Cosmological tests of the gravastar hypothesis

Abstract

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.