5 Conclusions and Future Prospects

The tremendous success to date of pulsars in testing different aspects of gravitational theory leads naturally to the question of what can be expected in the future. Improvements to the equivalence-principle violation tests will come from both refining the timing parameters of known pulsars (in particular, limits on eccentricities and orbital period derivatives) and the discovery of further pulsar–white-dwarf systems. Potentially coalescing pulsar–white-dwarf binaries, such as PSRs J1141–6545, J0751+1807 [89], and 1757–5322 [48Jump To The Next Citation Point], bear watching from the point of view of limits on dipolar gravitational radiation. Another worthy, though difficult, goal is to attempt to derive the full orbital geometry for ultra-low-eccentricity systems, as has been done for PSR J0437–4715 [139]; this would quickly lead to significant improvements in the eccentricity-dependent tests.

The orbital-period-derivative measurements of double-neutron-star binaries are already limited more by systematics (Galactic acceleration models for PSR B1913+16, and poorly known distance for PSR B1534+12) than by pulsar timing precision. However, with improved Galactic modeling and a realistic expectation of an interferometric (VLBI) parallax for PSR B1534+12, there is still hope for testing more carefully the prediction of quadrupolar gravitational radiation from these systems. The other timing parameters, equally important for tests of the quasi-static regime, can be expected to improve with time and better instrumentation, such as the wider-bandwidth coherent dedispersion systems now being installed at many observatories (see, e.g., [31128]). Especially exciting would be a measurement of the elusive Shapiro delay in PSR B1913+16; the longitude of periastron is now precessing into an angular range where it may facilitate such a measurement [144].

In the last few years, surveys of the Galactic Plane and flanking regions, using the 64-m Parkes telescope in Australia [107], have discovered several hundred new pulsars (see, e.g., [9250]), including several new circular-orbit pulsar–white-dwarf systems [484927] and the eccentric pulsar–white-dwarf binary PSR J1141–6545 [73]. A complete reprocessing of the Galactic Plane survey with improved interference filtering is in progress; thus there is still hope that a truly new system such as a pulsar–black-hole binary may emerge from this large survey. Several ongoing smaller surveys of small regions and globular clusters (see, e.g., [26111]) are also finding a number of new and exotic binaries, some of which may eventually turn out to be useful for tests of GR. The possible recent appearance of PSR J1141–6545 and the predicted disappearance of PSR B1913+16 due to geodetic precession make it worthwhile to periodically revisit previously surveyed parts of the sky in order to check for newly-visible exotic binaries. Over the next several years, large-scale surveys are planned at Arecibo [1] and the new 100-m Green Bank Telescope [58], offering the promise of over 1000 new pulsars including interesting binary systems. The sensitivity of these surveys will of course be dwarfed by the potential of the proposed Square Kilometre Array radio telescope [118], which will be sensitive to pulsars clear through our Galaxy and into neighbouring galaxies such as M31. The next 10 or 20 years promise to be exciting times for pulsar searchers and for those looking to set ever-more-stringent limits on deviations from general relativity.

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