Constraining Alternative Theories of Gravity Using Pulsar Timing Arrays
The opening of the gravitational wave window by ground-based laser interferometers has made possible many new tests of gravity, including the first constraints on polarization. It is hoped that, within the next decade, pulsar timing will extend the window by making the first detections in the nanohertz frequency regime. Pulsar timing offers several advantages over ground-based interferometers for constraining the polarization of gravitational waves due to the many projections of the polarization pattern provided by the different lines of sight to the pulsars, and the enhanced response to longitudinal polarizations. Here, we show that existing results from pulsar timing arrays can be used to place stringent limits on the energy density of longitudinal stochastic gravitational waves. However, unambiguously distinguishing these modes from noise will be very difficult due to the large variances in the pulsar-pulsar correlation patterns. Existing upper limits on the power spectrum of pulsar timing residuals imply that the amplitude of vector longitudinal (VL) and scalar longitudinal (SL) modes at frequencies of 1/year are constrained, A_(VL) < 4×10^(−16) and A_(SL) < 4×10^(−17), while the bounds on the energy density for a scale invariant cosmological background are Ω_(VL)h^2 < 4×10^(−11) and Ω_(SL)h^2 < 3×10^(−13).
© 2018 American Physical Society. Received 18 December 2017; revised manuscript received 20 March 2018; published 4 May 2018. We appreciate Laura Sampson's help with generating simulated data sets. We thank Xavier Siemens for suggesting that we extend our study to cover cosmological backgrounds, and we thank Justin Ellis for providing the posterior samples for the characteristic strain that were used to calibrate our likelihood model. L. O., N. J. C., and S. R. T. appreciate the support of the NSF Physics Frontiers Center Grant No. PFC-1430284. The research was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. N. Y. also acknowledges support from the NSF CAREER Grant No. PHY-1250636 and NASA Grants No. NNX16AB98G and No. 80NSSC17M0041.
Submitted - 1712.07132.pdf
Published - PhysRevLett.120.181101.pdf