Probing the Nanohertz Gravitational Wave Background

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AST 0206044PI BackerElectromagnetic pulses from a distant clock traversing a gravitational wave experience a variable delay depending on the gravitational wave phase at the emitter and at the receiver. This effect allows the use of a spatial array of the most precise pulsars, which are precision celestial clocks, as a sensitive gravity-wave detector at very low, nHz, frequencies using a measurement interval extending over several years. It is expected there should be a low frequency random background of gravitational radiation as a relic from an early epoch of the formation of the Universe, but that an astrophysically significant 'foreground' of gravity waves from the coalescence of massive black hole binaries in the nuclei of galaxies will be the dominant source at nHz frequencies. The global Pulsar Timing Array of selected pulsars is essential for positive detection of gravitational radiation and involves a continuation and improvement of the Arecibo experiments, the new Green Bank Telescope, and a collaboration with pulsar groups in Amsterdam, France, and Australia to assemble the "best" combined data sets for the multi-pulsar/multi-frequency/multi-telescope analysis. The project will also learn about the intrinsic properties of pulsars and the turbulence structure of the plasma in the line of sight to the pulsar.***