  We study oscillations and instabilities of relativistic stars using
perturbation theory in general relativity and take into account the
contribution of a dynamic spacetime. We present the oscillation spectrum as
well as the critical values for the onset of the secular CFS instability of
neutron stars, and propose universal relations for gravitational wave
asteroseismology, which may help constrain the neutron star radius and/or the
nuclear equation of state. The results are relevant for all stages during a
neutron star's life but especially to nascent or remnant objects following a
binary merger.

General Relativity and Quantum Cosmology