Implication of Pulsar Timing Array Experiments on Cosmological
  Gravitational Wave Detection
  Gravitational waves provide a new probe of the Universe which can reveal a
number of cosmological and astrophysical phenomena that cannot be observed by
electromagnetic waves. Different frequencies of gravitational waves are
detected by different means. Among them, precision measurements of pulsar
timing provides a natural detector for gravitational waves with light-year
scale wavelengths. In this review, first a basic framework to detect a
stochastic gravitational wave background using pulsar timing array is
introduced, and then possible interpretations of the latest observational
result of 12.5 year NANOGrav data are described.
