Matter creation and cosmic acceleration
  We investigate the creation of cold dark matter (CCDM) cosmology as an
alternative to explain the cosmic acceleration. Particular attention is given
to the evolution of density perturbations and constraints coming from recent
observations. By assuming negligible effective sound speed we compare CCDM
predictions with redshift-space-distortion based f(z) sigma_8(z) measurements.
We identify a subtle issue associated with which contribution in the density
contrast should be used in this test and then show that the CCDM results are
the same as those obtained with LambdaCDM. These results are then contrasted
with the ones obtained at the background level. For the background tests we
have used type Ia supernovae data (Union 2.1 compilation) in combination with
baryonic acoustic oscillations and cosmic microwave background observations and
also measurements of the Hubble parameter at different redshifts. As a
consequence of the studies we have performed at both the background and
perturbation levels, we explicitly show that CCDM is observationally degenerate
with respect to LambdaCDM (dark degeneracy). The need to overcome the lack of a
fundamental microscopic basis for the CCDM is the major challenge for this kind
of model.
