Gravitational-wave Limits from Pulsar Timing Constrain Supermassive
  Black Hole Evolution
  The formation and growth processes of supermassive black holes (SMBHs) are
not well constrained. SMBH population models, however, provide specific
predictions for the properties of the gravitational-wave background (GWB) from
binary SMBHs in merging galaxies throughout the Universe. Using observations
from the Parkes Pulsar Timing Array, we constrain the fractional GWB energy
density with 95% confidence to be ${\Omega}_{GW}(H_0/73 {\rm km} {\rm s}^{-1}
{\rm Mpc}^{-1})^2 < 1.3 \times 10^{-9}$ at a frequency of 2.8 nHz, which is
approximately a factor of six more stringent than previous limits. We compare
our limit to models of the SMBH population and find inconsistencies at
confidence levels between 46% and 91%. For example, the standard galaxy
formation model implemented in the Millennium simulations is inconsistent with
our limit with 50% probability.
