Stochastic Gravitational Wave Background originating from Halo Mergers
  The stochastic gravitational wave background (GWB) from halo mergers is
investigated by a quasi-analytic method. The method we employ consists of two
steps. The first step is to construct a merger tree by using the Extended
Press-Schechter formalism or the Sheth & Tormen formalism, with Monte-Carlo
realizations. This merger tree provides evolution of halo masses. From $N$-body
simulation of two-halo mergers, we can estimate the amount of gravitational
wave emission induced by the individual merger process. Therefore the second
step is to combine this gravitaional wave emission to the merger tree and
obtain the amplitude of GWB. We find $\Omega_{GW}\sim 10^{-19}$ for $f\sim
10^{-17}-10^{-16}$ Hz, where $\Omega_{GW}$ is the energy density of the GWB. It
turns out that most of the contribution on the GWB comes from halos with masses
below $10^{15} M_\odot$ and mergers at low redshift, i.e., $0<z<0.8$.
