Large-scale structure: the observed and reconstructed cosmic web as a volume, with flows, and a ΛCDM simulation to compare

This is a multi-part design issue spanning survey catalogues, reconstructed density grids, flow fields and ΛCDM simulation data, with no assignee or pull request yet. Start by reading the catalogue issues it references (#221/PR #223, #230) and the apps and MilkyWay.md notes it mentions, to see how the point-population engine and ray-marched volume are structured. Done means the app shows the observed galaxies and reconstructed web with named structures, and a simulated box at the same scale, with every brightness physical and sources documented.

Part of #117 (deep sky); the large-scale-structure half of #40. It follows the galaxy catalogues (#221 / PR #223 and the catalogue issues linked below), and as an analysis feature it lives in an app (see the apps issue linked below).

Goal

A volumetric view of the cosmic web around us: the real one, where Virgo, the Great Attractor, Coma, Perseus–Pisces, Shapley and the Local Void actually are, and a ΛCDM simulation of the same physics to compare it with or to fill in beyond what's mapped.

What's available now (2026)

Galaxy redshift and distance surveys (the observed web)

• 2MRS, the 2MASS Redshift Survey (Huchra et al. 2012): about 44,600 galaxies to K_s 11.75 over the whole sky. The standard all-sky map of the local universe.
• 2M++ (Lavaux & Hudson 2011): about 69,000 galaxies, combining 2MRS, 6dFGS and SDSS. It's the input of most reconstructions.
• Cosmicflows-4 (Tully et al. 2023): about 56,000 galaxy distances, so peculiar velocities. The density and velocity reconstructions from it (Courtois et al. 2023; Hoffman et al. 2024; Valade et al.) are the Laniakea pictures, and the Sketchfab model linked in #40 is one of them. Served from the Extragalactic Distance Database (edd.ifa.hawaii.edu).
• SDSS (about 1M main-sample redshifts) and DESI DR1 (2025, about 18M redshifts): deep but partial-sky. For later.

Reconstructed density fields (the real web as a grid)

• BORG on 2M++ (Jasche & Lavaux 2019): Bayesian initial conditions, so a posterior density field.
• Constrained simulations grown from those: CSiBORG, Manticore-Local (2025), and SIBELIUS (Sawala et al. 2022). An N-body simulation whose structures sit where the real ones do, out to about 150–200 Mpc. This is the closest thing to "a large volumetric simulation of the real local universe". Check which grids and licences are public before choosing.
• The CF4 density and velocity grids (Hoffman et al. 2024; Valade et al.) are linear-theory and smoother, but they come with the flows.

ΛCDM simulations (the statistical web)

• IllustrisTNG: TNG300 is a box 300 Mpc on a side, with public snapshots and density fields (tng-project.org).
• Uchuu (Ishiyama et al. 2021): a box 2.9 Gpc on a side, with public halo catalogues.
• Millennium and Millennium-XXL: public databases.
• AbacusSummit, FLAMINGO and Quijote: very large public suites.

Proposal

1. The web as points. 2MRS's galaxies are placed by distance: Cosmicflows-4 where measured, else redshift corrected by the CF4 flow model. They're drawn as points of their own K-band light converted to V on the point-population engine (#98), with labels for the major structures.
2. The web as a volume. A log-density cube of the real local universe, from a constrained reconstruction or simulation, about ±200 Mpc:
   • the grid is 256³ at 8 bits, about 16 MB, so it goes in Git LFS;
   • it's ray-marched with the Milky Way's integrated-light machinery (MilkyWay.md);
   • its light is the density times the mean luminosity density of the universe, so it's in exposure units like everything else, with no hand-set brightness.
3. Flows. CF4's velocity field as streamlines, showing Laniakea's basin and the flow toward the Great Attractor.
4. Real vs simulated. A TNG300 or Uchuu sub-box at the same scale and light, side by side or as a fill beyond the reconstruction's edge.

Steps 1–4 are an app in the widgets drawer (the apps issue linked below):

• its controls are points, volume, flows, structure labels, and real or simulated;
• its state is in apps.lss: tokens (design/URLs.md);
• its stop removes all of it.

The galaxies' labels stay a plain scene setting (e).

Hosts the sandbox will need

edd.ifa.hawaii.edu (CF4), and the chosen reconstruction's and simulation's hosts (tng-project.org, and the BORG/Manticore/Uchuu data hosts). Check them once the source is chosen. cdsarc.cds.unistra.fr (2MRS) is already allowed.

Done when

• From outside the Local Group, the app shows the observed galaxies and the reconstructed web around them, with the named structures where they are, and CF4's flows into Laniakea.
• A simulated ΛCDM box can be shown at the same scale for comparison.
• Every brightness is physical.
• The sources, grids and licences are documented.

Related: SPARC galaxies #221 / PR #223; globulars #228 (Harris, plus Baumgardt); deep-sky objects from OpenNGC #229; nearby galaxies #230; S4G and ATLAS3D #231; large-scale structure #232; catalogues as scene features, analysis as apps #233; galactic dynamics #106; the Milky Way #116.

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