StarTrack predictions of the stochastic gravitational-wave background from compact binary mergers

We model the gravitational-wave background created by double compact objects from isolated binary evolution across cosmic time using the \textbf{\textit{StarTrack}} binary population code. We include population I/II stars as well as metal-free population III stars. Merging and non-merging double compact object binaries are taken into account. In order to model the low frequency signal in the band of the space antenna LISA, we account for the evolution of the redshift and the eccentricity. We find an energy density of $\Omega_{GW} \sim 1.0 \times 10^{-9}$ at the reference frequency of 25 Hz for population I/II only, making the background detectable at 3 $\sigma$ after about 7 years of observation with the current generation of ground based detectors, such as LIGO, Virgo and Kagra, operating at design sensitivity. The contribution from population III is one order of magnitude below the population I/II for the total background, but dominates the residual background, after detected sources have been removed, in 3G detectors. It modifies the shape of the spectrum which starts deviating from the usual power law $\Omega_{GW}(f) \sim f^{2/3}$ after $\sim 10$ Hz. The contribution from the population of non merging binaries, on the other hand, is negligible, being orders of magnitude below. Finally, we observe that the eccentricity has no impact in the frequency band of LISA or ground based detectors.