  Cosmic chronometers may be used to measure the age difference between passively evolving galaxy populations to calculate the Hubble parameter H(z). The age estimator emerges from the relationship between the amplitude of the rest frame Balmer break at 4000 angstroms and the age of a galaxy, assuming that there is one single stellar population within each galaxy. However, recent literature has shown possible contamination (up to 2.4% of the stellar mass in a high redshift sample) of a young component embedded within the predominantly old population of the quiescent galaxy.   We compared the data with the predictions of each model, using a new approach of distinguishing between systematic and statistical errors (in previous works, these had incorrectly been added in quadrature) and evaluating the effects of contamination by a young stellar component. The ages inferred using cosmic chronometers represent a galaxy-wide average rather than a characteristic of the oldest population alone. The average contribution from the young component to the rest luminosity at 4000 angstroms may constitute a third of the luminosity in some samples, which means that this is far from negligible. This ratio is significantly dependent on stellar mass, proportional to M^{-0.7}. Consequently, the measurements of the absolute value of the age or the differential age between different redshifts are incorrect and make the previous calculations of H(z) very inaccurate. Some cosmological models, such as the Einstein-de Sitter model or quasi-steady state cosmology, which are rejected under the assumption of a purely old population, can be made compatible with the predicted ages of the Universe as a function of redshift if we take this contamination into account. However, the static Universe models are rejected by these H(z) measurements, even when this contamination is taken into account. 
Impact of young stellar components on quiescent galaxies: deconstructing   cosmic chronometers