  We present pop-cosmos: a comprehensive model characterizing the galaxy
population, calibrated to $140,938$ ($r<25$ selected) galaxies from the Cosmic
Evolution Survey (COSMOS) with photometry in $26$ bands from the ultra-violet
to the infra-red. We construct a detailed forward model for the COSMOS data,
comprising: a population model describing the joint distribution of galaxy
characteristics and its evolution (parameterized by a flexible score-based
diffusion model); a state-of-the-art stellar population synthesis (SPS) model
connecting galaxies' instrinsic properties to their photometry; and a
data-model for the observation, calibration and selection processes. By
minimizing the optimal transport distance between synthetic and real data we
are able to jointly fit the population- and data-models, leading to robustly
calibrated population-level inferences that account for parameter degeneracies,
photometric noise and calibration, and selection. We present a number of key
predictions from our model of interest for cosmology and galaxy evolution,
including the mass function and redshift distribution; the
mass-metallicity-redshift and fundamental metallicity relations; the
star-forming sequence; the relation between dust attenuation and stellar mass,
star formation rate and attenuation-law index; and the relation between
gas-ionization and star formation. Our model encodes a comprehensive picture of
galaxy evolution that faithfully predicts galaxy colors across a broad redshift
($z<4$) and wavelength range.
