Direct detection of gravitational waves

The direct detection of GWs is extremely challenging. Their interaction with masses on Earth, for instance by changing the size or separation of masses, is almost unmeasurable as it will always be smaller than 1 part in 1020 (e.g. Schutz 2003, Cambridge University Press). Nevertheless, GW detectors have been constructed at several facilities around the world (e.g. GEO600 and VIRGO detectors in Europe, or the LIGO detectors in the US; see Danzmann 1995, in Edoardo Amaldi Conference on Gravitational Wave Experiments, eds. Coccia, Pizzella & Ronga, 100; Acernese et al., 2004, Astroparticle Physics, 21, 1; Abramovici et al., 1992, Science, 256, 325) conducting extraordinarily precise experiments which attempt to measure the changing distance between a number of test-masses. Unfortunately, due to their sensitivity limitations, the current generation of ground-based detectors has not been able to detect GWs yet. The detection of GWs on Earth is not only difficult because the displacement of the masses is so small, but it is also aggravated by seismic noise that causes the masses in a terrestrial laboratory to move. Fortunately, in addition to the high-frequency GWs in principle detectable on Earth (> few Hz), GWs could be produced and propagate at any frequency. Indeed, the amplitude of low-frequency GWs is expected to be much larger, and a wide variety of possible GW sources exists.

Another method exists that can detect GW at low frequencies, being sensitive to a variety of astrophysical and cosmological sources, ranging from the effects of “cosmic strings” (as predicted by theories of gravity known as “string theory”) and to the binary motion and merging of super-massive black holes at the heart of galaxies in the early Universe. The latter objects will produce signals in the range of a few nHz, i.e. well below the frequencies to which LISA might be sensitive. This complementary method is the high precision timing observations of an ensemble of radio millisecond pulsars in a so-called “Pulsar Timing Array” (PTA).

Pulsars as gravitational wave detectors

While pulsars already provide the indirect evidence for the existence of GWs, they can also be used to achieve the first direct detection. The idea is not new but was first proposed by Sazhin (1978, Soviet Astronomy, 22, 36) and Detweiler (1979, ApJ, 234, 1100). In this experiment, the observed pulsars would be timed to high precision, i.e. the arrival times of their pulses on Earth would be recorded accurately and compared with a rotational “timing“ model counting every single rotation of the neutron star. Slight deviations from the expected arrival times would be visible as significant “timing residuals”. Such timing residuals would be caused by a passing gravitational wave as each pulsar and the Earth can be considered as free masses whose positions respond to changes in the space-time metric. A perturbation by a GW would therefore displace both pulsars and the Earth slightly, leading to timing residuals that depend on the GW amplitude and the total observing time. The sensitivity of GW detection using pulsars scales directly with the achieved timing precision; the timing residuals are the figure of merit used to determine this precision.