The Big Bang theory is a theory about the origin of the universe in astrophysics. 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 observed speed of galaxies moving away under Hubble's law. According to Friedman's general theory of relativity, space may expand. Extending to the past, these observations show that the universe began to expand from an 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. In a narrow sense, the word "Big Bang" refers to the drastic changes experienced in the early days of the universe, about13.7 billion (1.37×1kloc-0/0) years ago. 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. The history of the Big Bang theory was developed through experimental observation and theoretical deduction. In the experimental observation, in the 19 10' s, Vesta Slipher and Karl wertheim Vilt confirmed that most spiral galaxies were retreating from the earth, but they didn't think what this meant to cosmology, and they didn't think that the discovered nebula was actually 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? 0? 6 Trey-Robertson-Walker Universe, time is 1922.

1927, the Belgian Catholic priest Lemaistre independently deduced Friedman-Ma Le? 0? 6tre-Robertson-Walker equation, and based on the shrinking phenomenon of spiral nebula, the "explosion" of the universe from a "primary atom"-this is later called the Big Bang. From 65438 to 0929, Edwin Haber provided experimental conditions for Lemaistre's theory. Haber proved that these spiral nebulae are actually galaxies, and measured their distance by observing Cepheid variables. He found that the speed of galaxies leaving the earth is directly proportional to their distance from the earth, which is the so-called Harper's law. According to the principle of cosmology, there is no special direction and special point when observing a large enough space, so Harper'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, supported and perfected by Gaimov; 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, the name of this theory is due to Huo Yier's satire. He spread it in the form of BBC radio program on 1949, and his paper "The Essence of Things" was published on 1950. Many years later, these two theories coexisted, but the observed facts began to support a universe evolving in a dense state at high temperature.

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.