Signs, symbols, and codes in cosmology from ancient to modern times

The circle is a symbol of perfection. Classical Greek philosophy attributed perfection and circular motion to the heavens. The motion of the sun, the moon, and the planets against the background of the stars are signs to be interpreted. As a culminating work of classical Greek cosmology Ptolemy’s theory as embodied in the Almagest, was the code which allowed to read the signs and connect them to the symbols. For 1500 years that work provided the code for the mental construction of the universe. When Dante was led through the nine spheres, he read the signs and saw the symbols. But at the same time astronomers had clear ideas about the cosmological implications of the same nine spheres. When 1609 Galileo turned the telescope to the sky, he expanded the universe into the realm of the nebulae (now called galaxies). When Thomas Wright around 1750 saw in the nebulae the location of heaven, Kant, inspired by Newton’s new theory of gravity, saw them as huge agglomerations of stars, a view confirmed 1925. The spectrum of a far away galaxy with its colours might have become the symbol for what we do not know about the universe. Yet spectroscopy, built on quantum mechanics, became the code that allows us to read the spectrum and extract knowledge about stars and the universe. Thus spectra have become key signs, and we know how to read them. When Slipher discovered the redshift of the nebulae, a long discussion began, how these signals should be interpreted. Lemaitre saw in the General Theory of Relativity the appropriate code, and thus became the discoverer of the expanding universe (often attributed to Hubble). – When are we sure, that we have correctly interpreted the signs, and that we have found the right code?