Cosmic variance in the nanohertz gravitational wave background

Understand

We use large N-body simulations and empirical scaling relations between dark matter halos, galaxies, and supermassive black holes to estimate the formation rates of supermassive black hole binaries and the resulting low-frequency stochastic gravitational wave background (GWB).

• We find this GWB to be relatively insensitive ($\lesssim10\%$) to cosmological parameters, with only slight variation between WMAP5 and Planck cosmologies.
• We find that uncertainty in the astrophysical scaling relations changes the amplitude of the GWB by a factor of $\sim 2$.
• Current observational limits are already constraining this predicted range of models.