Local measurement of {\Lambda} using pulsar timing arrays
  We have considered the propagation of gravitational waves (GW) in de Sitter
space time and how a non-zero value of the cosmological constant might affect
their detection in pulsar timing arrays (PTA). If {\Lambda} is different from
zero waves are non-linear in Friedmann-Robertson-Walker coordinates and
although the amount of non-linearity is very small it gives noticeable effects
for GW originating in extragalactic sources such as spiraling black hole
binaries. The results indicate that the timing residuals induced by
gravitational waves from such sources in PTA would show a peculiar angular
dependence with a marked enhancement around a particular value of the angle
subtended by the source and the pulsars, depending mainly on the actual value
of the cosmological constant and the distance to the source. The position of
the peak could represent a gauge of the value of \Lambda. The enhancement that
the new effect brings about could facilitate the first direct detection of
gravitational waves while representing a local measurement of \Lambda.
