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Some of the means through which the possible presence of nearly deconfined quarks in neutron stars can be detected by astrophysical observations of neutron stars from their birth to old age are highlighted., Comment: 8 pages, 6 figures, Plenary talk in Quark Matter 2006 to appear in J. Phys. G: Nucl. Part. Phys. 34 (2007)

We study how the shear viscosity, η, entropy density, s, and η/s depend on growing hadron chemical potentials resulting from the loss of chemical equilibrium during the evolution of a relativistic heavy-ion collision. Our calculations here are for an interacting pion gas in a system of net baryon number zero. Time evolution of the temperature and pion chemical potential are taken from ideal fluid hydrodynamic calculations of RHIC and LHC collisions. We find that the shear viscosity decreases whereas the entropy density increases with increasing pion chemical potential resulting in values of η/s that are slightly reduced from the case of chemical potentials being zero when chemical equilibrium prevails. Our results indicate that the inclusion of additional mesons and baryons will likely lead to further reduction in the value of η/s.

The shear viscosity η and entropy density s of a hadron gas with zero baryon number density are calculated using the Chapman-Enskog and virial expansion approaches, respectively. Interactions are included via the K-matrix parametrization of cross sections preserving the unitarity of the S-matrix. In the four component mixture (π – K − N − η), a total of 57 resonances up to 2 GeV mass are included. Interactions forming resonances reduce the magnitude of η and increase s, both effects serving to progressively reduce η/s as the temperature nears the QCD phase transition temperature.