The current expansion rate of the Universe is captured by the so-called Hubble constant, or its dimensionless equivalent, “little h”, which is a key parameter in the, extremely successful, standard model of cosmology.
The Hubble constant relates measurements of the expansion history of the Universe to its components, and “little h” appears in all astrophysical quantities which measurement or calibration somewhat depend on the background cosmology.
There are many different ways to constrain H_0 or little h and they are fully equivalent only within a model. I will recap different approaches to measure h and discuss what they mean in both a model-dependent and model-independent way.