16 Feb. 2026 - Lyα telecon

Corentin Ravoux

Hydro simulations

• Nyx simulation code:
  • Dark matter particles with N-body and Particle Mesh
  • Baryons resolved with full hydrodynamic simulations
• GPU version: highly vectorizable
• Initialization with Zel'dovic approximation at z=200
• Computation of P3D/P1D with gimlet (MPI code)

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

The ACCEL² simulation suite

• Simulation grid: [160 - 640] Mpc/h, Resolution range: [25 - 100] kpc/h
• Proper modeling of Lyman-α forest needs a resolution four times higher than the Jeans scale (~25 kpc/h)
• Time-varying, but spatially uniform, ultraviolet background
• Rescaling of mean transmitted flux to match observations
• One thermal history for now (can be done in post-processing)

Lukic et al. 2015, Chabanier et al. 2023

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

P3D from gimlet

Best resolution

Largest simulation

• P3D computed with the whole simulation volume in 3 directions
• Profile caused by thermal broadening and Jeans smoothing

Chabanier, Ravoux et al. 2024

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

P3D fit results with Arynio

Physical resolution has more impact than the box size because of mode coupling

• Variation of bias and RSD parameters as a function of redshift
• Agreement for bias, large changes for RSD parameter

Chabanier, Ravoux et al. 2024

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

P3D fit results with Arynio

Chabanier, Ravoux et al. 2024

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

What is available on NERSC

• Full grids for the 6 simulations and all redshifts (on my STORE):
  • Density fields (baryons or dark matter)
  • Lyman-alpha flux field
• Measurements for all sims and redshifts:
  • P1D, P3D
  • Full P3D (for all k_par, k_perp): can be converted to correlation function (to do)

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

EFT project with ACCEL²: P3D EFT

Text

Roger de Belsunce

• Fit of EFT model directly to the measured P3D
• Improve constraints on EFT bias terms
• Comparison of EFT to linear model
• Estimation of the non-linear BAO shift

de Belsunce et al. 2025

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

Text

Kazuyuki Akitsu, Stephen Chen, Mikhail Ivanov

• Fit by forward modeling method directly on the simulation Lyman-α field
• Issue: need initial condition field from the simulation (not saved for the first ACCEL² grid)
• Aim: Field level fit & generation (could be used for fast mock generation)

EFT project with ACCEL²: EFT at field level

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

Text

• IC not saved for the first ACCEL² grid
• Several tests to "reverse-engineer" the IC: Comparison of correlation at the field level for different MPI rank --> not sucessful
• Rerunning on perlmutter (currently tested) & IC saving
  • 1536³ grid with 160 Mpc.h⁻¹
  • 3072³ grid with 160 Mpc.h⁻¹
  • Consider rerunning the 6144³ (need larger allocation & a lot of large memory nodes --> special call)

EFT project with ACCEL²: Initial condition issue

Corentin Ravoux                                                                                                                                                                     Lyα telecon - February 2026

2026

2027

2028

2029

2030

2031

DESI

DESI-I

DESI-Ib

DESI-II

LSST

2032

2033

2025

2036

4MOST

Main survey

Main survey

Comissioning

ZTF

ZTF3