# Iterative initial condition reconstruction

## TL;DR

This paper introduces an iterative algorithm to reconstruct initial cosmic density conditions from dark matter distributions, significantly improving the accuracy of large-scale structure analysis and BAO measurements.

## Contribution

The paper presents a novel iterative reconstruction method that effectively reverses nonlinear gravitational effects up to one-loop order, enhancing the recovery of initial conditions from dark matter data.

## Key findings

- Reconstruction achieves over 95% correlation with initial density at certain scales.
- Reduces differences between reconstructed and initial fields by over 100 times at specific scales.
- Improves BAO signal-to-noise ratio by approximately 2.5 to 2.7 times, approaching the linear density limit.

## Abstract

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 $k\le 0.35\; h\mathrm{Mpc}^{-1}$. The reconstruction also reduces the power in the difference between reconstructed and initial fields by more than 2 orders of magnitude at $k\le 0.2\; h\mathrm{Mpc}^{-1}$, 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.