The Unsettled Number: Hubble's Tension

International audienceOne of main sources of uncertainty in modern cosmology is the present rate of the Universe's expansion, H0, called the Hubble Constant. Once again different observational techniques bring different results causing a new "Hubble tension". In the present work we review the historical roots of Hubble constant from the beginning of the XX century, when modern cosmology started, to the present. We develop the arguments that gave rise to the importance of measuring the expansion of the Universe, its discovery and describing the different pioneering works to measure it. There has been a long dispute on this matter, even in the present epoch that is marked by high-tech instrumentation and therefore resulting in smaller uncertainties in the relevant parameters. It is again now necessary to conduct a careful and critical revision of the different methods, before one invokes new physics to solve the so-called Hubble tension