  The next generation ground-based gravitational wave interferometers will
possibly observe mergers of binary black holes (BBHs) and binary neutron stars
(BNSs) to redshift $z\gtrsim{}10$ and $z\gtrsim{}2$, respectively. Here, we
characterize the properties of merging BBHs, BNSs and neutron star-black hole
binaries across cosmic time, by means of population-synthesis simulations
combined with the Illustris cosmological simulation. We find that the mass of
merging compact objects does not depend (or depends very mildly) on the merger
redshift. Even the mass distribution of black holes depends only mildly on
redshift, because BBHs originating from metal-poor progenitors
($Z\leq{}4\times{}10^{-3}$) dominate the entire population of merging BBHs
across cosmic time. For a common-envelope efficiency $\alpha{}\ge{}3$, the main
difference between the mass distribution of BBHs merging in the last Gyr and
that of BBHs merging more than 11 Gyr ago is that there is an excess of heavy
merging black holes ($20-35$ M$_\odot$) in the last Gyr. This excess is
explained by the longer delay time of massive BBHs.
