Principles of Gravitational-Wave Detection with Pulsar Timing Arrays
  Pulsar timing uses the highly stable pulsar spin period to investigate many
astrophysical topics. In particular, pulsar timing arrays make use of a set of
extremely well-timed pulsars and their time correlations as a challenging
detector of gravitational waves. It turns out that pulsar timing arrays are
particularly sensitive to ultra-low-frequency gravitational waves, which makes
them complementary to other gravitational-wave detectors. Here, we summarize
the basics, focusing especially on supermassive black-hole binaries and cosmic
strings, which have the potential to form a stochastic gravitational-wave
background in the pulsar timing array detection band, and the scientific goals
on this challenging topic. We also briefly outline the recent interesting
results of the main pulsar timing array collaborations, which have found strong
evidence of a common-spectrum process compatible with a stochastic
gravitational-wave background and mention some new perspectives that are
particularly interesting in view of the forthcoming radio observatories such as
the Five hundred-meter Aperture Spherical Telescope, the MeerKAT telescope, and
the Square Kilometer Array.
