Friedmann's theory discovered the most grandiose natural phenomenon, the cosmological expansion. Having passed through a rigorous checkup in the discussion with Einstein, it very soon found proof and confirmation in astronomical observations.

This concluding chapter of the book deals with the astronomical investigations that set up an observational basis for the science of the Universe; it discusses the further development of Friedmann's legacy and the ideas of modern cosmology, a living and rapidly developing science that eagerly absorbs the latest findings provided by astronomical observations and the brightest ideas produced by fundamental physics.

Galaxies and the Universe

Speaking of stars in his 1917 cosmological article (see Chapter 9), Einstein referred to them as the basic elements constituting the Universe. That was the period when the astronomical concept that our stellar system was unique enjoyed wide popularity. It was assumed that the Milky Way with all its stars, the Sun being one of them, was the whole Universe. Einstein too may have believed this. In the early 1920s, however, the astronomical picture of the world was altered drastically. The leading role in cosmology turned to be played by another class of astronomical objects – the nebulae.

Nebulae had long been known to astronomers, and they were regarded, within the concept of the unique stellar system, as comparatively small gas clouds floating among the stars. The true nature of the nebulae was established when the first major telescopes appeared. Three hundred years before, Galileo had invented a telescope and used it to see individual stars in the Milky Way. He proved therefore that the whitish strip encompassing the whole sky consists of stars that the naked eye cannot distinguish.