Open N-Body Codes Unlock Precise Tests of Modified Gravity for Future Cosmic Surveys
Preprint introduces open PySCo-EFT and ECOSMOG-EFT codes for accurate nonlinear simulations in EFTofDE models of modified gravity. Validated to <0.5% on large scales and <2% on small via convergence tests, these tools will help test gravity theories with data from Euclid and LSST surveys by modeling Vainshtein screening.
The preprint by Ganjoo et al. (arXiv:2604.15434) delivers two open-source N-body codes, PySCo-EFT and ECOSMOG-EFT, for simulating structure formation in universes described by the Effective Field Theory of Dark Energy. Unlike previous efforts that often kept codes proprietary, these are made publicly available, marking a collaborative leap forward. EFTofDE, as introduced in the seminal 2013 work by Gubitosi, Piazza and Vernizzi (arXiv:1212.4526), provides a unified language for a vast range of modified gravity models using a limited set of parameters. Building on the original ECOSMOG code from 2011 (arXiv:1110.2215), the new implementations incorporate luminal gravitational wave speeds and focus on the nonlinear Vainshtein screening. The methodology involves particle mesh techniques in PySCo-EFT for rapid exploration and adaptive mesh refinement in the RAMSES-based ECOSMOG-EFT for high resolution in dense areas. Using multigrid and iterative solvers for the scalar field equation, the team demonstrates sub-0.5% consistency with linear theory on large scales across a series of convergence tests varying mass resolution, simulation volume, starting redshift and refinement thresholds. On nonlinear scales, the two codes agree to similar precision. They quantify limitations from these choices, with all contributing errors below 2% at k=10 h/Mpc. What much of the existing literature on EFTofDE missed was the quantitative assessment of how screening can either dominate or be negligible depending on parameter choices — a finding that could explain why some gravity tests pass while others show hints of tension. Previous coverage also underplayed the role of open tools in enabling the community to generate the thousands of simulation realizations needed for robust covariance estimates in next-generation surveys like Euclid, DESI, and LSST. In the context of the ongoing S8 tension, where observed clustering appears lower than ΛCDM predictions, these codes allow exploration of whether modified gravity can alleviate it on nonlinear scales without violating Solar System tests, thanks to screening. As a preprint, peer review may yet refine some numerical claims. Limitations include restriction to Horndeski models without additional baryonic or neutrino physics. Nevertheless, this tandem of codes bridges theoretical alternatives to gravity with the practical needs of upcoming observational campaigns, potentially reshaping our understanding of cosmic acceleration.
HELIX: These freely available simulation codes will allow cosmologists to accurately forecast how matter should cluster if gravity deviates from Einstein's predictions on cosmic scales, helping interpret results from upcoming telescopes and test if dark energy is really just modified gravity.
Sources (3)
- [1]PySCo-EFT and ECOSMOG-EFT: a tandem of N-body simulation codes for the Effective Field Theory of Dark Energy(https://arxiv.org/abs/2604.15434)
- [2]Effective Field Theory of Dark Energy(https://arxiv.org/abs/1212.4526)
- [3]ECOSMOG: An Efficient COde for Simulating MOdified Gravity(https://arxiv.org/abs/1110.2215)