According to the Big Bang theory, the universe expanded from a dense and hot singularity to its present state. The Big Bang theory is a theory about the origin of the universe in cosmic physics. According to the Big Bang theory, the universe evolved from a state of extremely high density and temperature about 654.38+0.4 billion years ago. This theory comes from the speed at which galaxies are moving away observed under Hubble's law. According to Friedman's general theory of relativity, space may expand. Extending (mathematically contrary to interpolation) to the past, these observations show that the universe began to expand from its initial state. In this initial state, the temperature and density of matter and energy in the universe are extremely high. As for what happened before this, general relativity thinks that there is a gravitational singularity, but physicists have different views on it.
The narrow sense of "Big Bang" refers to the drastic changes that the universe experienced at the early stage of its formation, about 65.438+03.7 billion years ago (654.38+0.37× 654.38+006.54.38+00). But in a broad sense, it refers to the popular theory that reveals the origin and expansion of the universe. The direct corollary of this theory is that the universe we live in today is different from that of yesterday or tomorrow. According to this theory, George Gamow predicted the existence of cosmic microwave background radiation in 1948. In the 1960 s, this kind of radiation was detected, which strongly supported the big bang theory, thus denying another popular steady-state universe theory.
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Development history
The Big Bang theory was developed through experimental observation and theoretical deduction. In the experimental observation, in the 19 10 era, Vesta Slipher and Karl wertheim Vilt confirmed that most spiral nebulae are moving away from the earth, but they don't think it means anything to cosmology, and they don't think that the discovered nebulae are actually other galaxies outside the Milky Way. At the same time, in theory, Einstein's general theory of relativity successfully established and deduced the universe without stable state. The universe described by metric tensor is either expanding or contracting. Einstein thought he had solved the mistake and added a cosmological constant to correct it. The first person who really applied general relativity to cosmology without using cosmological constants was Alexander Friedman. The universe described by his equations is called Friedman-Ma Le? The Tre-Robertson-Walker universe has been expanding constantly, and it is reasonable to imagine that it used to be smaller than it is now. If we can reverse the movie history of the universe, we should find that at some time long ago, all the stars were together, and the universe was originally a dense material core. 1927, the Belgian Catholic priest Lemaistre independently deduced Friedman-Ma Le? Tre-Robertson-Walker equation, and put forward that the universe "exploded" from a "primary atom" on the basis of the shrinking phenomenon of spiral nebula-this is the later big bang.
From 65438 to 0929, Edwin Hubble provided experimental conditions for Lemaistre's theory. Hubble proved that these spiral nebulae are actually galaxies, and measured the distance between them by observing the stars in the Cepheus Delta. He found that the speed of galaxies leaving the earth is just proportional to their distance from the earth, which is the so-called Hubble law. According to the principle of cosmology, there is no special direction and special point when observing a large enough space, so Hubble's law shows that the universe is expanding. There are two opposing possibilities in this view: one is the Big Bang theory put forward by Lemaistre and supported and perfected by Gamov; The other is Huo Yier's stable universe model. In the steady-state universe model, the space left by distant galaxies constantly produces new substances, so the universe remains basically unchanged. In fact, this theory was put forward to satirize Lemaistre's "The Big Bang Theory", which was first spread through the BBC radio program in 1949, and the paper "The Essence of Things" was published in 1950.
For many years after that, the two theories coexisted, but the observed facts began to support a universe that evolved into a thermally dense state. 1965 The discovery of cosmic microwave background radiation makes people think that the Big Bang theory is the best theory for the origin and evolution of the universe. Before 1970, many cosmologists thought that the universe might contract before it expanded, so as to avoid deducing an infinitely dense "absurd" singularity from Friedman's model. The representative model is Richard tolman's Oscillating Universe. At the end of1960s, Stephen Hawking and others proved that this hypothesis is not feasible, because singularity is a direct and important inference of Einstein's theory of gravity. After that, most cosmophysicists began to accept that the universe described by general relativity is limited in time. However, due to the lack of understanding of the law of quantum gravity, it is still uncertain whether this singularity is an infinitesimal point in the true sense, or whether the physical contraction process can go on indefinitely, thus indirectly reaching the infinity of the universe in time.
At present, almost all the research of cosmic physics is related to the Big Bang theory, or it is an extension or a further explanation, such as how galaxies were formed under the Big Bang theory, the physical processes that occurred during the Big Bang, and the new observations are explained by the Big Bang theory. At the end of 1990s and the beginning of the 20th century, due to the development of telescope technology and the large amount of data collected by artificial detectors, the Big Bang theory made a major breakthrough. The situation and data of the universe during the Big Bang can be calculated more accurately, which has produced many unexpected results, such as the acceleration of the expansion of the universe. See dark energy.
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theory
The Big Bang theory estimated the age of the universe as 137 2 billion years, which was obtained through the observation of type Ia supernovae, the measurement of cosmic background radiation intensity and the measurement of galaxy correlation function. The results of these three independent calculations are consistent, which is considered as strong evidence of the so-called Lambda-CDM model that describes the properties of galaxies in the universe in more detail. The early universe was full of homologous substances, and its temperature, pressure and energy were extremely high. With the expansion and cooling, the matter in the universe changes phase, which is similar to the condensation process of steam cooling and the solidification process of water, but the difference is that the former occurs at the more basic particle level.
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affect
The first 1 sec is a watershed in the history of the universe. After this moment, the temperature of the universe dropped to a certain extent, which can be described by our existing physical knowledge, thus obtaining a roughly accurate bird's eye view of the universe. Before 1 s, the dense and hot universe was a bunch of particles that humans could not understand, and the existing physical laws could not describe their behavior. This is the black box type of 1 sec.
Before 1 s, there should be equal amounts of protons and neutrons in the universe, because weak interaction will transform protons and neutrons into each other and maintain their balance. However, at 1 s, the expansion speed becomes too large, and the weak interaction can no longer maintain the balance of proton and neutron numbers. Because neutrons are slightly heavier than protons, it takes more energy to convert protons into neutrons, which is more difficult than converting neutrons into protons. Then, the weak interaction stops, and neutrons and protons no longer transform into each other in large quantities, leaving a certain proportion of the relative number of neutrons and protons-about 1 ratio 6.
Within 3 minutes after the initial 1 s, neutrons and protons violently polymerize to form deuterium, helium and lithium nuclei, mainly helium nuclei. This process consumes all neutrons and the remaining protons become hydrogen nuclei. After 3 minutes, the temperature of the universe dropped below1000 billion degrees, and the material density also dropped rapidly, so this nuclear reaction stopped. The calculation shows that in the first three minutes, about 22-24% of the substances form Helium 4, and almost all the remaining substances exist in the form of hydrogen, with only a few parts in 100,000 becoming Helium 3 and deuterium, and a few parts in 10 billion becoming lithium.
So the Big Bang model predicts that 22-24% of the matter in the universe should be helium, and the rest is mostly hydrogen. Hydrogen and helium formed in the first three minutes constitute more than 99% of the matter in the universe. The rich and colorful heavy elements that form planets and life account for less than 1% of the total mass of the universe, and most of them were formed long after the Big Bang.
The observation of helium, deuterium and other elements in the whole universe proves the universality of the above abundance values. There is a wonderful consistency between the simple big bang model and strict astronomical observation. This prediction was the greatest success of the Big Bang. About one million years ago, the temperature dropped to 107 ~ 6K, and the universe was filled with plasma composed of light element nuclei, electrons and protons. After 250 million years, when the temperature dropped to 103K, the radiation weakened, neutral atoms were formed, and the plasma recombined into normal gas. Galaxies began to form about/kloc-0.0 billion years ago, the first stars began to appear about 5 billion years ago, and the solar system was formed about/kloc-0.0 billion years ago.
With the passage of time, in the almost evenly distributed material space, the slightly denser areas attract nearby materials through gravity, thus increasing the density, forming today's astronomical structures such as gas clouds, stars and galaxies. The specific process depends on the form and quantity of cosmic matter, among which there may be three forms: cold dark matter, hot dark matter and baryon matter.