Neutron Stars as Probes for General Relativity and Gravitational Waves

Abstract

The discovery of the first radio pulsar in a binary system, by Russell Hulse and Joseph Taylor in 1974, initiated a completely new field for testing general relativity (GR) and alternative theories of gravity. To date there are a number of binary pulsars known, which can be utilized for precision test of relativistic gravity. Depending on their orbital properties and their companion, these pulsars provide tests for various different phenomena, predicted by GR and its alternatives. In many aspects, these tests go far beyond of what can be achieved in the solar system. A prime example is the verification of the existence of gravitational waves, as predicted by GR. It is the large fractional binding energy ( ∼ 0. 1) and the strong internal gravity of neutron stars, that make high-precision timing of binary pulsars ideal probes for various predictions of strong-field gravity. So far, GR has passed all these tests with flying colors. In the near future, in terms of radio pulsars, new radio telescopes, like the SKA, will soon greatly enhance our timing capabilities of known binary pulsars. Furthermore, new instrumentation and search techniques promise the discovery of many new systems, suitable for testing GR, among these hopefully also a pulsar in orbit around a black hole. Quite recently, ground-based gravitational wave detectors have made their first observations of gravitational waves. This has not only opened a new window on the universe, but has also taken our gravity tests to the highly dynamical strong-field regime. While the first gravitational wave signals came from merging black holes, it is expected that in the near future mergers of double neutron-star as well as neutron star-black hole systems will be among the observed gravitational wave signals. Moreover, pulsar timing arrays are expected to soon observe gravitational waves in the nano-Hertz band, emitted by supermassive black hole binaries.