From a cosmic population to a PTA signal
Adjust a semi-analytic merger-rate model and see how its black-hole-binary population changes across chirp mass and redshift—and what that implies for the nanohertz gravitational-wave background.
Build a black-hole-binary population
Change the merger-rate law and watch the PTA source population and its ensemble gravitational-wave background respond.
d²N / d log₁₀𝓜 dz · integrated over the PTA band
Circular, GW-driven binaries · Poisson population realization
The five controls define the semi-analytic mass–redshift population in fastropop.SemiAnalyticPopulation. The upper map can show either where PTA-band binaries are most numerous or where they contribute most strongly to the strain; massive nearby systems are rarer but individually louder. The lower plot contrasts the ensemble-mean background with one finite Poisson realization, revealing the bin-to-bin structure produced by discrete sources. A small z < 10−4 cutoff regularizes the idealized model’s divergent nearest-source variance.
What is being calculated?
The controls are the five parameters of fastropop’s semi-analytic population law: its normalization, mass slope and cutoff, and redshift evolution and cutoff. Together they determine how many binaries occupy each chirp-mass and redshift interval.
The upper panel separates two related questions. Source abundance identifies where PTA-band binaries are common, while GW contribution weights those systems by how strongly they affect the strain. The lower panel then compares the smooth ensemble expectation with one discrete population realization. Its irregular high-frequency structure comes from the small number of individually influential binaries in each bin, not from a change in the underlying mean population.