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

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Science  18 Oct 2013:
Vol. 342, Issue 6156, pp. 334-337
DOI: 10.1126/science.1238012

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Testing Black Holes

Gravitational waves, predicted by General Relativity, are expected to be produced when very massive bodies, such as black holes, merge together. Shannon et al. (p. 334) used data from the Parkes Pulsar Timing Array project to estimate the gravitational wave background produced by pairs of supermassive black holes (those with masses between 106 and 1011 that of the Sun) in merging galaxies. The results can be used to test models of the supermassive black hole population.


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 (ΩGW) with 95% confidence to be ΩGW(H0/73 kilometers per second per megaparsec)2 < 1.3 × 10−9 (where H0 is the Hubble constant) at a frequency of 2.8 nanohertz, which is approximately a factor of 6 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 Simulation Project is inconsistent with our limit with 50% probability.

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