Big Bang Scenario
Like any scheme that claims to explain the data on the spectrum of microwave cosmic radiation, the chemical composition of pre-galactic matter and the hierarchy of the scales of cosmic structures, the standard model of the evolution of the Universe is based on a number of initial assumptions (about the properties of matter, space and time) that play the role of original conditions for the expansion of the world. One of the working hypotheses of this model is the assumption of uniformity and isotropy of the properties of the Universe throughout all stages of its evolution.
In addition, based on data on the spectrum of microwave radiation, it is natural to assume that in the Universe in the past there was a state of thermodynamic equilibrium between plasma and radiation, the temperature of which was high. Finally, extrapolating to the past the laws of increasing the densities of matter and radiation energy, we will have to assume that already at a plasma temperature close to 1010 K, protons and neutrons existed in it, which were responsible for the formation of the chemical composition of cosmic matter.
Obviously, such a complex of initial conditions cannot be formally extrapolated to the very early stages of the expansion of the Universe, when the plasma temperature exceeds 1012 K, since under these conditions qualitative changes in the composition of matter would occur, associated, in particular, with the quark structure of nucleons. This period, preceding the stage with a temperature of about 1012 K, can naturally be attributed to the super-early stages of the expansion of the Universe, which, unfortunately, are still very little known.
The fact is that, as we deepen into the past of the Universe, we inevitably encounter the need to describe the processes of interconversion of elementary particles with more and more energy, tens or even thousands of times higher than the threshold of energies available to research at the most powerful modern accelerators. In such a situation, obviously, a whole complex of problems arises, firstly, with our ignorance of the new types of particles that are born under conditions of high plasma densities, and secondly, with the lack of a “reliable” theory that would allow us to predict the main characteristics of the cosmological substrate in this period.
However, even without knowing in detail the specific properties of superdense plasma at high temperatures, it can be assumed that, starting with a temperature slightly lower than 1012 K, its characteristics satisfied the conditions listed at the beginning of this section. In other words, at a temperature of about 1012 K, matter in the Universe was represented by electron-positron pairs (e -, e +); muons and antimuons (m -, m +); neutrinos and antineutrinos, both electronic (ve, ve), and muon (vM, vM) and tau-neutrinos (vt, vt); nucleons (protons and neutrons) and electromagnetic radiation.
The interaction of all these particles ensured a state of thermodynamic equilibrium in the plasma, which, however, changed as the Universe expanded for various types of particles. At temperatures less than 1012 K, muon-antimuon pairs with a rest energy of about 106 MeV8 were the first to “feel” it. Then, even at a temperature of the order of 5.109 K, the annihilation of electron – positron pairs began to prevail over the processes of their production during the interaction of photons, which ultimately led to a qualitative change in the plasma composition.
Starting with temperatures T <109 K, the main role in the dynamics of the expansion of the Universe began to play electronic, muonic and tau neutrinos, as well as electromagnetic radiation. How is the energy redistributed that was “stored” at the leptonic stage in massive particles?
It turns out that she went on to “heat” the radiation, and with it the particles that are at temperatures above 5.109 K in equilibrium with the radiation. Indeed, a slight increase in the density of photons caused by the annihilation of muons and antimuons automatically leads to an increase in the concentration of electron-positron pairs that interact with photons in the reaction Y + Y -> e – + e +. In turn, electrons and positrons can produce pairs of neutrinos and antineutrinos.
Thus, the entire excess energy of muons after their annihilation is redistributed between the various components of the plasma. A similar “transfer” of the energy of massive particles to lighter ones was to be carried out only until the lightest charged leptons — electrons and positrons, which last “heated” the radiation at a temperature of about 5 * 109 K — began to annihilate. At that moment, electromagnetic radiation played a dominant role in the expansion of the Universe, and the leptonic era of the “temperature” history of cosmic plasma was replaced by the era of radiation predominance.
In fact, it was during this period at plasma temperatures of about 5 * 109 K that the equilibrium spectrum of electromagnetic radiation formed, which came to us in the form of a microwave relic background. It was during the annihilation of electron-positron pairs that almost all the energy stored in this component was transferred to electromagnetic radiation.