The Big Bang Theory is the most influential theory in modern cosmology. Its main point is that the universe once had an evolutionary history from hot to cold. During this period, the cosmic system is constantly expanding, which makes the density of matter evolve from dense to sparse, just like a huge explosion. Basic introduction Chinese name: BIGBANG mbth: The universe? big bang ； The Big Bang Theory time: 13.82 billion years ago? Formation: The founders of dense and fiery singularity expansion and explosion theory: Lemaitre, Gamow, Hubble, etc. Brief introduction, production principle, basic assumptions, research history, initial stage, verification stage, mature stage, brief history of explosion, observation facts, related concepts, expansion space, horizon, microwave radiation (Nobel Prize in Physics in 1978), helium abundance, main evidence, theoretical awards, existing problems, modern debates. Introduction "The Big Bang Theory" holds that the universe was formed by the expansion of a dense and hot singularity after a big bang 13.7 billion years ago. In 1927, Belgian astronomer and cosmologist Lemaitre (Georges Lema? Tre) proposed the BIGBANG hypothesis for the first time. In 1929, according to the hypothesis, American astronomer Hubble put forward Hubble's law that the redshift of galaxies is proportional to the distance between galaxies, and deduced the inflationary universe theory in which galaxies are far away from each other. The most influential theory in modern cosmology. Its main point is that the universe once had an evolutionary history from hot to cold. During this period, the cosmic system is constantly expanding, which makes the density of matter evolve from dense to sparse, just like a huge explosion. One of the founders of this theory is Gamov. In 1946, American physicist Gamov formally put forward the Big Bang theory, arguing that the universe was formed by a big bang that happened about 14 billion years ago. At the end of last century, the observation of Ia supernovae showed that the universe was expanding at an accelerated pace, because the universe might be mostly composed of dark energy. At the beginning of the explosion, matter can only exist in the form of basic particles such as neutrons, protons, electrons, photons and neutrinos. The constant expansion after the explosion of the universe caused the temperature and density to drop rapidly. With the temperature decreasing and cooling, atoms, nuclei and molecules are gradually formed, and they are combined into common gases. The gas gradually condensed into nebulae, which further formed various stars and galaxies, and finally formed the universe we see today. The idea that the universe does not exist forever, but is created from nothingness can be said to be deeply rooted in western culture. Although Greek philosophers have considered the possibility of an eternal universe, all major western religions have always insisted that the universe was created by God at a certain time in the past. Basic Assumptions The establishment of the Big Bang theory is based on two basic assumptions: the universality of physical laws and cosmological principles. Cosmological principle means that the universe is homogeneous and isotropic on a large scale. At first, these viewpoints were introduced as transcendental axioms, and now there are related research works trying to verify them. For example, for the first hypothesis, it has been proved by experiments that the relative error of the fine structure constant will not exceed 1 (-5) for most of the time since the birth of the universe. In addition, through the observation of solar system and binary star system, the general theory of relativity has been verified by very accurate experiments; On a broader cosmological scale, the empirical success of the Big Bang theory in many aspects is also a strong support for general relativity. Assuming that the large-scale universe is isotropic from the earth, the cosmological principle can be derived from a simpler Copernican principle. The Copernican principle means that there is no preferred (or special) observer or observation position. According to the observation of microwave background radiation, the cosmological principle has been proved to be established on the order of 1 (-5), while the uniformity observed in the universe on a large scale is on the order of 1%. What many people don't know at the initial stage of the research process is that compared with today, when the Big Bang theory has become common sense, the attitude of the world scientific community was "scoffed at" for a long time after it was just put forward. This strange phenomenon is because the scientific community at that time was influenced by the philosophical trend of thought that the theory of evolution overthrew the theory of God's creation, blindly opposed the traditional theory, and refused to admit that the universe had a starting point, as the Bible said. During this period, the western scientific community generally insisted that the universe and matter were constant, without beginning or end. Therefore, all theories involving that the universe and everything "have a starting point" are not recognized. Including great scientists like Einstein, are also influenced by it. Einstein summed up the gravitational field equation, and found that this formula of Rμv-(1/2)Rgμv=kTμv would deduce that the universe is actually a dynamic universe with never-ending material changes, so he imposed a "cosmological constant" in this formula to maintain the calculation results of the static universe. That is to say, the original field equation is actually like this: ∧gμv+Rμv-(1/2)Rgμv=kTμv, where the constant ∧ is the cosmological constant. Verification stage But since American astronomer Edwin Hubble began to observe the "redshift phenomenon" in 1922, the view of "cosmic expansion" began to take shape. In 1929, Edwin Hubble summed up a landmark discovery, that is, no matter which direction you look, distant galaxies are rapidly leaving us, while nearby galaxies are approaching us. In other words, the universe is expanding. This means that the stars were closer to each other in the early days. In fact, it seems that they happened to be in the same place at some time about 1 billion to 2 billion years ago, so Hubble's discovery suggests that there was a moment called the Big Bang, when the universe was at a singularity with infinite density. Hearing this, Einstein soon came to Wilson Observatory where Hubble worked, and personally observed the redshift phenomenon under the leadership of Hubble. After the interview, Einstein publicly admitted the mistake that his subjective consciousness influenced the scientific conclusion, and removed the cosmological constant in the field equation, so there was the Einstein Field Equation that we are familiar with today. Mature stage Around 1948, Gamov was the first to establish the concept of thermal explosion. The Big Bang that created the universe is not the kind of explosion that happened at a certain point on the earth and then spread to the surrounding air, but the kind of explosion that happened everywhere at the same time and filled the whole space from the beginning. Every particle in the explosion flew away from every other particle. In fact, it should be understood as the rapid expansion of space. "Whole space" can refer to the whole infinite universe, or a finite universe that can bend back to its original position like a sphere. According to the Big Bang cosmology, the early universe was a large homogeneous gas composed of microscopic particles, with extremely high temperature, extremely high density and expanding at a great rate. These gases have a uniform temperature under thermal equilibrium. This unified temperature was an important symbol of the state of the universe at that time, so it was called temperature of the universe. The adiabatic expansion of gas will reduce the temperature, and make the nucleus, atom and even star system appear one after another. A brief history of explosion At the beginning of the Big Bang: About 15 billion years ago, the point with infinitely small volume, infinite density, infinitely high temperature and infinite curvature of spacetime was called a singularity. Space and time were born out of some kind of timelessness-some cosmologists call it quantum vacuum (pseudo-vacuum), which is full of quantum energy disturbance consistent with Heisenberg's uncertainty principle. 1 -43 seconds after the Big Bang (Planck time): about 1 32 degrees, the universe emerged from the background of quantum fluctuations, and this stage is called Planck time. Before that, the density of the universe may exceed 1.94 grams per cubic centimeter, which is 1.78 times higher than the density of protons. All forces in physics are one kind. (Supersymmetry) At this stage, the universe has cooled down to the point where gravity can be separated and begin to exist independently, and there are gravitons that transmit gravitational interaction. Other forces in the universe (strong, weak interaction and electromagnetic interaction) are still one. 1 -35 seconds after the Big Bang: about 1 27 degrees. During the skyrocketing period (the first push), gravity has separated and quarks, bosons and leptons have formed. At this stage, the universe has cooled to the point where the strong interaction can be separated, while the weak interaction and electromagnetic interaction are still unified in the so-called electric weak interaction. The universe has also skyrocketed, which lasted only 1 -33 seconds. At this moment, the universe experienced 1 times doubling (2,1), and the scale obtained was 1-3 times that of the previous scale (the skyrocketing is the universe itself, that is, space and time itself, and does not violate the light speed barrier). Before the inflation, the universe was still in the interconnected range of photons, which could smooth out all rough points. When the inflation stopped, what was detected today had stabilized in their respective small areas, which is called the inflation theory. 1 -12 seconds after the Big Bang: about 1 15 degrees, during the particle phase, protons and neutrons and their antiparticles formed, and bosons, neutrinos, electrons, quarks and gluons stabilized. The universe becomes cold enough, and the weak electric interaction is decomposed into electromagnetic interaction and weak interaction. Lepton families (electrons, neutrinos and corresponding antiparticles) need to wait for the universe to continue to cool for 1 -4 seconds before they can be separated from the equilibrium phase with other particles. Once the neutrinos are decoupled from matter, they will travel freely through space, and in principle, these native neutrinos can be detected. .1 second after the Big Bang: about 1 billion degrees, mainly photons, electrons and neutrinos, with proton neutrons accounting for only one billionth. The system is in thermal equilibrium, and the temperature and density are constantly decreasing. .1 seconds after the big bang: about 3 billion degrees, the neutron-proton ratio dropped from 1. to .61. One second after the big bang: about 1 billion degrees, neutrinos escaped outward, and the annihilation reaction of positive and negative electrons appeared, and the nuclear force was not enough to bind neutrons and protons. Ten seconds after the big bang: about 3 billion degrees, during the nuclear period, stable nuclei (chemical elements) such as hydrogen and helium were formed. When the universe cools below 1 9 Kelvin (about 1 seconds later), the particle transition is impossible. The calculation of nuclear synthesis indicates that baryon density only accounts for 2%~5% of the matter needed for a topologically flat universe, which strongly implies that other forms of matter energy (non-baryon dark matter and dark energy) are full of the universe. 35 minutes after the big bang: about 3 million degrees, the primary nuclear synthesis process stopped, and neutral atoms could not be formed. 1 11 seconds (1 4 years) after the Big Bang, the temperature was about 1 5 Kelvin, which was the material period. In the early history of the universe, light dominated all forms of energy. With the expansion of the universe, the wavelength of electromagnetic radiation is lengthened, and the corresponding photon energy is also reduced. The radiation energy density decreases in inverse proportion to the product of scale (R) and volume (4πR 3 /3), that is, An 1/R 4 decreases, while the energy density of matter simply decreases in inverse proportion to volume. Ten thousand years later, the density of matter caught up with the radiation density and surpassed it. From then on, the universe and its dynamics began to be dominated by matter. 3, years after the Big Bang: about 3, degrees, neutral atoms were formed by chemical combination, and the main components of the universe were gaseous substances, which gradually condensed into gas clouds with high density under the action of self-gravity until stars and star systems. Quantum vacuum reached its peak in the skyrocketing period, and then it permeated the whole universe in the form of dark energy, and with the rapid decrease of matter and radiation density, dark energy became more and more obvious. Dark energy may occupy 2/3 of the total energy density of the universe, thus promoting the accelerated expansion of the universe. The scientific nature of the big bang theory is compelling. The most direct evidence comes from the study of the light characteristics of distant galaxies. In the 192s, the astronomer Edwin Hubble studied the observations made by Vesto Slipher. He noticed that the colors of distant galaxies were slightly redder than those of nearby galaxies. Hubble carefully measured this reddening and made a picture. He found that this reddening (redshift) is systematic, and the farther away a galaxy is from us, the redder it appears. The color of light is related to its wavelength. In the white light spectrum, blue light is at the short wave end and red light is at the long wave end. The reddening of distant galaxies means that their wavelength of light waves has been slightly longer. Hubble confirmed this effect after carefully determining the positions of characteristic spectral lines in the spectra of many galaxies. He believes that the lengthening of light waves is the result of the expanding universe. This great discovery of Hubble laid the foundation of modern cosmology. The nature of the expanding universe puzzles many people. From the earth's point of view, it seems that distant galaxies are rapidly leaving us. However, this does not mean that the earth is the center of the universe. On average, the expansion images in different parts of the universe are the same. It can be said that every point is the center, and no point is the center (the best explanation is a painting: the cutting of three-dimensional space). We'd better think of it as the space between galaxies is stretching or expanding, rather than the galaxies moving in space. This is different from the explosion from one point that we see in our daily life. The fact that space can stretch seems strange, but it is a concept that scientists have been familiar with since Einstein's general theory of relativity was published in 1915. General relativity holds that gravity is actually a manifestation of the bending or deformation of space (strictly speaking, space-time). In a sense, space is elastic and can be bent or stretched in a certain way, depending on the arrangement of substances. This idea has been fully confirmed by observation. Related Concepts The basic concept of expansion space can be understood through a simple simulation. Imagine sewing a row of buttons on an elastic band. Suppose that the elastic band is stretched from both ends, and as a result, all the buttons are far away from each other. No matter which button we choose to look at, the buttons on its adjacent side seem to be moving away, and this expansion is the same everywhere, and there is no special center. Of course, when we draw this row of buttons, it has a central button, but this has nothing to do with the way the system expands. As long as this elastic band with buttons is infinitely lengthened or looped into a circle, this center will no longer exist. From any button, the nearest [url button regresses at a certain speed, and then the next button regresses by twice, and so on. In your opinion, the farther away the button is, the faster it retreats. Therefore, this expansion means that the regression speed is proportional to the distance-this is an extremely important relationship. With the help of this image, we can imagine that the light wave is. No wonder Hubble found that the redshift is proportional to the distance, which is completely consistent with the result of this simple image simulation. An important feature of the big bang of the horizon is the existence of the horizon: because the universe has a finite age and light has a finite speed, there may be some past events that cannot transmit information to us through light. From this analysis, we can see that there is such a limit or past horizon, and only here.