Dynamic Zoom Simulations: a fast, adaptive algorithm for simulating
  lightcones
  The advent of a new generation of large-scale galaxy surveys is pushing
cosmological numerical simulations in an uncharted territory. The simultaneous
requirements of high resolution and very large volume pose serious technical
challenges, due to their computational and data storage demand. In this paper,
we present a novel approach dubbed Dynamic Zoom Simulations -- or DZS --
developed to tackle these issues. Our method is tailored to the production of
lightcone outputs from N-body numerical simulations, which allow for a more
efficient storage and post-processing compared to standard comoving snapshots,
and more directly mimic the format of survey data. In DZS, the resolution of
the simulation is dynamically decreased outside the lightcone surface, reducing
the computational work load, while simultaneously preserving the accuracy
inside the lightcone and the large-scale gravitational field. We show that our
approach can achieve virtually identical results to traditional simulations at
half of the computational cost for our largest box. We also forecast this
speedup to increase up to a factor of 5 for larger and/or higher-resolution
simulations. We assess the accuracy of the numerical integration by comparing
pairs of identical simulations run with and without DZS. Deviations in the
lightcone halo mass function, in the sky-projected lightcone, and in the 3D
matter lightcone always remain below 0.1%. In summary, our results indicate
that the DZS technique may provide a highly-valuable tool to address the
technical challenges that will characterise the next generation of large-scale
cosmological simulations.
