Most supermassive black holes must be rapidly rotating

We use the integrated spectrum of the X-ray background and quasars' spectral energy distribution to derive the contribution of quasars to the energy output of the universe. We find a lower limit for the energy from accretion onto black holes of 7% of the total luminosity of the universe and probably more, with 15% quite possible. Comparing these values with the masses of black holes in the center of nearby galaxies, we show that the accretion process must be, on average, very efficient: at least 15% of the accreted mass must be transformed into radiated energy. This further implies that most supermassive black holes are rotating rapidly.