Secondly, what is cosmic background radiation? Where did it come from? Cosmic background radiation is a large group of photons, but the source of this group of photons is different from that of ordinary light sources. In the latter, the light source (if the light source is a blackbody, the photon group it emits is blackbody radiation) and the photon group it emits are clearly defined. For example, the sun and sunlight are two obviously different things. But the former, there is no light source at all At the beginning of the Big Bang, some form of energy was transformed into a hodgepodge of photons, quarks, leptons and many other elementary particles with vacuum phase transition. At this time, not only photons only contain kinetic energy, but also other particles with static mass account for the vast majority of their total energy-their properties are almost the same as photons (at this time, all high-energy particles are actually called "radiation", and the main difference between radiation and ordinary matter particles is the amount of kinetic energy), and they share each other frequently. It is precisely because of the strong interaction between them that the whole pot of hodgepodge is in a very uniform thermal equilibrium state, and the photon group in it is bound to be in a blackbody radiation state (see Annex 1).
Thirdly, the photon group in the background radiation has long been decoupled from the material particles-the interaction between them is very small, because with the cooling of the universe, all the particles lose a lot of weight and can no longer be transformed into each other. These two groups of matter are like isolated systems, and there is almost no energy exchange between them. Therefore, the decoupled background radiation is neither endothermic nor exothermic.
Finally, the energy density of radiation is proportional to the-4th power of the cosmic scale, and the energy density of matter particles is proportional to the-3rd power of the cosmic scale. Therefore, with the expansion of the universe, the average temperature of physical particles drops slower than that of radiation. So if we really want to calculate the weak interaction between them, we can say that the background radiation is endothermic. However, it should be noted that this heat absorption has nothing to do with the properties of ordinary blackbody, because the background radiation itself has nothing to do with the properties of ordinary blackbody, and nothing plays the role of ordinary blackbody at all in the process of background radiation generation.
Attachment 1: The microwave background radiation is very consistent with the blackbody spectrum.
A small hole in the cavity is used to simulate a black body in the laboratory. This black body is darker than the actual black object (such as carbon black), because a large number of photons entering the cavity from the small hole have to be absorbed and reflected many times, and only a tiny part of them have the chance to come out from the small hole again, which is equivalent to that the small hole absorbs almost all the incident light, so it is a good black body. Since it is a good blackbody, the radiation emitted from its pores is a good thermal radiation with blackbody spectrum by heating this cavity. It can be seen that when radiation and thermal objects interact for many times and reach thermal balance (think of photons hitting walls everywhere in the cavity … being absorbed and emitted continuously …), radiation becomes radiation with good blackbody spectral characteristics. In the early days of BIGBANG, the interaction between photons and physical particles was extremely frequent, and they reached a good state of thermal balance with each other, so the energy distribution of photons had good blackbody spectral characteristics at this time, but after they cooled down-the Doppler redshift caused by the expansion of the whole space widened each band in the same proportion-the distribution pattern of photons still maintained the blackbody spectrum.