2012MNRAS.420.2429L

We compute the number density of massive black holes (BHs) at the centre of galaxies at z = 6 in different dynamical dark energy (DDE) cosmologies, and compare it with existing observational lower limits, to derive constraints on the evolution of the dark energy equation of state parameter w. Our approach only assumes the canonical scenario for structure formation from the collapse of overdense regions of the dark-matter-dominated primordial density field on progressively larger scales; the BH accretion and merging rate have been maximized in the computation so as to obtain robust constraints on w and on its look-back time derivative w_a. Our results provide independent constraints complementary to those obtained by combining supernovae, cosmic microwave background and baryonic acoustic oscillations, while the latter concern combinations of w₀ and w_a leaving the time evolution of the state parameter w_ahighly unconstrained, the BH abundance mainly provide upper limits on w_a, only weakly depending on w₀. Combined with the existing constraints, our results significantly restrict the allowed region in DDE parameter space, ruling out DDE models not providing cosmic time and fast growth factor large enough to allow for the building up of the observed abundance of BHs; in particular, models with -1.2 ≤ w₀ ≤ -1 and positive redshift evolution w_a ≳ 0.8 – completely consistent with previous constraints – are strongly disfavoured by our independent constraints from BH abundance. Such range of parameters corresponds to `quintom' DDE models, with w crossing -1 starting from larger values.