Every cubic centimeter of space contains about 500 photons. Substances on the same volume accounted for a much lower: about 10'6 baryons (the so-called heavy elementary particles, including the protons and neutrons). Since photons don't disappear (the space between galaxies is transparent), the ratio of photons to the number of baryons in the course of expansion of the Universe is preserved. But the energy of the photons decreases over time because of the red shift.

Therefore, somewhere in the past, the energy density of radiation was more energy density of ordinary particles. This means that up to a certain point photons not only in the number but also "weight" (mass - it's just energy divided by the square of the speed of light) exceeded the baryons. In those days radiation fully defined the nature of the Universe expansion. On the era referred to as radiation stage in the evolution of the Universe. At this stage, the temperature of the substance and radiation was the same.

But one day, after about a million years after the beginning of the expansion of the Universe, everything has changed: there was a transition radiation from stage to stage substances. This event is called the moment regso^MND^them. The temperature is then dropped to a few thousand degrees. Of nuclear physics it is known that at this temperature begins Association (recombination) electrons, former to this free particles, protons and helium nuclei. It is at this stage in the Universe began the formation of atoms, mostly hydrogen and helium.

If recombination ionized matter and radiation actively interacted with each other, then after that the situation has changed dramatically: the light quanta almost ceased "see" neutral atoms. The universe became transparent to radiation, which began to travel freely. This is the radiation we now perceive as a relic. Figuratively speaking, the photons of the CMB "captured" epoch of recombination and have direct information about the distant past. However, since photons "red" because of the expansion of the Universe and have reduced their energy is approximately 1000 times.

After recombination substance for the first time began to evolve independently from radiation, and it began to receive the seal - embryos of future galaxies and their clusters. That is why it is important for scientists experiments to study the properties of the CMB - its spectrum and spatial inhomogeneities (fluctuations). Their efforts were not in vain: in the early 90s, the Russian space experiment "Relic-2" and the American "Kobe" found very little difference in the temperature of the cosmic background radiation and the neighboring areas of the sky. The deviation from the average temperature (2,73 To) is only about a thousandth of a percent! These temperature variations carry the information about the deviation of the density of a substance from the mean in the era of recombination. It is the variation of the density subsequently led to the formation of the observed Universe of large-scale structures, clusters of galaxies and of individual galaxies.

Immediately after recombination there were no stars, no galaxies, no other space objects; the substance is spread in the Universe almost evenly. The reason of homogeneous environment formed massive bodies (stars, planets, galaxies, and so on) is found in the power of gravity. There, where the density was slightly above average, was stronger and gravity, and therefore more dense education has become even closer. Conversely, areas of low density everything was done razreshenie, as the substance of them went into the denser region. Thus, initially almost homogeneous environment is eventually split into individual "clouds", which was formed galaxies.

In the modern view, the first galaxies were formed in the era, which corresponds to the red shifts z I; 4-8 (recall that the red shift is called a change in the wavelength of the electromagnetic radiation in relation to the original wavelength). Observing very distant galaxies with large red shifts confirm that this is the most young objects that we see soon after their birth.