of the Universe. M., 1982
Sunyaev R.A. Microwave background radiation. – In the book: Physics of Space: A Little Encyclopedia. M., 1986
Dolgov A.D., Zeldovich Ya.B., Sazhin M.V. Cosmology of the early Universe. M., 1988
Novikov I.D. Evolution of the Universe. M., 1990
In 2006, John Mather and George Smoot were awarded the Nobel Prize in Physics for their discovery of the blackbody spectrum and anisotropy of the cosmic microwave background radiation. These results were obtained based on measurements made using the COBE satellite launched by NASA in 1988. The results of J. Mather and J. Smoot confirmed the origin of the Universe as a result of the Big Bang. The extremely small difference in the temperature of the cosmic background radiation ΔT/T ~ 10 -4 is evidence of the mechanism of formation of galaxies and stars.
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J. Mather (b. 1946) |
 J. Smoot (b. 1945) |
Rice. 52. Blackbody spectrum of cosmic microwave background radiation.
The cosmic microwave background radiation (or cosmic microwave background radiation) was discovered in 1965 by A. Penzias and R. Wilson. At an early stage of the evolution of the Universe, matter was in the state of plasma. Such a medium is opaque to electromagnetic radiation; intense scattering of photons by electrons and protons occurs. When the Universe cooled to 3000 K, electrons and protons united into neutral hydrogen atoms and the medium became transparent to photons. At this time, the age of the Universe was 300,000 years, so the cosmic microwave background radiation provides information about the state of the Universe in this era. At this time, the Universe was practically homogeneous. The inhomogeneities of the Universe are determined by the temperature inhomogeneity of the cosmic microwave background radiation. This heterogeneity is ΔT/T ≈ 10 -4 −10 -5. The inhomogeneities of the cosmic microwave background radiation are witnesses of the inhomogeneities of the Universe: the first stars, galaxies, clusters of galaxies. With the expansion of the Universe, the wavelength of the CMB increased Δλ/λ = ΔR/R and currently the wavelength of the CMB is in the radio wave range, the temperature of the CMB is T = 2.7 K.
Rice. 53. Anisotropy of cosmic microwave background radiation. More dark color sections of the spectrum of the cosmic microwave background radiation that have a higher temperature are shown.
J. Mather: "In the beginning there was the Big Bang−so we now say with great confidence. The COBE satellite, proposed as a project in 1974 National agency Aeronautics and Space Research (NASA) and launched in 1989, provided very strong evidence in favor of this: the cosmic microwave background radiation (CMBR, or cosmic microwave background radiation) has a spectrum of an almost perfect black body with a temperature
2.725 ±0.001 K, and this radiation is isotropic (the same in all directions) with a relative standard deviation of no more than 10 per million at angular scales of 7° or more. This radiation is interpreted as a trace of an extremely hot and dense early stage of the evolution of the Universe. In such a hot and dense phase, the creation and destruction of photons, as well as the establishment of equilibrium between them and with all other forms of matter and energy, would occur very quickly compared to the characteristic time scale of the expansion of the Universe. Such a state would immediately produce blackbody radiation. An expanding Universe must retain the black-body nature of this spectrum, so measuring any significant deviation from the ideal black-body spectrum would either invalidate the whole Big Bang idea or show that some energy was added to the CMB after the rapid establishment of equilibrium (for example, from decay of some primary particles). The fact that this radiation is isotropic to such a high degree is key evidence that it comes from the Big Bang."
Rice. 54. Robert Wilson and Arno Penzias at the antenna where the cosmic microwave background radiation was recorded.
J. Smoot: “According to the theory of the hot Universe, the cosmic microwave background radiation is residual radiation formed at the earliest high-temperature stages of the evolution of the Universe at a time close to the beginning of the expansion of the modern Universe 13.7 billion years ago. The CMB itself can be used as a powerful tool for measuring the dynamics and geometry of the Universe. CMB was discovered by Penzias and Wilson at the Laboratory. Bella in 1964
They discovered persistent isotropic radiation with a thermodynamic temperature of about 3.2 K. At the same time, physicists at Princeton (Dick, Peebles, Wilkinson and Roll) were developing an experiment to measure the cosmic microwave background radiation predicted by the hot universe theory. The accidental discovery of the cosmic microwave background radiation by Penzias and Wilson ushered in a new era in cosmology, marking the beginning of its transformation from myth and speculation into a full-fledged scientific field.
The discovery of the anisotropy of the temperature of the cosmic microwave background revolutionized our understanding of the Universe, and its modern research continue the revolution in cosmology. The construction of the angular power spectrum of CMB temperature fluctuations with plateaus, acoustic peaks and a damped high-frequency end led to the approval of the standard cosmological model, in which the geometry of space is flat (corresponds to the critical density), dark energy and dark matter dominate and there is only a little ordinary matter. According to this successfully confirmed model, the observed structure of the Universe was formed by gravitational instability, which amplified quantum fluctuations generated in the very early inflationary era. Current and future observations will test this model and identify key cosmological parameters with outstanding precision and significance."
Cosmic electromagnetic radiation coming to Earth from all sides of the sky with approximately the same intensity and having a spectrum characteristic of black body radiation at a temperature of about 3 K (3 degrees on the absolute Kelvin scale, which corresponds to -270 ° C). At this temperature, the main share of radiation comes from radio waves in the centimeter and millimeter ranges. The energy density of the cosmic microwave background radiation is 0.25 eV/cm 3 .
Experimental radio astronomers prefer to call this radiation “cosmic microwave background” (CMB). Theoretical astrophysicists often call it “relict radiation” (the term was proposed by the Russian astrophysicist I.S. Shklovsky), since, within the framework of the generally accepted theory of the hot Universe today, this radiation arose at the early stage of the expansion of our world, when its matter was almost homogeneous and very hot. Sometimes in scientific and popular literature you can also find the term “three-degree cosmic radiation”. Below we will call this radiation “relict radiation”.
The discovery of the cosmic microwave background radiation in 1965 was of great importance for cosmology; it became one of the most important achievements of natural science of the 20th century. and, of course, the most important for cosmology after the discovery of the redshift in the spectra of galaxies. Weak relict radiation brings us information about the first moments of the existence of our Universe, about that distant era when the entire Universe was hot and no planets, no stars, no galaxies existed in it. Detailed measurements of this radiation carried out in recent years using ground-based, stratospheric and space observatories lift the curtain on the mystery of the very birth of the Universe.
Hot Universe theory. In 1929, American astronomer Edwin Hubble (1889-1953) discovered that most galaxies are moving away from us, and the faster the further the galaxy is located (Hubble's law). This was interpreted as a general expansion of the Universe, which began approximately 15 billion years ago. The question arose about what the Universe looked like in the distant past, when galaxies just began to move away from each other, and even earlier. Although the mathematical apparatus, based on Einstein’s general theory of relativity and describing the dynamics of the Universe, was created back in the 1920s by Willem de Sitter (1872-1934), Alexander Friedman (1888-1925) and Georges Lemaitre (1894-1966), about the physical nothing was known about the state of the Universe in the early era of its evolution. It was not even certain that there was a certain moment in the history of the Universe that could be considered the “beginning of expansion.”
The development of nuclear physics in the 1940s allowed the development of theoretical models for the evolution of the Universe in the past, when its matter was believed to be compressed to a high density at which nuclear reactions were possible. These models, first of all, were supposed to explain the composition of the matter of the Universe, which by that time had already been measured quite reliably from observations of the spectra of stars: on average, they consist of 2/3 of hydrogen and 1/3 of helium, and all other chemical elements taken together account for no more than 2%. Knowledge of the properties of intranuclear particles - protons and neutrons - made it possible to calculate options for the beginning of the expansion of the Universe, differing in the initial content of these particles and the temperature of the substance and the radiation that is in thermodynamic equilibrium with it. Each of the options gave its own composition of the original substance of the Universe.
If we omit the details, then there are two fundamentally different possibilities for the conditions in which the beginning of the expansion of the Universe took place: its matter could be either cold or hot. The consequences of nuclear reactions are fundamentally different from each other. Although the idea of the possibility of a hot past of the Universe was expressed by Lemaitre in his early works, historically it was the first to consider the possibility of a cold beginning in the 1930s.
In the first assumptions, it was believed that all matter in the Universe first existed in the form of cold neutrons. It later turned out that this assumption contradicts observations. The fact is that a neutron in a free state decays on average 15 minutes after its occurrence, turning into a proton, electron and antineutrino. In an expanding Universe, the resulting protons would begin to combine with the remaining neutrons, forming the nuclei of deuterium atoms. Further, a chain of nuclear reactions would lead to the formation of nuclei of helium atoms. More complex atomic nuclei, as calculations show, practically do not arise in this case. As a result, all matter would turn into helium. This conclusion is in sharp contradiction with observations of stars and interstellar matter. The prevalence of chemical elements in nature rejects the hypothesis that the expansion of matter begins in the form of cold neutrons.
In 1946 in the USA, a “hot” version of the initial stages of the expansion of the Universe was proposed by Russian-born physicist Georgy Gamow (1904-1968). In 1948, the work of his collaborators, Ralph Alpher and Robert Herman, was published, which examined nuclear reactions in hot matter at the beginning of cosmological expansion in order to obtain the currently observed relationships between the amounts of various chemical elements and their isotopes. In those years, the desire to explain the origin of all chemical elements by their synthesis in the first moments of the evolution of matter was natural. The fact is that at that time they mistakenly estimated the time that had elapsed since the beginning of the expansion of the Universe as only 2-4 billion years. This was due to the overestimated value of the Hubble constant, which resulted from astronomical observations in those years.
Comparing the age of the Universe at 2-4 billion years with the estimate of the age of the Earth - about 4 billion years - we had to assume that the Earth, Sun and stars were formed from primary matter with a ready-made chemical composition. It was believed that this composition did not change any significantly, since the synthesis of elements in stars is a slow process and there was no time for its implementation before the formation of the Earth and other bodies.
The subsequent revision of the extragalactic distance scale also led to a revision of the age of the Universe. The theory of stellar evolution successfully explains the origin of all heavy elements (heavier than helium) by their nucleosynthesis in stars. There is no longer any need to explain the origin of all elements, including heavy ones, at the early stage of the expansion of the Universe. However, the essence of the hot Universe hypothesis turned out to be correct.
On the other hand, the helium content of stars and interstellar gas is about 30% by mass. This is much more than can be explained by nuclear reactions in stars. This means that helium, unlike heavy elements, should be synthesized at the beginning of the expansion of the Universe, but at the same time in limited quantities.
The main idea of Gamow's theory is precisely that the high temperature of a substance prevents the transformation of all the substance into helium. At the moment of 0.1 seconds after the start of expansion, the temperature was about 30 billion K. Such hot matter contains many high-energy photons. The density and energy of photons are so high that light interacts with light, leading to the creation of electron-positron pairs. The annihilation of pairs can, in turn, lead to the production of photons, as well as to the emergence of neutrino and antineutrino pairs. In this “seething cauldron” there is an ordinary substance. At very high temperatures, complex atomic nuclei cannot exist. They would be instantly smashed by the surrounding energetic particles. Therefore, heavy particles of matter exist in the form of neutrons and protons. Interactions with energetic particles cause neutrons and protons to rapidly transform into each other. However, the reactions of combining neutrons with protons do not occur, since the resulting deuterium nucleus is immediately broken up by high-energy particles. Thus, due to the high temperature, the chain leading to the formation of helium breaks at the very beginning.
Only when the Universe, expanding, cools to a temperature below a billion kelvins, some amount of the resulting deuterium is already stored and leads to the synthesis of helium. Calculations show that the temperature and density of a substance can be adjusted so that by this moment the proportion of neutrons in the substance is about 15% by mass. These neutrons, combining with the same number of protons, form about 30% of helium. The remaining heavy particles remained in the form of protons - the nuclei of hydrogen atoms. Nuclear reactions end after the first five minutes after the expansion of the Universe begins. Subsequently, as the Universe expands, the temperature of its matter and radiation decreases. From the works of Gamow, Alpher and Herman in 1948 it followed: if the theory of the hot Universe predicts the emergence of 30% helium and 70% hydrogen as the main chemical elements of nature, then the modern Universe must inevitably be filled with a remnant (“relic”) of the primordial hot radiation, and the modern temperature This CMB should be around 5 K.
However, the analysis of different options for the beginning of cosmological expansion did not end with Gamow’s hypothesis. In the early 1960s, an ingenious attempt to return to the cold version was made by Ya.B. Zeldovich, who suggested that the original cold matter consisted of protons, electrons and neutrinos. As Zeldovich showed, such a mixture, upon expansion, turns into pure hydrogen. Helium and other chemical elements, according to this hypothesis, were synthesized later when stars formed. Note that by this time astronomers already knew that the Universe is several times older than the Earth and most of the stars around us, and data on the abundance of helium in prestellar matter was still very uncertain in those years.
It would seem that the decisive test for choosing between the cold and hot models of the Universe could be the search for cosmic microwave background radiation. But for some reason, for many years after the prediction of Gamow and his colleagues, no one consciously tried to detect this radiation. It was discovered completely by accident in 1965 by radio physicists from the American Bell company R. Wilson and A. Penzias, who were awarded the Nobel Prize in 1978.
On the way to detecting cosmic microwave background radiation. In the mid-1960s, astrophysicists continued to theoretically study the hot model of the Universe. The calculation of the expected characteristics of the cosmic microwave background radiation was carried out in 1964 by A.G. Doroshkevich and I.D. Novikov in the USSR and independently by F. Hoyle and R. J. Taylor in the UK. But these works, like the earlier works of Gamow and his colleagues, did not attract attention. But they have already convincingly shown that cosmic microwave background radiation can be observed. Despite the extreme weakness of this radiation in our era, it, fortunately, lies in that region of the electromagnetic spectrum where all other cosmic sources generally emit even weaker radiation. Therefore, a targeted search for the cosmic microwave background radiation should have led to its discovery, but radio astronomers did not know about it.
This is what A. Penzias said in his Nobel lecture: “The first published recognition of cosmic microwave background radiation as a detectable phenomenon in the radio range appeared in the spring of 1964 in a short article by A.G. Doroshkevich and I.D. Novikov, entitled Average radiation density in the Metagalaxy and some issues of relativistic cosmology. Although an English translation appeared the same year, but somewhat later, in the widely known journal Soviet Physics - Reports, the article apparently did not attract the attention of other specialists in the field. This remarkable paper not only deduces the spectrum of the CMB as a black-body wave phenomenon, but also clearly focuses on the twenty-foot horn reflector at Bell Laboratory at Crawford Hill as the most suitable instrument for detecting it!” (quoted from: Sharov A.S., Novikov I.D. The Man Who Discovered the Explosion of the Universe: The Life and Work of Edwin Hubble M., 1989).
Unfortunately, this article went unnoticed by both theorists and observers; it did not stimulate the search for cosmic microwave background radiation. Historians of science are still wondering why for many years no one tried to consciously look for radiation from the hot Universe. It is curious that past this discovery - one of the largest in the 20th century. - Scientists walked by several times without noticing him.
For example, cosmic microwave background radiation could have been discovered back in 1941. Then the Canadian astronomer E. McKellar analyzed the absorption lines caused by interstellar cyanogen molecules in the spectrum of the star Zeta Ophiuchi. He came to the conclusion that these lines in the visible region of the spectrum can only arise when light is absorbed by rotating cyanogen molecules, and their rotation should be excited by radiation with a temperature of about 2.3 K. Of course, no one could have thought then that the excitation of rotational levels of these molecules caused by cosmic microwave background radiation. Only after its discovery in 1965 were the works of I.S. Shklovsky, J. Field and others published, in which it was shown that the excitation of the rotation of interstellar cyanogen molecules, the lines of which are clearly observed in the spectra of many stars, is caused precisely by relict radiation.
An even more dramatic story occurred in the mid-1950s. Then the young scientist T.A. Shmaonov, under the guidance of famous Soviet radio astronomers S.E. Khaikin and N.L. Kaidanovsky, carried out measurements of radio emission from space at a wavelength of 32 cm. These measurements were made using a horn antenna similar to the one that was used many years later by Penzias and Wilson. Shmaonov carefully studied possible interference. Of course, at that time he did not yet have at his disposal such sensitive receivers as the Americans later acquired. The results of Shmaonov’s measurements were published in 1957 in his candidate’s thesis and in the journal “Instruments and Experimental Techniques”. The conclusion from these measurements was as follows: “It turned out that the absolute value of the effective temperature of background radio emission... is equal to 4 ± 3 K.” Shmaonov noted the independence of the radiation intensity from the direction in the sky and from time. Although the measurement errors were large and there is no need to talk about any reliability of the number 4, it is now clear to us that Shmaonov measured precisely the cosmic microwave background radiation. Unfortunately, neither he himself nor other radio astronomers knew anything about the possibility of the existence of cosmic microwave background radiation and did not attach due importance to these measurements.
Finally, around 1964, the famous experimental physicist from Princeton (USA), Robert Dicke, consciously approached this problem. Although his reasoning was based on the theory of an “oscillating” Universe, which repeatedly experiences expansion and contraction, Dicke clearly understood the need to search for cosmic microwave background radiation. On his initiative, in early 1965, the young theorist F. J. E. Peebles carried out the necessary calculations, and P. G. Roll and D. T. Wilkinson began building a small low-noise antenna on the roof of the Palmer Physical Laboratory in Princeton. It is not necessary to use large radio telescopes to search for background radiation, since the radiation comes from all directions. Nothing is gained from having a large antenna focus the beam onto a smaller area of the sky. But Dicke’s group did not have time to make the planned discovery: when their equipment was already ready, they only had to confirm the discovery that others had accidentally made the day before.
Despite the use of modern devices and the latest methods studying the Universe, the question of its appearance still remains open. This is not surprising given its age: according to the latest data, it ranges from 14 to 15 billion years. It is obvious that since then there has been very little evidence of the grandiose processes of the Universal scale that once took place. Therefore, no one dares to assert anything, limiting themselves to hypotheses. However, one of them has recently received a very significant argument - cosmic microwave background radiation.
In 1964, two employees of a well-known laboratory, carrying out radio observations of the Echo satellite, having access to the appropriate ultra-sensitive equipment, decided to test some of their theories regarding the own radio emissions of certain space objects.
In order to filter out possible interference from ground-based sources, it was decided to use 7.35 cm. However, after turning on and tuning the antenna, a strange phenomenon was recorded: a certain noise, a constant background component, was recorded throughout the Universe. It did not depend on the position of the Earth relative to other planets, which immediately eliminated the assumption of radio interference from these or on the time of day. Neither R. Wilson nor A. Penzias even realized that they had discovered the cosmic microwave background radiation of the universe.
Since none of them assumed this, attributing the “background” to the peculiarities of the equipment (suffice it to remember that the microwave antenna used was the most sensitive at that time), almost a whole year passed until it became obvious that the recorded noise was part of the Universe itself. The intensity of the detected radio signal turned out to be almost identical to the intensity of radiation with a temperature of 3 Kelvin (1 Kelvin is equal to -273 degrees Celsius). For comparison, zero Kelvin corresponds to the temperature of an object made of motionless atoms. ranges from 500 MHz to 500 GHz.
At this time, two theorists from Princeton University- R. Dicke and D. Pibbles, based on new models of the development of the Universe, mathematically calculated that such radiation should exist and permeate all space. Needless to say, Penzias, who accidentally learned about lectures on this topic, contacted the university and reported that the cosmic microwave background radiation had been registered.
Based on theory Big Bang, all matter arose as a result of a colossal explosion. For the first 300 thousand years after this, space was a combination of elementary particles and radiation. Subsequently, due to expansion, temperatures began to fall, which made it possible for atoms to appear. The detected relict radiation is an echo of those distant times. While the universe had boundaries, the density of particles was so high that the radiation was “bound”, since the mass of particles reflected any kind of waves, preventing them from propagating. And only after the formation of atoms began, space became “transparent” for waves. It is believed that this is how the cosmic microwave background radiation appeared. IN currently Each cubic centimeter of space contains about 500 initial quanta, although their energy has decreased by almost 100 times.
CMB radiation in different parts of the Universe has different temperatures. This is due to the location of the primary matter in the expanding Universe. Where the density of atoms of future matter was higher, the share of radiation, and therefore its temperature, was reduced. It was in these directions that large objects (galaxies and their clusters) subsequently formed.
The study of cosmic microwave background radiation lifts the veil of uncertainty over many processes occurring at the beginning of time.
