Iterative initial condition reconstruction

Motivated by recent developments in perturbative calculations of the nonlinear evolution of large-scale structure, we present an iterative algorithm to reconstruct the initial conditions in a given volume starting from the dark matter distribution in real space. In our algorithm, objects are first moved back iteratively along estimated potential gradients, with a progressively reduced smoothing scale, until a nearly uniform catalog is obtained. The linear initial density is then estimated as the divergence of the cumulative displacement, with an optional second-order correction. This algorithm should undo nonlinear effects up to one-loop order, including the higher-order infrared resummation piece. We test the method using dark matter simulations in real space. At redshift z=0, we find that after eight iterations the reconstructed density is more than 95% correlated with the initial density at k≤0.35 hMpc-1. The reconstruction also reduces the power in the difference between reconstructed and initial fields by more than 2 orders of magnitude at k≤0.2 hMpc-1, and it extends the range of scales where the full broadband shape of the power spectrum matches linear theory by a factor of 2–3. As a specific application, we consider measurements of the baryonic acoustic oscillation (BAO) scale that can be improved by reducing the degradation effects of large-scale flows. In our idealized dark matter simulations, the method improves the BAO signal-to-noise ratio by a factor of 2.7 at z=0 and by a factor of 2.5 at z=0.6, improving standard BAO reconstruction by 70% at z=0 and 30% at z=0.6, and matching the optimal BAO signal and signal-to-noise ratio of the linear density in the same volume. For BAO, the iterative nature of the reconstruction is the most important aspect.

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26 pages, 14 figures, published version. Code available at https://github.com/mschmittfull/iterrec

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