  If supermassive black holes (SMBHs) are the energy sources that power quasars
and active galactic nuclei, then QSO SDSS 1148+5251, the quasar with the
highest redshift (z_QSO=6.43), hosts a supermassive black hole formed within
0.9 Gyr after the Big Bang. This requirement places constraints on the
cosmological formation of SMBHs, believed to grow from smaller initial seeds by
a combination of accretion and mergers. We focus on gas accretion onto seeds
produced by the collapse of Pop III stars at high redshift. We incorporate the
results of recent relativistic, MHD simulations of disk accretion onto Kerr
black holes to track the coupled evolution of the masses and spins of the
holes. We allow for an additional amplification of ~10^4 in the mass of a
typical seed due to mergers, consistent with recent Monte Carlo simulations of
hierarchical mergers of cold, dark matter halos containing black hole seeds. We
find that the growth of Pop III black hole seeds to ~10^9 M_sun by z_QSO = 6.43
favors MHD accretion disks over standard thin disks. MHD disks tend to drive
the holes to a submaximal equilibrium spin rate a/M ~ 0.95 and radiation
efficiency epsilon_M ~ 0.2, while standard thin disks drive them to maximal
spin (a/M = 1) and efficiency (epsilon_M = 0.42). This small difference in
efficiency results in a huge difference in mass amplification by accretion at
the Eddington limit. The MHD equilibrium efficiency is consistent with the
observed ratio of the QSO plus AGN luminosity density to the local SMBH mass
density. Our prototype analysis is designed to stimulate the incorporation of
results from relativistic stellar collapse and MHD accretion simulations in
future Monte Carlo simulations of hierarchical structure formation to better
determine the cosmological role of SMBHs and their mass and spin distributions.
