High-redshift objects and the generalized Chaplygin gas
  Motivated by recent developments in particle physics and cosmology, there has been growing interest in an unified description of dark matter and dark energy scenarios. In this paper we explore observational constraints from age estimates of high-$z$ objects on cosmological models dominated by an exotic fluid with equation of state $p = -A/\rho^{\alpha}$ (the so-called generalized Chaplygin gas) which has the interesting feature of interpolating between non-relativistic matter and negative-pressure dark energy regimes. As a general result we find that, if the age estimates of these objects are correct, they impose very restrictive limits on some of these scenarios. 
J. S. Alcaniz (University of Washington), Deepak Jain (University of   Delhi), Abha Dev (University of Delhi)