  We review the main scientific pictures of the universe developed from ancient
times to Albert Einstein and underline that all of them treated the universe as
a stationary system with unchanged physical properties. In contrast to this,
100 years ago Alexander Friedmann predicted that the universe expands starting
from the point of infinitely large energy density. We briefly discuss the
physical meaning of this prediction and its experimental confirmation
consisting of the discovery of redshift in the spectra of remote galaxies and
relic radiation. After mentioning the horizon problem in the theory of the hot
universe, the inflationary model is considered in connection with the concept
of quantum vacuum as an alternative to the inflaton field. The accelerated
expansion of the universe is discussed as powered by the cosmological constant
originating from the quantum vacuum. The conclusion is made that since
Alexander Friedmann's prediction of the universe expansion radically altered
our picture of the world in comparison with the previous epochs, his name
should be put on a par with the names of Ptolemy and Copernicus.
