Present cosmological data (eg., supernovae type Ia, CMB temperature fluctuations, galaxy clustering, high-z clusters of galaxies, etc.) strongly suggests that we live in a flat and accelerating universe, which contains at least some sort of cold dark matter and an extra component which acts as having a negative pressure, as for example the energy of the vacuum (or in a more general setting the so called dark energy), to explain the observed accelerated cosmic expansion.
The fundamental importance of the detection and quantification of dark energy for our understanding of the cosmos and for fundamental theories implies that the results of the different experiments should not only be scrutinized, but alternative independent methods to measure dark energy should be developed and applied as well.

Also the accurate and precise measurement of the Hubble constant provides crucial, empirical constraints to help guide the emerging cosmological model and is considered one of the most fundamental tasks on the interface between Astronomy and Cosmology. Its importance steams from the fact that the accurate determination of the local expansion rate of the Universe is a prerequisite for independent constraints on the mass-energy content of the Universe, ie., its is a necessary prior in order to break the existing degeneracies among the cosmological parameters.

I would describe the use of Giant HII regions and HII galaxies as an alternative cosmic tracer to measure the Hubble constant [H0], the Dark Energy Equation of State and the matter content of the Universe [w(z) & Ωm] helping to break the dark-energy models degeneracy since the proposed tracer can be followed with 10m class telescopes up to z ~ 3.5.