Many cosmological models predict that the Universe is presently
filled with a stochastic gravitational wave background (GWB)
produced during the big bang era [122]. The idea to use pulsars as natural detectors of gravitational
waves was first explored independently by Sazhin and Detweiler in
the late 1970s [137,
54]. The basic concept is to treat the solar system barycentre and
a distant pulsar as opposite ends of an imaginary arm in space.
The pulsar acts as the reference clock at one end of the arm
sending out regular signals which are monitored by an observer on
the Earth over some time-scale
T
. The effect of a passing gravitational wave would be to cause a
change in the observed rotational frequency by an amount
proportional to the amplitude of the wave. For regular monitoring
observations of a pulsar with typical TOA uncertainties of
, this ``detector'' would be sensitive to waves with
dimensionless amplitudes
and frequencies as low as
[30,
37]. This method, which already yields interesting upper limits on
the GWB, is reviewed in §
5.1
. The idea of more sensitive detector based on an array of pulsar
clocks distributed over the sky is discussed in §
5.2
.