Early growth of massive black holes in dynamical dark energy models with negative cosmological constant

Recent results from combined cosmological probes indicate that the dark energy component of the Universe could be dynamical. The simplest explanation envisages the presence of a quintessence field rolling into a potential, where the dark energy density parameter Ω_ DE =Ω_Λ+Ω_ x results from the contribution of the ground-state energy, Ω_Λ, and the scalar field energy, Ω_x. Provided that Ω_ DE ≈ 0.7, negative values of Ω_Λ can be consistent with current measurements from cosmological probes and could help explain the large abundance of bright galaxies observed by the JWST at z> 10, largely exceeding the pre-JWST expectations in a Λ cold dark matter universe. We aim to explore the extent to which such a scenario can also account for the early presence of massive black holes (BHs) with masses of M_ BH ≳ 10^7,M_⊙ observed at z≳ 8 and for the large over-abundance of active galactic nuclei (AGNs) with respect to pre-JWST expectations. Our aim is not to provide a detailed description of BH growth, but rather to compute the maximal BH growth that can occur in cosmological models with negative Ω_Λ under the simple assumption of Eddington-limited accretion onto initial light BH seeds with masses of seed ∼ 10^2,M_⊙ that originated from Pop III stars. To this aim, we developed a simple analytic framework to connect the growth of dark matter halos to the maximal growth of BHs within the above assumptions. We show such models can account for present observations assuming values of Ω_Λ≈ -1, simultaneously boosting both galaxy and AGN number counts without invoking any additional physics. This would allow us to trace the observed excess of bright and massive galaxies and the early formation of massive BHs and the abundance of AGNs to the same cosmological origin.