ABSTRACT
This chapter introduces useful cosmological units of measurement. It discusses those fundamental cosmological observations, the pillars of cosmology that a model should be able to address in a unified picture, leading ideally to new phenomenological and testable predictions. The chapter introduces some convenient units of measurement and conventions. Indeed the electromagnetic radiation offers an extensive variety of options and corresponding methods of observation depending on the various frequency ranges. There are different ways to measure the distance of an electromagnetic radiation astronomical source. The existence of gravitational radiation was first predicted by A. Einstein as a consequence of the theory of general relativity in 1916. A luminous astronomical source with a known absolute luminosityL, the total emitted power, is called standard candle. A precise determination of the astronomical unit is done with great precision with techniques based on radar reflection off the surfaces of planets. This determination can be regarded as a first rung in the so-called cosmic distance ladder.
