Gravitational-wave Limits from Pulsar Timing Constrain Supermassive Black Hole Evolution

Understand

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%.