Generally accepted scenario of galaxy origin.
When the fundamental law of energy conservation was formulated in nineteenth century, the astronomers faced a question about sources of stellar energy and origin of stars. A start emits a huge amount of energy and this process cannot continue forever. Therefore, the lifetime of stars is limited and the stars must appear by some mechanisms. What is the origin of stars?
Later one, at the beginning of twentieth century astronomers discovered that stars are grouped into giant clusters – galaxies, the problem of star origin evolved into the problem of galaxy origin. These problems became more interesting when science recognized that the Universe is expanding and the age of the Universe is less than 15-16 billion years.
For today the generally accepted scenario of formation of galaxies and stars is the following.
Firstly, it is believed that about fifteen billions years ago (this number can changed in future) all of matter was in a superdense state and all has began from a “Big Bang”. Nobody knows why this happened (although there are many hypotheses on this subject); this moment is postulated as a primary instant of Universe evolution.
Secondly, it is assumed that almost all energy of the Universe emerging after the Big Bang was in the form of heat. Often this hypothesis is called a theory of hot Universe.
If we extrapolate Universe expansion back to its origin and assume the thermal form of energy, we can estimate what was the initial temperature in early Universe. For example, astronomers believe that after one hundredth second after the Big Bang the Universe was as extremely hot: up to 100 billion degrees [34,p.113]. At this high temperature the particle energy was so high that it was impossible to form any atomic nuclei and the entire matter had consisted of a chaotic mix of elementary particles.
While the Universe was expanding and the temperature was dropping, helium nuclei were formed – as most stable among atomic nuclei. This theory teaches that almost one fourth of nucleons (a general name for protons and neutrons) merged into helium nuclei during the first three minutes of existence, but the later spreading of the Universe had dropped the temperature and density even more and this synthesis stopped. This brings to an assumption that atomic nuclei heavier than helium could be synthesized only within stars (much later).
After these events the Universe kept expanding and cooling until (this was thousand years after big boom) the average temperature dropped to about 3000° К, so that negative electrons were able to meet positive nuclei and form up neutral atoms. Until the Universe was composed of charged particles only, it remained opaque for radiation, because radiation tends to interact intensively with charged particles. But when the matter in the Universe became mostly neural, the majority of radiation split away from matter.
In 1964 astronomers A. Penzias and R. Wilson were busy with adjustment of a new radio antenna providing very low level of noise; the antenna was engineered for research of radio-waves emitted by our Galaxy at the wavelength of 7.35 cm. They discovered a new type of radiation that seemed to come from sky in a completely isotropic fashion. The further study revealed that the spectrum of this noise radiation corresponds to black-body radiation spectrum with the temperature of 2.7° К.
Today this discovery by A. Penzias and R. Wilson is interpreted as cosmic microwave background (CMB) radiation, which appeared in early Universe (when the temperature was about 3000° К and the Universe size was by thousand times smaller than now). That is why this radiation is called “relic radiation”. The temperature (energy) of relic radiation while moving in the expanding Universe decreased steadily and nowadays it reached the level of 2.7° К.
This discovery of relic radiation put the theory of hot Universe on the scene, and interest of cosmologists to alternative theories (e.g., theory of stationary Universe) has fade away.
And what about origin of galaxies in the framework of hot Universe theory?
It is assumed also that the early Universe had fluctuations in density – small deviations from the uniform level. These deviations enhanced with growth of Universe, so they evolved into galaxies. Тhat is, galaxies were formed due to gravitational condensation of rarefied mass of gas around the areas with local increased density.
This scenario seems quite verisimilar. However, all attempts of going deeper and quest for details have failed.