From the "singularity", about 10-37 seconds after the expansion, the phase transition occurred, which led to the skyrocketing of the universe. During this period, the expansion of the universe increased exponentially. When the boom ended, the matter that made up the universe included quark-gluon plasma and all other elementary particles. At this time, the universe is still so hot, and particles are doing relativistic high-speed random motion. During this period, particle-antiparticle pairs are constantly produced and annihilated through collisions, making the number of particles and antiparticles in the universe equal (the total baryon number in the universe is zero). Until a certain moment later, an unknown reaction process that violates the conservation of baryon number appears, making the number of quarks and leptons slightly exceed the number of antiquarks and leptons-the range is about one-third of the order of magnitude. This process is called baryon number generation.

With the expansion of the universe and the further decrease of temperature, the energy of particles will generally decrease gradually. When the energy is reduced to1tev (1012ev), symmetry breaking occurs, which makes the elementary particles and the elementary interaction form what we see today. The birth of the universe is 10- 18. The components guessed in the Big Bang model are further reduced, because the particle energy at this time has been reduced to the range that can be achieved by high-energy physics experiments. 10-6 seconds later, quarks and gluons combine to form protons, neutrons and other baryons. Because the number of quarks is slightly higher than that of antiquarks, the number of baryons is also slightly higher than that of antiquarks. At this time, the temperature of the universe has been lowered enough to produce new proton-antiproton pairs (similarly, new neutron-antiparticle pairs cannot be produced), which immediately leads to the mass annihilation between particles and antiparticles, making the original number of protons and neutrons only one billion, and the corresponding antiparticles are all annihilated. After about 1 s, a similar process occurred between electrons and positrons. After this series of annihilations, the velocities of the remaining protons, neutrons and electrons drop below the theory of relativity, and at this time,

Minutes after the Big Bang, temperature of the universe dropped to about one billion Kelvin, and the density dropped to about the level of air density. A few protons combine with all neutrons to form nuclei of deuterium and helium. This process is called initial nuclear synthesis. However, most protons do not combine with neutrons to form hydrogen nuclei. With the cooling of the universe, the energy density of the universe mainly comes from the contribution of gravity generated by static mass. About 379,000 years later, electrons and nuclei combined to form atoms (mainly hydrogen atoms), while substances emitted radiation through decoupling, which spread relatively freely in space, and the remnants of this radiation formed today's cosmic microwave background radiation.

Although the matter in the universe is almost evenly distributed on a large scale, there are still some areas with slightly higher density, so for a long time, the matter in these areas attracts the nearby matter through gravity, thus becoming more dense, forming the gas clouds, stars, galaxies and other structures that can be observed in astronomy today. The details of this process depend on the form and quantity of matter in the universe. There may be three forms: cold dark matter, hot dark matter and baryon matter. The best observation results from WMAP show that the dominant form of matter in the universe is cold dark matter, while the other two forms of matter account for less than 18% in the universe. On the other hand, independent observations of type Ia supernovae and cosmic microwave background radiation show that the universe is dominated by an unknown energy form called dark energy. Dark energy is thought to permeate every corner of space. Observations show that 72% of the total energy density of the universe exists in the form of dark energy. It is speculated that dark energy existed when the universe was very young, but at this time the scale of the universe was very small and the distance between substances was very close, so the role of gravity was remarkable at that time, thus slowing down the expansion of the universe. But after decades of expansion, the increasing dark energy began to slow down the expansion of the universe. The simplest way to express dark energy is to add the so-called cosmological constant term to Einstein's gravitational field equation, but this still can't answer the questions about the composition and formation mechanism of dark energy, and some more basic problems associated with it, such as the details of its state equation and its internal relationship with the standard model in particle physics. These unsolved problems still need to be further studied in theory and experimental observation.

All the evolution of the universe after the skyrocketing period can be described very accurately by λλCDM model in cosmology, which comes from the independent framework of general relativity and quantum mechanics. As mentioned above, within about 10- 15 seconds after the big bang, there was no widely supported model to describe the universe. It is generally believed that a quantum gravity theory integrating general relativity and quantum mechanics is needed to break through this problem. How can we understand this problem?

At the beginning of the universe, there was nothing but energy. After the Big Bang, matter was transformed from energy (mass-energy conversion E=mcc). Contemporary particle physics tells us that at a sufficiently high temperature (called "threshold temperature"), the collision of photons can produce matter particles. The following is the detailed process of material evolution in the universe:

At the time scale of 65438 th+0/65438 th+00000 seconds after the birth of the universe, the temperature reached several hundred trillion kelvin, which was higher than the threshold temperature of hadrons and leptons. Photon collisions produce positive and negative hadrons and leptons, some of which are annihilated into photons. When the equilibrium state is reached, the total number of particles is roughly equal to the total number of photons, and the annihilation hadrons are broken into "quarks", at which time quarks are in a state of no mutual protection.

On the time scale of 0.0 1 sec, the temperature is 1000 billion kHz, which is lower than the threshold temperature of hadron and higher than that of lepton. The reaction of photons producing hadrons stopped, and hadrons were no longer decomposed into quarks, with protons and neutrons accounting for half, but the number of hadrons decreased due to the continuous annihilation of positive and negative protons. Neutrons and protons are constantly transforming each other, and the temperature is 65438+ at 1.09 seconds.

Time scale 13.82 seconds, temperature less than 3 billion kelvin, completing the task of material creation. Neutron decay occurs, which is proton plus electron plus antineutrino. At this time, proton: neutron = 83: 17.

The time scale is 3 minutes and 46 seconds, the temperature is 900 million, and all antiparticles are annihilated. Photon: matter particle = 1 billion: 1, neutron does not decay, proton: neutron = 87: 13 (until now); At this time, a very important evolution took place: two protons and two neutrons produced 1 helium nucleus, and the neutron was preserved because of the constraint of nuclear force. The universe has entered the era of nuclear synthesis. If there is no helium nucleus, all neutrons will decay, and other nuclei in nothingness will also decay in the future.

The time scale is 300,000-700,000 years, the temperature is 4,000-3,000 K, and energy and matter are in thermal balance. Stable hydrogen and helium nuclei began to appear, and the universe entered the era of recombination. In the later period, the universe gradually changed into an era dominated by matter. (Photons can spread freely with the decrease of temperature, which is the background radiation of today's three-opening universe! ).

The time scale is 400-500 million years and the temperature is 100. Matter particles begin to condense and gravity gradually increases. After the "dark ages", the first stars and galaxies formed.

With the formation of the first stars, the atoms in the stars undergo nuclear fusion reaction, and then nuclei of helium, carbon, oxygen, magnesium, iron and other elements appear. Nuclear fusion refers to a form of nuclear reaction in which atoms with small mass, mainly deuterium or tritium, polymerize with each other under certain conditions (such as ultra-high temperature and high pressure) to generate new nuclei with heavy mass, accompanied by huge energy release.