  We introduce a new model of the evolution of the concentration of dark matter
halos, c(t). For individual halos, our model approximates c(t) as a power law
with a time-dependent index, such that at early times, concentration has a
nearly constant value of c=3-4, and as cosmic time progresses, c(t) smoothly
increases. Using large samples of halo merger trees taken from the Bolshoi-P
and MDPL2 cosmological simulations, we demonstrate that our 3-parameter model
can approximate the evolution of the concentration of individual halos with a
typical accuracy of 0.1 dex for t>2 Gyr for all Bolshoi-P and MDPL2 halos of
present-day mass greater than 10^11.5 Msun. We additionally present a new model
of the evolution of the concentration of halo populations, which we show
faithfully reproduces both average concentration growth, as well as the
diversity of smooth trajectories of c(t), including capturing correlations with
halo mass and halo assembly history. Our publicly available source code,
Diffprof, can be used to generate Monte Carlo realizations of the concentration
histories of cosmologically representative halo populations; Diffprof is
differentiable due to its implementation in the JAX autodiff library, which
facilitates the incorporation of our model into existing analytical halo model
frameworks.
