  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.
