Cosmological tests of the gravastar hypothesis
  Gravitational vacuum stars (gravastars) have become a viable theoretical
alternative to the black hole (BH) end-stage of stellar evolution. These
objects gravitate in vacuum like BHs, yet have no event horizons. In this
paper, we present tests of the gravastar hypothesis within flat Friedmann
cosmology. Such tests are complementary to optical and gravitational wave
merger signatures, which have uncertainties dominated by the poorly constrained
gravastar crust. We motivate our analysis directly from the action principle,
and show that a population of gravastars must induce a time-dependent dark
energy (DE). The possibility of such a contribution has been overlooked due to
a subtlety in the de facto definition of the isotropic and homogeneous stress.
Conservation of stress-energy directly relates the time evolution of this DE
contribution to the measured BH comoving mass function and a gravastar
population crust parameter $\eta$. We show that a population of gravastars
formed between redshift $8 \lesssim z \lesssim 20$ readily produces the
present-day DE density over a large range of $\eta$, providing a natural
resolution to the coincidence problem. We compute an effective DE equation of
state $w_\mathrm{eff}(a)$ as a function of present-epoch BH population
observables and $\eta$. Using a BH population model developed from the cosmic
star formation history, we obtain $w_0$ and $w_a$ consistent with Planck
best-fit values. In summary, the gravastar hypothesis leads to an unexpected
correlation between the BH population and the magnitude and time-evolution of
DE.
